Important Questions for CBSE Class 9 Science Chapter 4 – Structure of the Atom

Important Questions Class 9 Science Chapter 4 – Structure of the Atom

Science is an integral part of our life.  Students may find it easy or difficult to understand the concepts if students have no idea how to approach it . However, students face problems understanding the fundamental concepts of Biology, Chemistry, and Physics studied in previous classes. Their interest and curiosity will not only encourage them to take an active interest in the world around them, as well as help them decide which stream they would like to pursue in higher classes.

 At Extramarks, we completely understand the importance of ensuring students understand concepts and the important questions and their role in preparation for the exam. Our subject matter experts have created important questions in Class 9 Science Chapter 4 at Extramarks. The primary purpose behind creating the Class 9 science Chapter 4 important questions is to enable students to clear their concepts of this chapter introduced in the CBSE syllabus

Extramarks experts have used a very easy-to-understand language while preparing the important questions. The list of important questions in Class 9 Science Chapter 4 helps students quickly revise most of the syllabus and score high grades in their examinations. Our experts have compiled more than thirty questions, including all types of questions such as Multiple-choice questions, long answers and short answer types. Chapter 4,  Structure of the Atom, is significant; referring to these questions will help the students revise the chapter well. 

Each question is a compilation of the NCERT textbook, NCERT exemplar, and other reference books. In addition, it has questions from past years’ exam papers. Science Class 9 Chapter 4 important questions are the best resource for preparing students for the final exams. Further, students can refer to other course material on our Extramarks website, including NCERT solutions, NCERT books, and past years’ questions papers. 

Get Access to CBSE Class 9 Science Important Questions 2022-23 with Solutions

Also, get access to CBSE Class 9 Science Important Questions for other chapters too:

CBSE Class 9 Science Important Questions

Sr No Chapters Chapter Name
1 Chapter 1 Matter in Our Surroundings
2 Chapter 2 Is Matter Around Us Pure
3 Chapter 3 Atoms and Molecules
4 Chapter 4 Structure of Atom
5 Chapter 5 The Fundamental Unit of Life
6 Chapter 6 Tissues
7 Chapter 7 Diversity in Living Organisms
8 Chapter 8 Motion
9 Chapter 9 Force and Laws of Motion
10 Chapter 10 Gravitation
11 Chapter 11 Work and Energy
12 Chapter 12 Sound
13 Chapter 13 Why Do We Fall ill
14 Chapter 14 Natural Resources
15 Chapter 15 Improvement in Food Resources

Class 9 Science Chapter 4 Important Questions With Solutions 

Long Answer Questions 

Q1. Define ionisation energy and electron affinity.

Answer: The ionisation energy of a part is the amount of energy required to supply energy to at least one mole (or more) of the element in the gaseous condition to produce one mole of caters. The electron affinity point refers to the energy change that occurs when one mole (or more) of the element’s atoms is formed within the gaseous condition.

Ionisation energy is the difficulty or trouble removing an electron from an atom, ion, or other substance. It is also the tendency for an atom to give up an electron. In general, electron elimination occurs in the ground state for chemical types.

Technically, ionisation energy can also be described as the minimum amount of energy that an electron must consume to escape the effects of the nucleus. It is also known as the ionisation potential. Often, this ionisation process in nature is endothermic. One can determine the compounds’ reactivity and chemical properties based on the Ionisation energy.

This concept can also be used to control the strength of chemical bonds. The units of measurement for ionisation energy are either electron volts or kJ/mol.

Q2. What are the benefits of the Periodic Table?


The benefits of long-form periodic tables are:

  • The periodic table enlists all the elements and facilitates the study of elements and their compounds. Periodic table groups are made up of elements that have similar properties. Knowing the properties of one element of a group will predict the properties of the other elements of the same group. This is why studying other elements of the same group is unnecessary.
  • You can predict the properties of an element by looking at its position in the periodic table.
  • For example, an element in the first group is more reactive than an element in the 18 th group or zero groups.
  • Periodic table elements are presented in tabular form. It is easy to recall the properties of elements when the element’s position is known. If the element’s position is known, the compounds made by them are also predictable. The study of chemistry is made more accessible and more systematic by the periodic table.

