General Representation of an Atomic Mass Number

(A,Z) Y

A = the mass number

Z = atomic number

Atom has three particles, protons which is the atomic number and the mass number which consists of the electrons and the neutrons. Example; Ca

With the representation above which is Calcium, Calcium has 20 protons (atomic number). Calcium has 20 electrons (the number is always the same as the proton number). Calcium has 20 neutrons.

The nucleons are protons and neutrons and that forms the mass number.

The nucleon is in a total of 40. Thus 40 – 20 = 20 Therefore Neutron is 20.

Example:

The atomic number of an element is 17 and its mass number is 35, find the number of (i) protons (ii) electrons (iii) neutrons.

Answer

Mass number (A) = 35; Atomic number (Z) = 17

The number of protons is 17; this is because the atomic number is the same as the proton number.

The number of electron is 17; this is because proton number is equal to number of electrons.

The number of neutrons

(N) = mass number (A) – Atomic number (Z)

  = 35 – 17
  = 18