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.

People also ask

• How would you describe the Structure of an Atom
• What was Rutherford’s Original Hypothesis
• What did Bohr Contribute to the Theory of an Atom
• What are the Characteristics of Electron, Proton and Neutron
• Explain Bohr Bury rules for Distribution of Electrons into Different Shells
• What are the Isotopes, Isobars and Isotones of an Element
• What did Dalton Contribute to the Understanding of the Atom
• What is Atomic Mass
• How has the Model of the Atom Changed Over the Years?

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         

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.