Q3. Why does Argon have zero valencies?

Answer: The Element Argon has 8 electrons in its valence shells. Since it has the maximum number of electrons in its valence shells, it does not have any tendency to combine with other elements. So, its valency is equal to zero.

Q4. Explain the drawbacks of Rutherford’s model of an atom?

Answer: Rutherford concluded from the a-particle scattering experiment that-

  • Maximum space within an atom is empty because most a-particles pass through the gold foil without getting deflected.
  • Significantly few particles were deflected from their path, indicating that the atom’s positive charge occupies very little space.
  • A tiny fraction of a-particles was deflected by 1800, thus establishing that the positive charges and the gold atom’s mass were concentrated in a minimal volume within the atom. From the data, he also calculated that the radius of the nucleus is about 105 times less than the radius of the atom.

Q5. Write any two observations which support the fact that atoms are divisible. 

Answer: The discovery of electrons and the discovery of protons supports the idea that atoms can be divided. A chemical reaction can transfer or share electrons among different atoms, which results in the rearrangement. It is possible to have isotopes of an element because of the differences in the number and size of neutrons. This supports the fact that atoms can be divisible.

Q6. Why did Rutherford select a gold foil in his a-ray scattering experiment?

Answer: Rutherford wanted a thin metal sheet to be used in the scattering experiment. Gold is the most malleable metal known. It is easy to make thin sheets from it. Rutherford used gold foil in his experiment with alpha-ray scattering.

Rutherford experiment

  • Rutherford carried out his gold foil experiment in 1911, and he could definitively establish the existence and position of protons in the atom.
  • He suggested that electrons are in a particular position and behave accordingly.
  • They used fast-moving particles of alpha to bombard fragile e-gold foil sheets.
  • Alpha particles are positive particles with a mass four times greater than hydrogen. Alpha particles are a type of natural radioactive particle.
  • Although the test did not reflect most alpha particles, about 1 in 8000 particles bounced off gold foil at extended angles.
  • Some of the alpha particles were even redirected to the source.

Q7. What are ions? What are the two types of ions?

Answer: An ion can be described as a small, electrically charged particle. Ions can be either single charged atoms (simple or polyatomic ions) or small charged “molecules”.

  • Simple ions are Na + and Ca 2+. Cl is -.
  • Polyatomic ions are (NH4)+ and (CO3)2-.

Contrary to protons, neutrons and electrons can easily be removed from an atom and added to it. This is how ions are formed. When an atom gains electrons or anions, it forms negative ions. This indicates that an atom’s number of protons and electrons is not balanced.

Q8. Please elaborate upon the difference between atomic weight and number in predicting elements’ chemical properties?

Answer: Atomic number refers to the number of protons within an atom. However, through a reaction, the number of protons does not change. Several electrons are also atomic numbers. The shell contains electrons that participate in chemical reactions. 

They also decide the chemical properties of elements. Relative atomic mass refers to the sum of the number of protons as well as the number of neutrons. This is important for predicting elements’ chemical properties.

Q9. The helium atom has 2 electrons in its valence shell, but its valency is not 2. Explain. 

Answer: He(z = 2) = 1s2

  1. Two valence electrons are present in the electronic configuration of a Helium atom. It is a noble gas.
  2. The outer shell of Helium contains 2 electrons, which completes the duplet configuration. Therefore, the valency for this atom is zero.

Q10. Why do the elements Helium, Neon, and Argon have zero valencies? 

Answer: A combining ability of an element is called valency. The valency of noble gases is zero because there are no free electrons, and the elements are already stable. Because the atoms in noble gases have their complete outer shells, they don’t lose, gain, or share electrons. The noble gases are therefore inert and don’t participate in chemical reactions. Noble gas doesn’t tend to lose electrons. Therefore, inert gas valency is zero.

Q11. Using Thompson’s model, explain how an atom can be neutral as a whole.

