Describe the composition of the nucleus of an atom.

Question: Describe the composition of the nucleus of an atom.

Solutions: The alpha-particle scattering experiment of Rutherford suggested the existence of a positively charged region within an atom. This positively charged region is known as the nucleus of the atom. Almost all the mass of an atom is concentrated within this small volume. Initially, the nucleus was assumed to contain positively charged particles called protons and negatively charged particles called electrons. However, it was later established that electrons cannot exist inside the nucleus.

The discovery of the neutron (a neutral particle having no charge) led to the present concept of the nucleus being composed of protons and neutrons. Protons and neutrons in a nucleus are collectively known as nucleons.

Define Atomic Number

The number of protons present in the nucleus of an atom is called the atomic number. It is denoted by Z.

Define Atomic Mass Number

The sum of the number of protons (Z) and the number of neutrons (N) is called the mass number. It is denoted by A.

Thus,

A=Z+N

or

N=A-Z

A distinct kind of atom or nucleus characterized by the structure of its nucleus (e.g., number of protons, number of neutrons, energy content) is called a nuclide or nuclear species. Nuclide is an internationally accepted term.

Representation of Nuclides

Nuclides are represented by:

${}^{A}_{Z}X \quad \text{or} \quad _{Z}X^{A}$

Where:

X = Symbol of the element

Z = Atomic number of the element

A = Mass number of the element

Example

The nucleus of sodium is denoted as:

${}^{23}_{11}\mathrm{Na}$

It has 23 nucleons

Protons (Z) = 11 

Neutrons=N = A-Z= 23-11 = 12