Define Atomic Number, Atomic Weight, Atomic Mass and Mass Number
Chemistry Science

Define Atomic Number, Atomic Weight, Atomic Mass and Mass Number

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Definition of Atomic Number, Atomic Weight, Atomic Mass and Mass Number

Atomic Number

The term atomic number was introduced by Moseley. Atomic number of an element is its serial number in the periodic table and it also equal to the number of protons present in an atom of the element.

Since, An atom as a whole is neutral hence the atomic number of the element is also equal to the number of electrons present in the atom of the element.

Elements are arranged in the periodic table according to increasing order of their atomic number. In the periodic table also indicates its atomic number.

Atomic Weight

The weight of an atom equal to the sum of weights of proton and neutron present in its nucleus.

In order to express the atomic weight of an atom, atomic weight of hydrogen or oxygen is taken as unit.

Hence, atomic weight of an element shows that the atom of the element is how many times heavier than hydrogen atom of lighter that oxygen atom.

Atomic weight of an element = Weight of one atom of the element / Weight of one atom of hydrogen

Atomic Mass and Atomic Mass Unit

The atomic mass of an element is the relative mass of one atom of the element compared with an atom of carbon as the standard.

In the middle of 19th century it was the oxygen atom which was selected as standard of mass.

In 1961 one more amendment was prescribed. After that the standard or atomic scale was changed to the carbon which is given mass of 12. It is known as the atomic mass unit.

Mass Number

The total number of protons and neutrons present in the nucleus of an atom is known as its mass number.

It is not fractional but integral.

e.g. The atomic weight of oxygen is 15.999 but mass number is 16.

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