Answer: According to Thompson’s model for an atom

(i) An atom has a positively charged sphere in which the negatively charged electrons have been implanted.

(ii) The magnitudes of electrons and protons are equal, so an atom is electrically neutral.

Q12. Write the distribution of electrons within the Carbon and Sodium atoms.

Answer: 6 electrons are found in a carbon atom. The following equation shows the electron distribution within a carbon atom. First orbit, or K-shell = 2, second orbit, or K-shell = 2, third orbit or Ku-shell = 2, and fourth orbit, or K-shell = 4.

L-shell, or second orbit = 4 electrons

The electron distribution of a carbon atom can be expressed as 2, 4.

There are 11 total electrons in a sodium atom. The electron distribution of the sodium atom can be described as follows: 

K-shell or first orbit = 2 electrons; 

K-shell or second orbit = 2 electrons. 

K-shell or third orbit = 2 electrons. 

K-shell or third orbit = 2 electrons. 

K-shell or fourth orbit = 2 electrons.

8 electrons = L-shell, second orbit

M-shell, third orbit = 1 electron

Alternatively, the electron distribution of a sodium atom can be expressed as 2, 8, 1.

Q13. What makes metals electropositive while non-metals electronegative in nature?

Answer: Electropositive metals can be found in nature. Electropositive metals give up electrons from the outermost shell to complete the octet and thus become electropositive. Metals are now positively charged. Because non-metals are not electronegative, they must gain electrons to complete the octet. Non-metals are therefore negatively charged.

Q14. Describe Bohr’s model of an atom.

Answer: Rutherford’s Atomic Model has some limitations. Neil Bohr (1912) proposed a model for the atom to help overcome these problems and explain the structure in detail. Below are the postulates for Bohr’s model.

  • An electron revolves around a nucleus in an orbit of an atom with fixed energy.
  • The energy of orbits increases as the outer shells become inner shells. 
  • The orbit closest to the nucleus has the lowest energy value. 
  • When energy is supplied, electrons move from lower orbit to higher orbit. If an electron jumps from a higher orbit (energy level) to a lower orbit(energy level), then energy is emitted as electromagnetic waves.
  • Each shell or orbit represents an energy level represented by an integer number such as n=1,2,3,….. These orbits can be characterised by K, L, M.N ……….., and are titled from the nucleus to the outside.

Short Answer Questions 

Q15. What is meant by canal rays?

Answer: The radiation that is positively charged is known as canal rays. The discovery was crucial in discovering another subatomic particle that was positively charged – proton. 

Q16. Name the three atomic particles of an atom?

Answer: An atom has three subatomic, atomic particles, including: 

  • Portion- positively charged. 
  • Electron – negatively charged.
  • Neutrons – neutral 

Q17. What are the limitations of J.J. Thomson’s model of the atom?

Answer: These are the limitations of J.J. Thomson’s model of an Atom.

  • Rutherford conducted alpha particle scattering experiments that the model did not explain. The model did not show why most of these alpha particles pass through the gold foil, but others are diverted through small angles and large angles. Others rebound entirely and return to their original path.
  • It provided no experimental evidence and was based on imagination.

Q18. Please compare all the proposed models of an atom: 


Thomson Rutherford Bohr
  • Sphere is positively charged
  • Electrons are negatively charged and scattered throughout the inside of the sphere.
  • Negatively charged = positively charged
  • The net charge that an atom carries is zero
  • The nucleus, located at the centre of the mass, is positively charged and holds the whole mass.
  • Electrons can be negatively charged and revolve along a clearly defined path.
  • The atom’s size is much larger than the nucleus.
  • The centrifugal force acting against the nucleus balances the attraction of electrons to it. The electrons are therefore not attracted to the nucleus.
  • The nucleus is positively charged and is found at the centre.
  • Electrons can be negatively charged and revolve around but not radiate any energy.
  • The orbits are labelled K, L, M and N.

Q19. Write the distribution of electrons in sodium and carbon atoms?

Answer: Atomic number of carbon = 6. 

Several protons = the number of electrons. 

