General properties of nuclei - Atomic number, Mass Number, Neutron Number, Isotopes and Isotones

Atomic Number (Z)

It is the number of protons present in the nucleus. For example, nitrogen has 7 protons, so Z for nitrogen is 7, Z for uranium is 92 and for hydrogen, Z is 1.

Mass Number (N)

It is the total number of protons and neutrons present in the nucleus. For example, carbon has 6 protons and 6 neutrons in its nucleus, so its mass number is 12; uranium has 92 protons and 143 neutrons, therefore, mass number of uranium is 235, ordinary hydrogen has only 1 proton in its nucleus, so its mass number is 1. It is obvious that A can never be less than Z.

Neutron Number (N)

It is the total number of neutrons present in the nucleus and is equal to A – Z. A nucleus X with atomic number Z, mass number A and neutron number N is represented as ^A_ZX. For example uranium with Z = 92; A = 235 and N = 143 is written as ²³⁵₉₂U.

Isotopes

Nuclei of an element, having the same atomic number but different mass numbers are called isotopes of that element. There are two types of isotopes: (i) stable and (ii) radioactive. Stable isotopes are those which do not show radioactivity, for example, ¹⁴C and ¹⁵N. Some radioactive isotopes are ⁴⁰K, ⁶⁰Co, etc.

Isotones

Nuclei having same N and different Z are known as isotones. The stable isotones with N = 1 are ²₁H and ³₂He₁.

Nucleons

The term nucleons refers to protons or neutrons present inside the nucleus. Thus, a nucleus with mass number A having N neutrons and Z protons has A nucleons.