What are molecules and ions in chemistry-A detailed introduction

Written by Adeel Abbas

What are molecules and ions in chemistry?

What are Molecules in chemistry?

The smallest particle of elements or compounds that can exist independently is known to be a molecule.

Definition of Molecule

It is made up of two or more atoms of similar or different elements. Cl2, O2, NH3, C6H12O6. Anyhow, molecules of inert gases consist of a single atom.

What is Atomicity in chemistry?

The number of atoms present in a molecule refer to its atomicity.

Definition of atomicity

For example in case of methane CH4 there are 4 Hydrogen atoms and 1 carbon atom so its atomicity is 5. Similarly the atomicity of Cl2 is 2, CO2 is 3, NH3 is 4 and of C6H12O6  24.

You can count the number of atoms in glucose molecule 3d structure.

3d model of glucose structure
3d model of glucose structure

Types of molecules on the basis of the number of atoms

Molecules can be categorized into multiple types on the basis of numbers of atoms present in them.

For example…

i) Monoatomic molecule

          The molecules formed as a result of the combination of only one atom are known to be monoatomic molecules.

Definition of monoatomic molecule

For example, He, Ne, Ar, Kr, Xe, Sodium vapors

ii) Diatomic molecules

          The molecules formed by the combination of two atoms are known to be diatomic molecules.

Definition of diatomic molecules

For example, H2, N2, O2, Cl2, Br2, HCl, HBr, CO, NO are known as diatomic molecules.

iii) Polyatomic molecules

          The molecules formed by the combination of more than two atoms are known to be polyatomic molecules.

Definition of Polyatomic molecules

For example, H2O, NH3, CCl4, H2SO4, KMnO4, C6H12O6 are the examples of polyatomic molecules.

Types of molecules on the basis of type of atom

1) Homoatomic or Homonuclear diatomic molecules

The molecules formed by the combination of two same atoms are known to be homoatomic or homonuclear diatomic molecules.

Definition of homoatomic molecules

For example, in the case of H2, N2, O2, Cl2 these have been formed by the combination of two same atoms.

2) Heteronuclear diatomic molecules

The molecules formed by the combination of two different atoms are known to be heteronuclear diatomic molecules.

Definition of heteronuclear molecules

For example, in case of HF, HCl, HBr, HI, CO, NO these have been form by the combination of two different atoms.

Size of molecules

Depending on the number of atoms , molecules may be of varied sizes.

i) Molecules of substances are very small

ii) The molecules are bigger than the atoms

iii) Some of molecules are so large that they are called macromolecules


The molecules of high molecular mass usually greater than 10,000 are known as macromolecules.

Definition of Macromolecules

For example…

I) Proteins  ii) Haemoglobin  iii) Some natural and synthetic polymers

Haemoglobin is a blood protein. It is a macromolecules. It helps to carry oxygen from our lungs to all parts of body to form oxy-haemoglobin. Each molecule consists of nearly 10,000 atoms. It is 68,000 times heavier than hydrogen atom.

Haemoglobin structure
Haemoglobin structure

What are ions in chemistry?

Ion is defined as a species that bears positive of negative charge.

Definition of ion

Types of Ions

There are following types of ions

1) Positive ion or cation

2) Negative ion or anion

1: Positive ion or Cation

A species which has positive charge is called positive ion or cation.

Cation definition

How cation is formed?

When atoms under the influence if energy lose one or more electrons, cations are formed.

Process of cation formation
Process of cation formation

When sufficient amount of energy is supplied to the neutral atom then the atom loses electon to form positive ion.

Cations have +1, +2, +3, +4 charges

The most common and simple cations are the metal ions, such as

Na+, K+, Ca2+, Mg2+, Al3+, Fe3+, Sn4+ .

The cations having group of atom are less common e.g. NH4+, H3O+ .

Negative Ion or Anion

A species with developed negative charge is called negative ion or anion.

Anion definition

Process of anion formation?

When one or more electrons are added to an isolated neutral atom then anions are formed.

Process of anion formation
Process of anion formation

Energy is usually liberated when an electron is added to a neutral isolated atom to form a uni-negitive ion.

Anion have -1, -2, -3, -4 charge.

The common and simple anions are F-, Cl-, I-, S2-, P3- etc.

There are many examples of anions having groups of atoms like OH-, MnO4-, CO32-, PO43-, SO42-, Cr2O72- etc.

What is a Molecular ion?

Any molecular species with either positive or negative charge is said to be molecular ion.

Definition of molecular ion


CO+, C2H4+, O2+, CH4+

Formation of the molecular ion

When beam of electron having energy 10-15eV is bombarded on a molecule of a compound in the vapour phase, an electron is removed and a radical cation is formed. This radical cation is called molecular ion.

This is done in the ionization chamber of the mass spectrometer. The fast-moving alpha particles can also ionize gas molecules to give molecular ions. A metal atom M is converted to M+ by electron bombardment.

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