Download PDF – Molecules and ions

Molecules:

A molecule is in general a group of two or more atoms that are chemically bounded together by certain attractive forces.

For example –  Two atom of hydrogen (H₂) and one atom of Oxygen (O)  combine with each other to form one molecule of water (H₂O)

Atoms of the same element or a different element can join together to form molecules.

Molecules of Elements :

The molecules of an element are constituted by the same type of atoms.

For example – Helium (He) consists of only one atom while chlorine (Cl) consists of two atoms.

The number of atoms constituting a molecule is known as its atomicity. For example – helium is monoatomic and oxygen is diatomic.

1.  Monoatomic – when an element is made up of only one atom. Ex – helium (He), all metals

2.  Diatomic – when an element is made up of two atoms. Ex – oxygen (O₂), all gases

3.  Triatomic – when an element is made up of three atoms. Ex – ozone (O₃)

4.  Tetra-atomic – when an element is made up of four atoms. Ex – phosphorus (P₄)

5.  Poly- atomic – when an element is made up of more than two atoms. Ex – sulphur (S₈)

Molecules of Compounds :

Atom of different element combines together in definite or fixed proportion to form molecules of compounds.

For example – One atom of carbon (C) and two atom of Oxygen (O₂) combine with each other to form one molecule of water (CO₂).

 Ions:

Compounds are made up of metals and non-metals. These elements contain charged species, which are known as ions.

These are basically charged particles which can be both negatively (-) or positively (+) charged.

The element which has negatively charged ion is called an ‘anion’ and the element which has positively charged ion is called ‘cation’.

For example – NaCl , i.e., (Na⁺ ) (  cl ^–)

 A group of an atom carrying a charge is called a polyatomic ion.