The nucleus consists of protons and neutrons while the proton is positively charged with a unit mass, the neutron also has a unit mass but with no charge. This means that the nucleus has a net positive charge with a mass equal to the mass of both the protons and the neutrons. The third particle that makes up the atom is the electron. It has a negligible mass and a negative charge which is equal and opposite to that of a proton in the nucleus. The electron does not form part of the nucleus rather, it moves in orbits around the nucleus. The sum of the protons and the neutrons in the nucleus is known as the mass number. For an atom to be neutral, the number of protons in the nucleus must be equal to the number of the electrons revolving around the nucleus. Elements can be arranged in a series on the basis of their atomic numbers. The number of protons present in the nucleus of an atom is known as the atomic number and must be equal to the number of electrons revolving around the nucleus for neutrality.
Arrangement of Electrons Around the Nucleus
In 1913, a scientist from Denmark called Niels Bohr put forward a theory to describe the arrangement of the electrons around the nucleus. The summary of the theory is that there exist certain circular orbits around the nucleus of an atom. These orbits possess certain energy which are quantized (definite) and are located at definite distances from the nucleus. It is in these orbits that electrons are found. These orbits can be called energy levels or shells and the orbit nearest to the nucleus has the lowest energy and has quantum number of 1. This is called the K-shell. The next shell will be quantum number 2 and is known as the L-shell and so on. Each main shell has other sub shells.
Each main shell must not contain more than 2n2 electrons where n is the quantum number.
For n = 1, the atom has one shell called K-shell and can only contain 2n2 electron or less i.e. 2×1 = 2 electrons.
For n = 2, the atom has two shells, K and L. The L-shell can only contain 2n2 electrons or less i.e. 2 x 22 = 8 electrons.
For n = 3, the atom has three shells, K, L and M. The M shell can only contain 2n2 electrons or less i.e. 2 x 32= 18 electrons and so on.
The maximum number of electrons which can be accommodated in the K, L, M and N shells are 2, 8, 18 and 32 respectively. The electronic arrangement of the first twenty elements are shown in the Table below.