Sa2 Cbse 9th Grade Science Notes

Energy level, Electron shell, Valence (chemistry)

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...
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