Concept of Atom

An atom consists of a central small nucleus which contains the proton and the neutron and is surrounded by electrons in an orbit. There is an electrostatic force of attraction between the positively charged nucleus and the negatively charged electrons. The maximum number of electrons a shell can accommodate is given by 2n², n is an integer

• n = 1,   2 × 1²  = 2
• n = 2,   2 × 2² = 8
• n = 3,   2 × 3² = 18

An atom is represented by the symbol:

\( ^{A}_{Z} \normalsize X \)

where: A – mass number, Z – atomic number 

Atomic number of an atom is the number of protons in the nucleus of an atom.

The mass number is the number of protons and neutrons in the nucleus of an atom 

The number of neutrons, n = A-Z,  

The proton is positively charged, the neutron has no charge but the electron is negatively charged.