The distribution of electrons within carbon atoms is determined by: First orbit, or K-shell = 2 electrons. Second orbit, or L-shell = 4. Or, you can simply write the distribution as 2, 4,

Atomic number for sodium = 11= Number protons = Number electrons. 

The distribution of electrons within a sodium atom can be described as:

First orbit or K-shell = 2 electrons.

Second orbit, or L-shell= 8 electrons.

Third orbit, or M-shell = 1. Or, you could write the distribution of electrons inside a sodium atom at 2, 8,1

Q20. If an atom’s K and L shells get full, what will be the total number of electrons in the atom?

Answer: The maximum number of the electron in the K-shell = 2

Maximum number of the electron in L-shell = 8

If the K and L-shell of an atom are complete, then the total number of electrons in the atom will be (2+8) = 10 electrons. 

Q21. How do you find the valency of chlorine, sulphur, and magnesium?

Answer: In elements where the number of electrons in the outer layer shell is less than four, then the valency of an atom = number of the electron in the outermost shell of the atom. 

  1. In the case of magnesium,

Thus, the valency of magnesium = 2

  1. If the number of electrons in the outermost shell is less than four then, 

Valency of an atom = 8 – Number of electrons in the outermost shell. 

  1. In the case of sulphur, then the valency of sulphur = 8 – 6 = 2
  2. In the case of chlorine, then the valency of chlorine = 8 – 7 = 1

Q22. Na+ has filled K and L shells. Give a reason. 

Answer: The atomic number of Na = 11= Total number of electrons. 

The electronic configuration of Na = 2, 8, 1. 

The electronic configuration of Na+ ion = 2 (K-shell), 8 (l-shell).

Thereby Na+ion has wholly filled K and L shells. 

Q23. An electron can be considered a universal particle. Explain.

Answer: Whatever an electron’s emission source, the value of the charge (e) and the mass (m) remain the same. The electrons can be released from the discharge tube either through the cathode or the gas contained in the tube. These values remain the same regardless of the metal that forms the cathode and the gas in the discharge tube. The electron is therefore regarded as a universal particle.

Q24. What observation would you make if the a-particle diffusion experiment was done using a foil made of other metal than gold?

Answer: If the metal is heavy, like gold (e.g. silver, platinum etc.) Similar results can be achieved if the metal is very light (e.g. sodium, magnesium etc. The massive and fast-moving a-particles (4u Mass) might push the nucleus aside and allow slight deviations to pass through.

Q25. The rules for writing the distribution of electrons in different shells for the first eighteen elements are summarised.

Answer: The electrons are of negligible mass, so the total mass of an atom is the total of the protons and neutrons in its nucleus. Each proton and neutron have a mass equal to 4 U. Therefore, the mass of an atom must equal the sum of the protons and neutrons in its nucleus. This is called the mass number of an element. This is represented by the symbol A’. It can be described as:

The sum of the protons and neutrons found in an atom’s nucleus.

Mass number (A) = No. number of protons + no. of neutrons

Q26. Distinguish the properties of electrons, protons, and neutrons. 


Electron Proton Neutron
  • It is present outside of the nucleus of an atom. 
  • Carry negative charge.
  • Its weight is negligible. 
  • Proton is present inside the nucleus of an atom.
  • It carries a positive charge. 
  • It weighs around 2000 times the mass of electrons. 
  • A neutron is present inside the nucleus of an atom. 
  • It is neutral. 
  • Weight is the same as a proton. 

Q27. Explains with examples: 

  1. Atomic number
  2. Mass number
  3. Isotopes
  4. Isobars


  1. Atomic number: The total number of protons in an element’s atom is known as the atomic number. 

Example: Oxygen contains 8 protons. The atomic number for oxygen, therefore, is 8.

  1. Mass number: This is the sum of the number of protons and neutrons in an element’s atom. It is also known as the mass number. Example: Oxygen contains 8 protons and eight neutrons.
  2. The mass number of oxygens is therefore 8 8 16. Isotopes are atoms with the same atomic numbers but different mass numbers. Three Hydrogen isotopes are an example:

       1) Protium

       2) Deuterium

       3) Tritium

  1. Isobars refer to atoms with the same mass but different atomic numbers. i.e. isobars are elements with the same mass as atoms made from different elements. Example: 40 40 20 18 Ca, Ar are two isotopes.

