CBSE Class 9 Science Revision Notes Chapter 4

CBSE Class 9 Science Revision Notes Chapter 4 – Structure of The Atom

An atom is the smallest part of matter that possesses all the chemical characteristics of an element. Atoms combine to create molecules which connect to create solids, liquids, or gases. 

Extramarks is a platform that offers precise Class 9 Science Chapter 4 Notes to assist students in reviewing the chapter to obtain higher marks in exams.

Students may recall from Class 9 about the structure of an atom. It has two regions: the nucleus, which is located in the centre and contains neutrons and protons, and the outer region, which maintains the electron’s orbit around the nucleus. The negative charge of neutrons matches the positive charge of a proton.

Following are the topics that are covered in Class 9 Chapter 4 Science Notes – Structure of the Atom:

1. Atomic Model by Thomson
2. Atomic Model by Rutherford
3. Atomic Number
4. Bohr’s Model of Atom
5. Electronic Arrangement in an Atom 
6. Isobars
7. Isotopes
8. Mass Number
9. Orbit
10. Sub-Atomic Particles of an Atom and Their Discovery
11. Valency

Summary of Structure of Atoms

• Atoms, the fundamental units of matter, are the units that give components their unique structures.
• The atom’s structure is made of three fundamental particles, namely – protons, electrons, and neutrons.
• E. Goldstein discovered the proton – a positively charged particle.
• J.J. Thomson discovered the electron – a negatively charged particle.
• Chadwick discovered the neutrons – a neutral particles or a particle with no charge.
• Protons and neutrons are found in the atom’s nucleus, which is located in its centre.
• The term “shell” or “energy level” refers to the outer region of the atom that houses the electrons in their orbits around the nucleus.
• Subshells are further broken down into these shells.
• The term “valence electrons” refers to the electrons that make up an atom’s outermost shell.
• The arrangement of electrons distributed among orbital shells and subshells is represented by the element’s electron configuration. The valence electrons are what determine the element’s particular chemistry.
• An element’s atomic number is determined by the number of protons present in its nucleus. Atoms are electrically neutral, thus they contain the same number of electrons as protons. The atomic number is symbolised by the letter Z. 
• The mass number of an atom is equal to the number of nucleons in its nucleus. Nucleons are the aggregate name for protons and neutrons, indicating that A is the mass number.
• In the notation of an atom, the mass number is written on the left of the element symbol as a superscript, and the atomic number is placed there as a subscript.
• Isotopes are defined as elements that have a different number of neutrons but the same number of protons and electrons.
• For example, the hydrogen isotopes proton, deuteron, and triton. Both of them have a single proton Z=1 and a single electron, but their neutron counts vary. Hydrogen has no neutron, while Deuterium has one neutron, and tritium has two.
• Elements with various atomic numbers but the same mass number are known as isobars because they share the same physical characteristics but differ in their chemical properties.
• Since both calcium and chlorine have a mass number of 40 and different atomic numbers—20 for calcium and 17 for chlorine—isobars are created when these two elements combine. 
• Isotones are substances that have distinct atomic numbers but the same amount of neutrons. 
• For example, both chlorine and potassium have 20 neutrons each, making them isotones, their atomic numbers are 37 and 39, respectively.

Chapter 4 Science Class 9 Notes: Atomic Number and Mass Number

According to the Class 9 Science Notes Chapter 4, the mass number is equal to both the number of protons and neutrons, whereas the atomic number refers to the total number of protons in an element.

Atomic Number

An atomic number is the number of chemical elements that are present in the periodic system and are arranged in order of increasing numbers of protons in their nuclei. The term “atomic number” is also used to describe how many protons and electrons there are in a neutral atom. For example, iron has an atomic number of 26 due to the presence of 26 protons in its nucleus. Protons and electrons are present in equal amounts in a neutral atom.

Mass Number

Class 9 Science Chapter 4 Notes explain that an element’s mass number is formed by adding the total number of protons and neutrons. The mass number is calculated after disdaining the electron’s contribution to mass. By deducting the total number of protons from the mass number, an approximation of the mass is used to determine how many neutrons an element has. Protons and neutrons both have masses of about 1 atomic mass unit or amu. The number of atoms in an isotope of the same element will be the same, but the mass will differ.

Class 9 Science Ch 4 Notes: Details of Isotopes

The Class 9 Science Chapter 4 Notes provide a brief review of isotopes. Isotopes have the same amount of protons but have different neutron counts. Despite having the same atomic number, they have distinct mass numbers. Every element is made up of different isotopes. They are all pure substances. They differ physically but share the same chemical properties. An element’s mass would be equal to the sum of all the neutrons and protons present if there were no isotopes in it. According to the Class 9 Science Chapter 4 Notes, if an element contains isotopes, the percentage of each isotope form should be known in order to calculate the average mass.

Science Class 9 Chapter 4 Notes: Details of Isobars

Isobars are the nuclides of various chemical elements. Since they both contain the same number of nucleons, their masses are also equal. But their atomic numbers are different. The term “isobars” was suggested by Alfred Walter Stewart. Its root words are bars, which means “weight,” and isos, which means “equal.”

Class 9 Science Notes Chapter 4: A Summary of The Structure of An Atom

• The nucleus, which sits in the middle of an atom and contains protons and neutrons, and the outer region, which houses the electron in its orbit around the nucleus, make up an atom.
• A proton’s positive charge is equal to the negative charge that all electrons have.
• Uncharged particles known as neutrons are present in the nucleus.