What is the Definition of Atom and Molecule

What is the Definition of Atom and Molecule 

Atom:
All the matter is made up of atoms. An atom is the smallest particle of an element that can take part in a chemical reaction. Atoms of most of the elements are very reactive and do not exist in the free state. They exist in combination with the atoms of the same element of another element.
Atoms are very, very small in size. The size of an atom is indicated by its radius which is called ‘atomic radius’. Atomic radius is measured in ‘nanometres’. The symbol of a nanometre is nm.
1 nanometre =  \(\frac { 1 }{ { 10 }^{ 9 } } \) metre
or         1 nm =   \(\frac { 1 }{ { 10 }^{ 9 } } \) m
or         1 nm = 10–9 m
Hydrogen atom is the smallest atom of all. They cannot be viewed by simple optical microscopes. However, through modern techniques such as scanning tunneling microscope it is possible to produce magnified images of surfaces of elements showing atoms.

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Molecules:
A combination of atoms is called a molecule. The forces which hold the atoms together in a molecule are called covalent bonds.
A molecule is the smallest particle of a substance which has the properties of that substance and can exist in the free state.
There are two types of molecules.

1. Molecules of elements
The molecule of an element contains two (or more) similar atoms chemically combined together.
Example:    A molecule of hydrogen element contains 2 hydrogen atoms combined together, and it is written as H2.

2. Molecules of compounds
The molecule of a compound contains two (or more) different types of atoms chemically combined together.
Example:    Hydrogen chloride is a compound. The molecule of hydrogen chloride (HCl) contains two different types of atom. Hydrogen(H) and chlorine atom (Cl)

Molecules of some compounds         

Compound

Combining elementsFormulaRatio by mass

Water

Hydrogen and oxygenH2O1:8
 AmmoniaNitrogen and HydrogenNH3

14:3

Carbon dioxide

Carbon and OxygenCO2

3:8

Atomicity
The number of atoms present in one molecule of an element is called its atomicity.
Example:    Noble gases (helium, neon, argon, krypton, etc.) have one atom each in their molecules such as He, Ne, Ar and Kr. So, the atomicity of noble gases is 1.
Example:    Hydrogen (H2), nitrogen (N2), oxygen (O2), chlorine (Cl2), bromine (Br2), and iodine (I2), all have 2 atoms each in their molecules. So, the atomicity of hydrogen, nitrogen, oxygen, chlorine, bromine and iodine is 2 each.

Atomicity of some common elements.

Type of element

NameSymbolAtomicity

Non-metal

HeliumHeMonoatomic

Argon

ArMonoatomic

Neon

NeMonoatomic
HydrogenH2

Diatomic

ChlorineCl2

Diatomic

Nitrogen

N2Diatomic
OxygenO2

Diatomic

Phosphorus

P4Tetratomic
SulphurS8

Polyatomic

Metals

Sodium

NaMonoatomic
IronFe

Monoatomic

Aluminium

Al

Monoatomic

CopperCu

Monoatomic