Q28. For the following statements, write which is true or false.

  1. a) JJ Thomson proposed that the nucleus of an atom contains only nucleons. 
  2. b) A neutron is formed by an electron and a proton combining. Therefore, it is neutral. 
  3. c) Mass of an electron is about 1/2000 times that of a proton. 
  4. d) Isotope of iodine is used for making tincture iodine, which is used as a medicine. 


  1. a) JJ Thomson proposed that the nucleus of an atom contains only nucleons. (False)
  2. b) A neutron is formed by an electron and a proton combining. Therefore, it is neutral. (False)
  3. c) Mass of an electron is about 1/2000 times that of a proton. (True)
  4. d) Isotope of iodine is used for making tincture iodine, which is used as a medicine. (True)

Multiple Choice Questions 

Q29. Dalton’s Atom Theory was successfully explained

(i) Law on conservation of mass

(ii) Law on constant composition

(iii) Law of radioactivity

(iv) Law on multiple proportions


(a) (i),(ii), and (iii).

(b) (i),(iii), and (iv).

(c) (ii),(iii), and (iv).

(d) (i),(iii) or (iv).

Answer: (d) (i),(iii) or (iv).


Dalton’s theory describes the laws of conservation and mass, Laws of constant composition, and Laws of multiple proportions. It does not provide any information about the Law of radioactivity.

Q30. Which of these statements are true about Rutherford’s model of an atom?

(i) The nucleus is positively charged

(ii) It was found that a-particles were four times heavier than a hydrogen atom.

(iii) Can be compared with the solar system

(iv) Agreed to Thomson’s model


(a) (iii), (iv)

(b) (ii), and (iii).

(c) (i), and (iv).

(d) Only (i)

Answer: (a) (iii), (iv)


The nucleus was able to deflect positively charged alpha particles. This proves that the nucleus has a positive charge. Rutherford also suggested that electrons are organised in an atom around a nucleus, like how planets are arranged around the sun.

Q31. Which statements are correct in Thomson’s model for atoms?

(i) The mass of an atom is assumed uniformly distributed across the atom

(ii) The positive charge is assumed uniformly distributed throughout the atom

(iii) The electrons are evenly distributed in the positively charged space

(iv) The electrons attract one another to stabilise the atom


(a) (i),(ii), and (iii).

(b) (i), and (iii).

(c) (i), and (iv).

(d) (i),(iii), and (iv).

Answer: (a) (iii), (iv)


Thomson suggested that positively charged protons stabilise negatively charged electrons within the nucleus. The assumption that the positive charge is uniformly distributed across the atom is an incorrect statement.

Q32.Rutherford’s alpha particle scattering experiment resulted in the discovery of:

  1. a) Electron
  2. b) Proton
  3. c) Nucleus in the atom
  4. d) Atomic mass 

Answer: c) Nucleus in the atom.

Explanation: The presence of a nucleus at the centre of an atom is demonstrated by the discovery that some alpha particles returned to their original route. 

Q33. The ion of an element has three positive charges. The atom’s mass is 27, and the number of neutrons is 14. Choose what the number of electrons in the ion is?

  1. a) 13
  2. b) 10
  3. c) 14
  4. d) 16

Answer: b) 10

Explanation: Mass number of the element = 27

Number of neutrons in the atom = 14

Hence, the number of electrons in an atom = 14

Hence, the number of electrons in atom = mass number – number of neutrons in atom

= 27 – 14 = 13 

Since the ion of the elements has three positive charges, the number of electrons in the ion is 13 – 3. The ion has 10 electrons. 

Miscellaneous Questions 

Q34. Answer the following questions:

  1. a) What are cathode rays? What is the nature of a charge on cathode rays?
  2. b) Why are cathode rays formed from the gas taken in the discharge tube?
  3. c) What conclusion is obtained from all the gases from the cathode ray?


