Chapter-3
Atoms and molecules

Atoms:-All the elements and molecules are made up of very tiny particles called atoms.Atom is the smallest unit that take place in any chemical reaction.

Laws of chemical combination

There are three laws of chemical combination

1 Law of conservation of mass

The law of conservation of mass was given by Antonie Lavoiser in 1774.

According to law of conservation of mass matter is neither created nor distroyed in a chemical reaction.

The law of conservation of mass means that total mass of product is equal to the total mass of reactants.

For example:-If 100 grams of calcium carbonate are decomposed completely then 56 grams of calcium oxide and 44 grams of carbondioxide are formed.

2 Law of constant proportion

The law of constant proportion was given by Joseph Proust in 1779.

The law of constant proportion state that a chemical compound always consist of the same element combined togather in the same proportion by mass.

For example:-Water is a compound which always consists of the same two elements hydrogen and oxygen combined togater in same constant proportion of 1:8 by mass.

Dalton atomic theory:-

According to dalton atomic theory all matter weather an element, compound or a mixture is composed of small particles called atoms.

Postulates

All the matter is made up of very small particles called atoms.

Atoms con not be created nor distroyed in any chemical reaction.

All the atoms of given element are same that is identical in size, mass, and chemical properties.

Atoms of different elements are different in size, mass and chemical properties.

Atoms of two different elements join togather during chemical reaction to form compound.

Atoms of the same element can combine in more than one ratio to produce more than one compound.

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All the atoms of given element are similar but they differ from atoms of element.

Diameter of an atom is about 1Ã—10^-10meter

Eg- Each atoms of sodium shows properties of sodium but differ from atom of clorine.

Symbol of atoms:-A symbol is an amplivation for the full name of element.

Rules for assigning symbol

The amplivation used to represent element is generally the first letter in capital of english name of an element.

When the names of two or more element begin with the same initial letter the initial letter followed by the next letter is used to symbolised the element.

Example:-Ba - Barellium
Br - Bromine

Symbol of some elements are derived from their latin name.

Example:-Sodium - Natrium - Na

The symbol of an element represents following :-

It represent one particular element.
It represent one atom of the element.
It represent one atomic mass of an element.

Ex:-

Molecule

The smallest particle of matter (element or compound) which exist in free state is called molecule.

Properties of molecules :-

The molecules of any given substance are similar thus all the molecule of water are similar.
The molecule of different substance are different. The molecule of water ate entirely different in properties from molecule of carbon dioxide.
The properties of substance are the properties of molecules.

Atomicity :-The number of atom vontained in a molecule of a substamce (element or compound) are calle its atomicity.

Ion :-

An ion is a charged particles. It can be either posetively charged or negatively charged.
A negatively charged ion is called anion.
A posetively charged ion is called cation.

Ionic compound :-Compound containing cahrged species are known as ionic compound . Such compound are formed from combination of metals and non-metals.
For example Sodium cloride.

Valency

It is the combining capacity of an element.
It can be used to find out how many atoms of an element will combine with other element to form a chemical formula.

Element	Charges	Valency
Potassium	K⁺	1
Barrium	Ba⁺²	2
Aluminium	Al⁺³	3
Calcium	Ca⁺²	2
Colbat	Co⁺²	2
Copper	Cu⁺/Cu⁺²	1/2
Cloride	Cl^-1	-1
Floride	F^-1	-1
Lead	Pb⁺¹/Pb⁺²	1/2
Zinc	Zn⁺²	2
Iodine	I^-1	1
Sulphide	S^-2	2

Polyatomic-ion:-Those ions which are formed from groups of joined atoms are called polyatimic ion.

Name of ion	Symbols
Ammonium	NH₄⁺
Hydroxide	OH^-
Nitrate	NO₃^-
Carbonate	CO₃^-2
Sulphate	SO₄^-2
Bicarbonate	HCO₃^-
Phosphate	PO₄^-3

How is molecular formula of a compound written?

In order to write formula of a compound comprise of cation and anion , the valency must be known.

Let us consider a compound of cation A with valency x⁺ and anion B with valency y^-.
When a compound of a metal and a non-metal the symbol of non-metal is written in left and symbol of non-metal is written in right. Inter change between the ion and their valency and these are placed on the lower side of each redical used as sub script.

Molecular mass:-Molecular mass of a substance is relative mass of ¹²carbon taken as 12.
For example:- 1Molecule of water H₂O contain 2 atoms of hydrogen and 1 atom of oxygen will make 1 molecule of water.