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.

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.

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.

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

Class 9 Science Chapter 4 Structure of the Atom is part of the CBSE Class 9 Science syllabus. It explains how our idea of the atom developed, from Thomson's plum pudding model to Rutherford's alpha particle scattering experiment and then to Bohr's model. Students learn about protons, neutrons and electrons, atomic number and mass number, how electrons are arranged in shells, valency, and about isotopes, isobars and isoelectronic species.

These CBSE important questions for Class 9 Science Chapter 4 cover Rutherford's nuclear model, isoelectronic species and isosters, isobars, valency from electron shells, reading the notation for atomic and mass number, the Bohr model, the plum pudding model and the conclusions of the alpha particle scattering experiment. Every question has a clear step-by-step answer in simple English with figures, so students can practise on their own, parents can check the answers, and teachers can use them for class tests.

Q1. Describe Rutherford's nuclear model of the atom.

Rutherford alpha particle scattering experiment with gold foil

Answer: The main points of Rutherford's nuclear model are:
1. The entire positive charge and almost all the mass of an atom is concentrated at its centre, called the nucleus.
2. The nucleus is very small compared with the atom. The radius of the nucleus is of the order of 10⁻¹³ cm, while the radius of the atom is about 10⁻⁸ cm. The rest of the atom is empty space.
3. The nuclei of different elements carry different amounts of positive charge, but all the atoms of one element have the same nuclear charge.
4. Electrons revolve around the nucleus at very high speed in circular paths called orbits.
5. The atom as a whole is electrically neutral, because the total negative charge of the electrons equals the total positive charge of the nucleus.

Q2. Which of the following forms a pair of isoelectronic species?
(a) Li⁺ and Na⁺
(b) Li⁺ and Mg²⁺
(c) Be²⁺ and Li⁺
(d) Be²⁺ and Al³⁺

Answer: (c) Be²⁺ and Li⁺
Isoelectronic species have the same number of electrons. Lithium has 3 electrons, so Li⁺ has 2. Beryllium has 4 electrons, so Be²⁺ also has 2. Both have 2 electrons, so they are isoelectronic.

Q3. Identify the species which is isoelectronic with a carbon atom.
(a) Na⁺
(b) Al³⁺
(c) O²⁻
(d) N⁺

Answer: (d) N⁺
A carbon atom has 6 electrons. Nitrogen has 7 electrons, so N⁺ has 7 − 1 = 6 electrons. Na⁺ and Al³⁺ have 10 electrons each and O²⁻ also has 10, so only N⁺ matches carbon.

Q4. An example of isosters is
(a) CO₂ and CO
(b) CO₂ and N₂O
(c) CO and NO
(d) NO and NO₂

Answer: (b) CO₂ and N₂O
Isosters are molecules that have the same number of atoms and the same total number of electrons. Both CO₂ and N₂O have 3 atoms and 22 electrons each, so they are isosters.

Q5. The given pair of elements, ⁴⁰Ar (Z = 18) and ⁴⁰K (Z = 19), have
(a) the same number of neutrons
(b) the same number of protons
(c) the same number of electrons
(d) the same number of nucleons

Answer: (d) the same number of nucleons
Protons and neutrons together are called nucleons, and their total is the mass number. Both have mass number 40, so both have 40 nucleons. Their atomic numbers are different (18 and 19), so protons, electrons and neutrons are not the same. Such atoms of different elements with the same mass number are called isobars.

Q6. Which of the given structures have the same valency?
(a) I and II only
(b) I, II and III
(c) I and III only
(d) II and III only

Three atomic structures I, II and III showing electrons in shells

Answer: (c) I and III only
Valency is decided by the number of electrons in the outermost shell. Structure I is helium (2 electrons in the only shell), which has a complete shell, so its valency is 0. Structure III also has a complete outermost shell, so its valency is 0 as well. Structure II is magnesium, which has 2 electrons in its outermost shell and loses them easily, so its valency is 2.