(a) Thomson used a discharge tube to pass electricity through gas at high voltage in an experiment. The cathode emitted a stream of tiny particles. The cathode emits a stream of tiny particles.

(b) Cathode rays are formed when electricity at high voltage passes through a gas in a discharge tube. The cathode releases tiny particles. The cathode is formed because the discharge tube contains atoms and all atoms contain electrons. The high voltage electrical current excites some electrons and pushes them out of the gas atoms. These electrons are fast-moving and form cathode radiations.

(c) The formation of cathode radiations shows that the negatively-charged electrons are one of the subatomic particles in all atoms.

Q35. Compare an electron and proton in terms of mass and charge. 

Answer: The mass of a proton is 1u; the proton’s mass is almost the same as that of a hydrogen atom. The relative mass of an electron is 1/1840u. Besides, the mass of an electron is 1/1840 of the mass of a hydrogen atom. On the other hand, a proton is positively charged, and an electron is negatively charged. 

Q36. Fill in the blanks in the following statements: 

  1. a) The number of protons in the nucleus of an atom is known as ______
  2. b) Total number of protons and neutrons in the nucleus of an atom is known as ______
  3. c) An atom with atomic mass numbers 23 and 11 will have ____ electrons. 
  4. d) An atom of an element has 11 protons, 11 electrons and 12 neutrons. The atomic mass of the atom is ____
  5. e) Nucleus of an atom has atomic number 17, mass number 37, and there are 17 electrons outside the nucleus; the number of neutrons in it is____
  6. f) Mass of an atom is concentrated in a small region of space called the _____
  7. g) Cathode rays are a beam of fast-moving_____
  8. h) Anode rays obtained from hydrogen gas consist of particles called____
  9. i) Maximum number of electrons which can be accommodated in the L shell is _____
  10. j) Maximum number of electrons which can go into the M shell is _____
  11. k) Subatomic particle does not present in a hydrogen atom is ____
  12. l) The electron has _____ charge, the proton has ____ charge, and the neutron _____ charge. 


  1. a) The number of protons in the atom’s nucleus is denoted as this element’s atomic number.
  2. b) The sum of the protons and neutrons in the nucleus of an atom is known as the mass number
  3. c) An atom with atomic mass number 23 and atomic number 11, the atom will have 11
  4. d) An atom of an element has 11 protons, 11 electrons and 12 neutrons. The atomic mass of the atom is
  5. e) The nucleus of an atom has the atomic number 17, mass number 37, and there are 17 electrons outside the nucleus; the number of neutrons in it is
  6. f) Mass of an atom is concentrated in a small region of space called the
  7. g) Cathode rays are a beam of fast-moving
  8. h) Anode rays obtained from hydrogen gas consist of particles called
  9. i) the Maximum number of electrons that can be accommodated in the L shell is
  10. j) The maximum number of electrons which can go into the M shell is
  11. k) A subatomic particle not present in a hydrogen atom is a neutron.
  12. l) The electron has a negative charge, the proton has a positive charge, and the neutron has no

Key Topics Covered In Class 9 Chapter 4 Science Important Questions

Students must recall the wide range of topics and concepts they have read. In addition, the chapter requires proper comprehension of the science concepts.

Introduction to the structure and function of an atom

An atom’s arrangement is made up of protons, electrons and neutrons. These essential components provide the weights and charges of atoms. The nucleus comprises protons and neutrons, while the electron orbits around them.


The fundamental building blocks of matter are atoms. They are the smallest part of matter and consist of three subatomic particle types: a proton, a neutron, and an electron.

Cathode ray experiment

  • The characteristics of the cathode (electrons) are independent of the materials used for electrodes and the natural gas in the cathode tube.
  • The study showed that the atom was not an unconstrained, indistinct particle but had at least one subatomic particle, the electron.


  • Subatomic particles called electrons have a negative charge within an atom.
  • It is believed that the electron’s mass and charge are insignificant.
  • The symbol for electrons, e-, is the symbol
  • The electrons are incredibly tiny.
  • They can be found outside the nucleus.

