The three fundamental sub atomic particles are protons, neutrons and electrons.

The proton is a positively charged particle with a mass equal to that of a hydrogen atom. The discovery of protons is credited to Ernest Rutherford though earliest prediction was done by Eugene Goldstein. The electron is negatively charged with a mass of 1/1840 the mass of a proton. It was discovered by J.J Thompson in his work with cathode rays. The neutron which was discovered by James Chadwick who bombarded a thin sheet of beryllium with alpha particles is neutrally charged and has a mass equal to that of a proton. R.A Millikan in his oil drop experiment found the charge of an elec tron to be -1.60 × 10^-19.

All atoms have the same number of electrons and protons hence an atom is electrically neutral due to the charges cancelling out. Protons and neutrons are collectively called nucleons

Rutherford's gold foil experiment showed that the atom is mostly empty space with a tiny, dense, positively-charged nucleus. Based on these results, Rutherford proposed the nuclear model of the atom. Thomson's plum pudding model of the atom had negatively-charged electrons embedded within a positively-charged "soup." This discovery showed that the atom consists of a positively charged nucleus and electrons which orbits around the nucleus.

In 1913, physicist Niels Bohr proposed a model of the atom in which the electron was able to occupy only certain orbits around the nucleus. This means that electrons evolve round the nucleus along certain imaginary circular path called Orbitals. He also predicted that these orbitals have certain energy levels.

Atomic orbitals describe where an electron is likely to be found in an atom. There are four basic orbitals: Sharp (s), Principal (p), Diffuse (d) and Fundamental (f) orbitals. The spdf orbitals denotes the subshells in which electrons are placed. Each Orbital can take a maximum of two electrons

Subshell	Shape	Number of orbitals	Number of maximum electrons
s	Spherical	1	2
p	Dumb bell	3	6
d	Double dumbbell	5	10
f	Complex shape	7	14

Main shells denoted by letters K, L, M,N, O have definite energy levels with the K shell having the least energy level. Each shell can take a maximum of electrons according to the formula 2n² where n = energy level; with the lowest energy level starting from 1. Each shell except the first shell can hold a maximum of 8 electrons while the first shell can hold a maximum of 2 electrons.