Sa2 Cbse 9th Grade Science Notes

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Synopsis – Grade 9 Science Term II

Chapter 1: Atoms and Molecules

Law of conservation of mass: Mass can neither be created nor can it be destroyed in a chemical reaction.

Law of constant proportion: A chemical substance always contains the same elements in a fixed proportion by mass, irrespective of the source of compound. 
Atom: The smallest particle which is the building block of matter. 
The symbol of the element is made from one or two letters of the English or the Latin name of the element.

Atomic mass: The sum of the protons and neutrons in an element gives its atomic mass. The atomic mass of an atom of an element is also known as its relative atomic mass, since it is determined relative to the mass of C-12 isotope.


Molecule: It is formed when two or more atoms of the same element or different elements get combined chemically.

Atomicity: The number of atoms that combine to form a molecule is called the atomicity of the molecule.

Ion: A charged species in which an atom or a group of atoms possess a net electric charge (positive or negative).
 Cations → Positively charged ion
 Anion → Negatively charged ion

Chemical formula: Representation of the composition of a molecule in terms of the symbols of elements present in that molecule.

Molecular mass: It is the sum of the atomic masses of all the atoms present in a molecule of that substance.

Formula unit mass: It is the sum of the atomic masses of all the atoms present in a formula unit of that substance.

Mole: The mole is a unit of measurement for the amount of substance. One mole of a substance is the quantity of the substance containing 6.022  1023 numbers of particles (atoms, molecules, or ions).

Chapter 2: Structure of the Atom

Atoms are not indivisible and are composed of three fundamental particles. These particles are electrons, protons, and neutrons.

Electrons: These are the negatively charged particle and were discovered by J. J. Thomson, by cathode ray experiment.

Canal rays are positively charged radiations consisting of protons. 
Protons: These are the positively-charged particles and were discovered by E. Goldstein. 
Neutron: These are electrically-neutral particle and were discovered by J. Chadwick. 
Various atomic models:

 Thomson's atomic model: Thomson thought that an atom is a sphere of positive charge in which electrons are embedded. An atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude.

 Rutherford's atomic model: All the positive charges (i.e., protons) were present at the centre of the atom, inside the nucleus, and the electrons were present in circular orbits around the nucleus. The electrons are not at rest and keep moving continuously in these circular orbits. The size of the nucleus is very small as compared to that of the atom.  Bohr’s atomic model: The electrons present around the nucleus revolve in specific orbits called energy levels. He also stated that the electrons do not release energy while revolving. The shells in which the electrons are present are known as K, L, M, N, and so on (or 1, 2, 3, 4, and so on), as proposed by Bohr and Bury. Each shell contains a specific number of electrons, which can be calculated using the formula 2n2.

Atomic Models

Dalton’s atomic model Thomson’s atomic model Rutherford’s atomic model Bohr’s atomic model


Valency: It is defined as the combining capacity of the atom of an element. It depends upon the number of electrons present in the outermost shell of its atom. 
Atomic number of an element is equal to the number of protons present in the atom and atomic mass is equal to the sum of the number of protons and...
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