Thomson’s model for an atom

  • Thomson states that an atom comprises a positively charged sphere and contains electrons. Both the positive and negative charges have equal power. The entire atom is therefore electrically neutral.
  • It is necessary to propose and consider the first theory of an atom.
  • The watermelon’s red-coloured edible portion can be compared to the atoms’ positive charge.
  • Contrast the black seeds of watermelon with the electrons in it.


  • The process by which an unstable nucleus within an atom is stripped of energy through the exchange of particles is called radioactivity.
  • This is done by emitting particles such as alpha and beta particles.
  • This is an entirely spontaneous process.
  • An atom could be unstable if the nucleus is unstable, which is a difference between protons and neutrons.

Rutherford Model

Alpha particles were created to fall on thin gold foil in the experiment. 

  • A large portion of the α-particles that were hurled towards a gold sheet passed through without deflection. The majority of the space within an atom remains unoccupied.
  • In some cases, α-particles were dispersed by the small angles of the gold sheets. Therefore, it is possible for the positive charge within an atom not to be evenly distributed.
  • The atoms retain the most amount of the positive charge.
  • Most of the α-particles had their electrons deflected. Only a few particles could be deflected at almost 180 degrees. The volume of positively charged particles in an atom is much smaller than its overall size.

Rutherford’s model for an atom

Rutherford was able to conclude that the model for the atom he created from the a-particle scattered test is the following:

  • A positively charged centre can be found within an atom. This is known as the nucleus. The nucleus is where the majority of an atom’s mass is found.
  • The orbits in which electrons travel around the nucleus are precise.
  • It is much smaller than the atom.

Neil Bohr Model

Bohr developed these theories to counter criticisms of Rutherford’s model.

  • Stable orbits allow electrons to orbit the nucleus and emit no radiant energy. Every orbit is characterised by particular energy, also known as an energy shell (or energy level).
  • A shell is a symbol that indicates the energy level of an orbit. It can be identified as N shells or K L shells. An electron in the ground state is at its lowest energy level.
  • When an electron moves from one orbital to another energy level, it releases or absorbs energy.
  • It releases energy if it can leap from an energy-rich level to one that is less. It absorbs energy when it moves from low energy to high energy levels.


Orbits are energy shells around the nucleus that spin electrons around. Bohr, Bury, and others proposed electron distribution in different orbits.

  • 2n2 is the formula that determines the number of electrons in a shell. “n” refers to the number of orbits and energy index.
  • From lower to higher energy levels, the shells are filled in a stepwise manner. If the previous shells are not filled, electrons will not be filled in the next one.


The electrons found in the outermost shell atoms are called valence electrons that are valence.

  • Valency for an atom is the ability to combine atoms, or their ability to combine and create molecules from atoms of the same elements or other elements.
  • The chemical activity of element atoms with their outermost shells filled is minimal.
  • Their capacity or valency equals 0.
  • We know, for example, that the number of electrons in the outermost hydrocarbon shell is one and in magnesium, it’s 2.
  • Therefore, hydrogen’s valency is one because it can rapidly lose one electron and become stable.
  • However, magnesium is number 2 because it can lose two electrons in minutes and attain stability.

Atomic Number

  • The atomic number of an atom is the number of protons present in the atom’s nucleus. The letter Z indicates it. 
  • The mass number and the representation of an Atom
  • The nucleus contains protons and neutrons. The mass number is the sum of these protons and neutrons.

Isotopes & Isobars

Isobars are the nuclides (atoms) of different chemical elements. Although they have different chemical properties, they share many similar physical properties. Therefore, we can define isobars as elements with different chemical numbers but the same mass.

Isotopes refer to elements whose number of neutrons and protons are different. Based on the above definitions of atomic mass, we can see that isotopes are elements with different mass numbers.

Isobars Isotopes
Isobars can be described as chemical elements with the same mass. The atomic structure of an element’s isotopes can be different
Equal atomic masses Different atomic masses exist
Many times, the physical properties of different substances are identical Physical properties can often be different
Atomic numbers can be different The atomic number is the same
Chemical elements can be different Different mass numbers can be assigned to chemical elements.

