Class 9 Science Solutions English Medium

NCERT Solutions Class 9 Science English Medium

3. Atoms and Molecules

Topic: Laws of Chemical Combination

3. Atoms And Molecules

Element: The basic substance of matter that can not be simplified is an element. 

Example: Hydrogen, Carbon, Oxygen, Iron, Silver and Gold etc. 

Atom: The smallest tiny particles of matter which cann't be divided further is called atom (Parmanu).

Molecules: A group of two or more than two atoms of the same element or different elements that chemically bonded together is called molecules. 

Example: O₂, H₂, N₂, H₂O, CO₂, MgCl₂ etc. 

Compound: A molecule that contains more than one element is compound.

Example: H₂O, CO₂, NH₃, BrCl₂, CH₄ etc. 

The smallest tiny particles of element is atom. like atom of Hydrogen (H), atom of oxygen (O), atom of Carbon (C), atom of Magnesium (Mg) etc.

So all matters are made by these tiny particles of elements i.e atoms.  

Formation of Matter

Atoms of elements

⇓

Molecules Or atoms

⇓

Compounds

⇓

Matters 

Laws of Chemical Combination:

The laws of chemical combination were established by Lavoisier and
Joseph L. Proust. 

The laws of chemical combination are two;

1. Law of conservation of mass: Law of conservation of mass states that
mass can neither be created nor destroyed in a chemical reaction.

2. Law of constant proportions: In a chemical substance the elements are
always present in definite proportions by mass.

Dalton's Atomic Theory:

British chemist John Dalton provided the basic theory about the nature of matter.
Dalton picked up the idea of divisibility of matter, which was till then just a philosophy. He took the name ‘atoms’ as given by the Greeks and said that the smallest particles of matter are atoms. His theory was based on the laws of chemical combination.

The postulates of dalton's theory:

(i) All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which cannot be created or destroyed in a
chemical reaction.
(iii) Atoms of a given element are identical in mass and chemical properties.
(iv) Atoms of different elements have different masses and chemical properties.
(v) Atoms combine in the ratio of small whole numbers to form compounds.
(vi) The relative number and kinds of atoms are constant in a given compound.

The name of elements  and its symbol:

 IUPAC (International Union of Pure and Applied Chemistry) approves names
of elements. Many of the symbols are the first one or two letters of the element’s name in English. The first letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase).

Example: 

Hydrogen: H

Carbon : C

Aluminium: Al 

Magnesium: Mg

Symbols of some elements are formed from the first letter of the name and a letter, appearing later in the name. Examples are:
(i) chlorine, Cl,

(ii) zinc, Zn etc.

Each element has a name and a unique chemical symbol.

Some examples of elements and their symbol.

Atomic Mass: An atomic mass unit or  amu is one twelfth of the mass of an unbound atom of carbon -12 isotope. This is approximetely 1.67377 x 10 ^-27 kilogram (kg). 

The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom.

Atomic mass unit is abbreviated as 'amu' but on IUPAC latest recommendation, it is writen as 'u'. 

Existance of atoms:

Atoms never exist independly. They form molecules or ions. 

We can say, atoms exist in group. 

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