Atomic Number and Mass Number

In our previous blog we have come across charged particles, atomic structures, electrons distributions and valency of the atom. Today we are going to talk about some other terms of atoms we use in our studies. The first one is;

Atomic Number

The atomic number is defined as the total number of protons present in the nucleus of an atom. It is denoted by ‘Z’. We know that all the protons are present only inside the nucleus of an atom. All atoms of an element have the same atomic number, Z. In fact, elements are defined by the number of protons they possess. For hydrogen, Z = 1, because in hydrogen atom, only one proton is present in the nucleus. Similarly, for carbon, Z = 6.

Mass Number

In 1932, J. Chadwick discovered another subatomic particle which had no charge and a mass nearly equal to that of a proton. It was eventually named as neutron. Neutrons are present in the nucleus of all atoms, except hydrogen. In general, a neutron is represented as ‘n’.

After studying the properties of the subatomic particles of an atom, we can conclude that mass of an atom is practically due to protons and neutrons alone. These are present in the nucleus of an atom. Hence protons and neutrons are also called nucleons. Therefore, the mass of an atom resides in its nucleus. For example, mass of carbon is 12 u because it has 6 protons and 6 neutrons, 6 u + 6 u = 12 u. Similarly, the mass of aluminum is 27 u (13 protons+14 neutrons). The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom.

Fore more details refer to Beyond Reaction: Fundamental Chemistry

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