Benefits of Solving Important Questions Class 9 Science Chapter 4

Science concepts can be complex for students to grasp. One can overcome this difficulty by practising important questions in Chapter 9, Science. The important questions in Class 9 Science Chapter 4 cover all the key topics. These questions are designed from an exam perspective, so they are the most likely to be asked during the exam. Solving important questions gives students a competitive edge.

These are the benefits of solving Important Questions Class 9 Science Chapter 4:

  • The syllabus will be reviewed and covered by students. These questions were created exclusively by subject experts for entrance exams.
  • These questions are more likely to be asked on the exam. This gives students a general idea of the type of questions they might face in exams and helps them stay prepared.
  • This comprehensive guide to essential questions combines all the questions and helps students prepare for the annual exams.
  • Chapter 4 Class 9 Science important questions help students gain a deeper understanding of concepts and prepare them for exams in a balanced way.
  • Students can choose between the different types of questions based on their exam requirements.
  • Each section is subdivided into sub-sections. Each section can be further divided into types of questions such as objective type, short answer, and long answer, respectively.
  • These solutions provide detailed information on the practicalities of clearing the question paper. This can significantly benefit students’ final exam preparation, future career pathways, and achievements.

Q.1 (a) Justify the statement Infectious diseases are also called communicable diseases.
(b) How penicillin affects bacteria
(c) Expand SARS.


(a) Infectious diseases can be communicated from diseased to healthy persons; hence they are also called communicable diseases.
(b) Penicillin is an antibiotic that blocks the bacterial process responsible for cell wall formation. As a result, the growing bacteria is unable to make a cell wall and hence dies.
(c) SARS: Severe Acute Respiratory Syndrome

Q.2 Describe the five Fs rule of good health.


The five Fs rule is developed to achieve good health. The five F stands for fluid, fingers, flies, fields, and floods.
1. Fluid: It includes protecting water sources and treating water before drinking.
2. Fingers: It includes washing hands before and after eating food and after defecation.
3. Flies: It includes controlling flies and covering food.
4. Fields: It includes washing fruits and vegetables before consuming them and avoiding open defecation.
5. Floods: It includes maintenance of drainage system and treatment of sewage water.

Q.3 Based on the given graph, select the CORRECT statement.

A. Increased mosquito population can bring deadly pandemics in the world.

B. Humans and mosquitoes can co-exist in the world.

C. Increased mosquito population may not have any impact on human health.

D. Mosquitoes can help in eradicating microbes that are deadly to humans.


Increased mosquito population can bring deadly pandemics in the world.

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FAQs (Frequently Asked Questions)

1. Based upon Chapter 4 of Class 9 Science, what is the structure and function of an atom?

In Chapter 4 of Class 9 Science, an atom consists of a nucleus surrounded by electrons and a nucleus made up of protons and neutrons. While electrons have a negative charge, protons are positive, and neutrons are uncharged. Scientists have established that the electrons travel in circular orbits of specified radii corresponding to discrete energy states. The nucleus, at the centre, is a simplistic version of the atomic structure based on the Bohr model. The atom can thus be understood as circular shells of electrons, with the lowest shell corresponding to the lowest energy state, whereas the higher-level shells have more energy.

2. Why should we study the important questions in Class 9 Science Chapter 4?

The list of important questions is essential for every exam. They cover all important concepts and play a vital role in the preparation. The NCERT-based questions follow a specific pattern, which is essential because the exam also follows the latest CBSE guidelines. Students must practise their papers to help them understand concepts, avoid any errors in their exams, and practise the set of essential questions multiple times.

3. Where can I access the list of important questions for Class 9 Science Chapter 4?

Students can access the list of questionnaires for Class 9 Science Chapter 4 on our Extramarks website. Our experienced subject experts  have compiled the important questions from NCERT exemplar, NCERT textbooks, NCERT solutions and other reference books. It also covers formulas and CBSE additional questions, and the study is updated with the latest CBSE syllabus. Students can start their preparation with other study material available on our website.