3. Compound

                                               Atomicity
The number of atoms constituting a Molecule is known as its atomicity.

Compounds

A pure substance made up of two or more elements chemically combine together in a fixed ratio under fixed condition is called compound. Example: Calcium carbonate, Common salt, Sugar. 

Properties of compound

 1.Compounds can be separated into the constituent only by chemical methods.  
2. Properties of compound differ from the properties of their constituents. 
3. During the formation of a compound energy is absorbed or released.
4. The compound are homogeneous.
 

Valency

The combining capacity of an element is known as its valency. Valency is used to find out how the atom of an element will combine with the atom of another element to form a chemical compound.
(Every atom wants to become stable, to do so it may lose, gain or share electrons.)
•    If an atom consists of 1, 2 or 3 electrons in its valence shell then its valency is 1, 2 or 3 respectively,
•    If an atom consists of 5, 6 or 7 electrons in the outermost shell, then it will gain 3, 2 or 1 electron               respectively and its valency will be 3, 2 or 1 respectively.
•    If an atom has 4 electrons in the outermost shell than it will share this electron and hence its valency         will be 4.
•    If an atom has 8 electrons in the outermost electron and hence its valency will be 0.

Ions
An ion is an electrically charged atom or group of atoms.
An ion is formed by the loss or gain of electrons by an atom, so it contains an unequal number of electrons and protons.

There are two types of ions:

Cation: A positively charged ion is known as cation.
For Ex: Na+, Mg2+ 
A cation is formed by the loss of one or more electrons by an atom.

Na – e– ——–>Na+
Z=11                 Z=10
2,8,1                  2,8
K, L, M                  K, L
 

Mg – e– ——–> Mg2+
Z=12                 Z=10
2,8,2                  2,8
K,L,M                  K,L
 

Anion: A negatively charged ion is known as anion.
For Ex: Cl–, O2- 
An anion is formed by gain of one or more electron by an atom.

Cl + e– ——–> Cl–
Z=17                 Z=18
2,8,7                  2,8,8
K, L, M                 K, L, M
 

O + e– ———> 02-
Z=8                   Z=10
2,6                    2,8
K, L                    K, L
 

Radicals

An atom or group of atoms having a charge, i. e. either negative or positive, on it.
The radicals having positive charge are called cations.eg. Sodium ion.
The radicals having negative charge are called anions. eg. Chloride ion.