Q7. Select the correct name, atomic number and mass number of the element 'X' according to the given information: ⁴⁰₁₈X
(a) X: Ar, Atomic Number: 18, Mass Number: 40
(b) X: Al, Atomic Number: 18, Mass Number: 40
(c) X: Ca, Atomic Number: 18, Mass Number: 22
(d) X: K, Atomic Number: 18, Mass Number: 22

Answer: (a) X: Ar, Atomic Number: 18, Mass Number: 40
In the notation ^A_ZX, the lower number Z is the atomic number and the upper number A is the mass number. So Z = 18 and A = 40. The element with atomic number 18 is argon (Ar).

Q8. Study the given table and identify the element(s) for which the information mentioned is INCORRECT.

Element	Protons	Neutrons	Electrons	Valency
Lithium	3	3	3	1
Nitrogen	7	7	7	3
Sulphur	16	16	16	2

(a) Lithium only
(b) Lithium and nitrogen only
(c) Nitrogen and sulphur only
(d) Lithium, nitrogen and sulphur

Answer: (a) Lithium only
Lithium has 3 protons and 3 electrons, which is correct, but its most common isotope has 4 neutrons, not 3 (mass number 7). So the entry for lithium is wrong. The entries for nitrogen (7, 7, 7, valency 3) and sulphur (16, 16, 16, valency 2) are correct.

Q9. In reference to Rutherford's model, which of the conclusions about the rays A, B and C are CORRECT?
(I) Ray A: Most of the space inside the atom is empty.
(II) Ray B: Positive charge occupies very little space in an atom.
(III) Ray C: All the positive charge and mass of the gold atom is concentrated in a very large volume within the atom.
(a) I, II and III
(b) I and II only
(c) II and III only
(d) I and III only

Answer: (b) I and II only
Most alpha particles passed straight through, which shows the atom is mostly empty space, so I is correct. Only a few were deflected, showing the positive charge takes up very little space, so II is correct. Statement III is wrong because the positive charge and mass are concentrated in a very small volume (the nucleus), not a large one.

Q10. Select the option that accurately depicts the labels for A, B and C according to the Bohr model.
(a) A = Shells, B = Nucleons, C = Nucleus
(b) A = Shells, B = Protons, C = Electrons
(c) A = Orbits, B = Electrons, C = Nucleus
(d) A = Shells, B = Nucleus, C = Electrons

Bohr model of the atom with parts A, B and C marked

Answer: (c) A = Orbits, B = Electrons, C = Nucleus
In Bohr's model the negatively charged electrons revolve around the positively charged nucleus in fixed circular paths. These paths are called orbits or shells. So A points to the orbits, B to the electrons moving in them, and C to the nucleus at the centre.

Q11. Which of the following statements are INCORRECT regarding the Plum Pudding model?
A. The model was proposed by J.J. Thomson.
B. The overall charge on an atom is negative.
C. Atom is a negatively charged sphere in which positively charged particles are embedded.
D. Electrons revolve around the nucleus of an atom in discrete orbits.
(a) A and D only
(b) B and C only
(c) C and D only
(d) B, C and D only

Answer: (d) B, C and D only
Statement A is correct, as J.J. Thomson did propose this model.
B is wrong: the atom as a whole is electrically neutral, not negative.
C is wrong: the model says the atom is a positively charged sphere with electrons embedded in it, like seeds in a watermelon.
D is wrong: electrons revolving in discrete orbits is Bohr's model, not the plum pudding model.

Q12. Read the Assertion (A) and Reason (R) and choose the correct option.
Assertion (A): Some alpha particles were seen to be deflected through small and large angles in the alpha particle scattering experiment.
Reason (R): The atom contains a lot of empty space.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Both statements are true facts of the experiment. But the empty space explains why most particles went straight through, not why some were deflected. The deflection happened because those alpha particles came close to the heavy, positively charged nucleus, which repelled them. So R does not explain A.

Related Study Material on Extramarks

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

Marks:3
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(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.

Marks:5
Ans

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.