Concept of atom is based on the combining the merits of Rutherford’s atomic model and Bohr’s atomic model. All substances are made up of atoms. All atoms are consists of –

Nucleus

Electrons

Nucleus of Atom

Nucleus is located at the center of atom. Nucleus is itself consists of two particles–

Proton
Neutron

Proton

Protons are positively charged particles. Charge on each proton is 1.6 × 10^-19 coulomb. The number of protons in nucleus of an atom represents the atomic number of atom.

Neutron

Neutron does not have any electrical charge. Means, Neutrons are electrically neutral particles. The mass of each neutron is equal to mass the of hydrogen nucleus. The diameter of nucleus is about 1/10000 of diameter of whole atom. Almost whole mass of atom is concentrated in its nucleus. Nucleus is positively charged due to presence of positively charged protons. In any material, the weight of the atom and radioactive properties are associated with the nucleus.

Electrons

An electron is negatively charged particle present in material.Charge on each electron is -1.6 × 10^-19 coulomb. These electrons surround the nucleus. Some facts about electrons in an atom are listed and explained below-

If an atom is having same no. of protons and electrons, then the atom is electrically neutral as the negative charge of electrons neutralize the positive charge of protons.
The electrons revolves around the atoms in shells (also called orbits).
A force of attraction is excreted on negatively charged electrons by positively charged nucleus. This force of attraction works as centripetal force required for electrons revolution around the atom.
The electrons which are near to nucleus are tightly bound with nucleus and it is more difficult to pull out (remove) these electrons from the atom than those which are far away from the nucleus.
The structure of an aluminum atoms is shown in figure below-

A definite amount of energy is required to remove the electron from its orbit. The energy required to remove the electron from first orbit is much more as compare to energy required to remove the electron from the outer orbit. This is due to force of attraction excreted by nucleus on electrons in first orbit is much more as compare to the force of attraction excreted on electrons of outer orbit. Similarly the energy required to remove the electron from second orbit will be less as compare to first orbit and greater than third orbit. Hence, we can say that the electrons in the orbit are associated with a definite amount of energy. Thus the orbits or shells are referred to as energy level.
The energy levels are denoted by the letters K, L, M, N, etc. Where K is nearest orbit to nucleus and having lowest energy level. Conversely the outer most orbit is having highest energy level.
The maximum number of electrons in any energy level is given by, ‘2n²’, where, n is an integer and represents the “principal quantum number”. For different energy levels the value of ‘n’ and maximum number of electrons as given in table below

Sl. No.	Energy level or Orbit (shell)	Principal quantum number ‘n’	Maximum Number of electrons (2n²)
1	K	1	2X1² = 2
2	L	2	2X2² = 8
3	M	3	2X3² = 18
4	N	4	2X4² = 32

The above formula (2n²) used for determining the maximum number of electrons in any shell, has some limitations. The Number of electrons in outer (highest energy level) cannot exceeds beyond 8 electrons. For example let’s consider the atom of calcium, it is having 20 electrons orbiting around its nucleus. As per formula of above rule i.e. 2n², the electrons distribution will be as 2 electrons in K level, 8 electrons in L level and balance 10 electrons. But the electrons in outer most energy level cannot exceeds 8 electrons. Hence, there will be 8 electron in M level and balance 2 electrons will go to next energy level i.e. 2 electrons will go the N- level. The electron configuration of Calcium atom is shown in figure below-

The electrons in outer most energy level are called “valence electrons”. The maximum possible number of “valence electrons” is 8 electrons. If the number of electrons in outer most orbit are 8, the atom becomes stable. The material whose atoms naturally having 8 electrons in their outer most shell (orbit), does not react with other material. i.e. inert gases: Helium, Neon, Argon, krypton etc. are having completely filled outer most shells and do not react with other materials.
The “valence electrons” of an atoms are loosely bound with atoms nucleus and can be freed / removed by different method i.e. by heating, by applying electric voltage etc.
The “valence electrons” form the bonds to hold the atoms of material together. The most of the properties of materials are determined by these bonds in materials.
The each main shell (energy level) is subdivided into subshells. These subshell are called orbitals. These subshell /orbitals are designated by s, p, d, f etc. with corresponding quantum number, l = 0, 1, 2, 3, 4,….(n-1) etc. The number of subshells in any main shell is equal to principal quantum number ‘n’. The electron capacity of any main shell can be determined by adding the electron capacity of subshells. The maximum electron capacity of subshells is governed formula of 2(2l +1 ) . The capacity of subshells are given in table below-

Sl. No.	Subshell	Quantum number (l)	Electron capacity of subshell 2(2l + 1)
1	s	0	2(2 X 0 + 1) = 2
2	p	1	2(2 X 1 + 1) = 6
3	d	2	2(2 X 2 + 1) = 10
4	f	3	2(2 X 3 + 1) = 14