Atomic Number, Mass Number, Relative Atomic Mass and amu - Home of Knowledge

Atomic Number, Mass Number, Relative Atomic Mass and amu

Atomic number or proton number or identification number:

The number of protons present in the nucleus of an atom is called proton number or atomic number.

Explanation: It is represented by symbol ‘Z’ . As all atoms of an element have the same number of protons in their nuclei, they have the same atomic number. Hence, each element has a specific atomic number termed as its identification number.

For example, all hydrogen atoms have 1 proton, their atomic number is Z=l. All atoms in carbon have 6 protons, their atomic number is Z=6. Similarly, in oxygen all atoms have 8 protons having atomic number Z=8 and sulphur having 16 protons shows atomic number Z = 16.

Mass number:The mass number is the sum of number of protons and neutrons present in the nucleus of an atom. It is represented by symbol ‘A’.


Caculation: It is calculated as A=Z+n where n is the number of neutrons and Z is number of protons.

Each proton and neutron has lamu mass. For example, hydrogen atom has one proton and no neutron in its nucleus, its mass number A=l+0 =1. Carbon atom has 6 protons and 6 neutrons, hence its mass number A=12. Atomic numbers and mass numbers of a few elements are given in Table.

Element Number of Protons

Number of Neutrons

(n= A-Z)

Atomic Number (Z) Mass Number (A)
Hydrogen 1 0 1 1
Carbon 6 6 6 12
Nitrogen 7 7 7 14
Oxygen 8 8 8 16
Fluorine 9 10 9 19
Sodium 11 12 11 23
Magnesium 12 12 12 24
Potassium 19 20 19 39
Calcium 20 20 20 40

Relative Atomic Mass:The relative atomic mass of an element is the average mass of the atoms of that element as compared to 1/12th (one-twelfth) the mass of an atom of carbon- 12 isotope (an element having different mass number but same atomic number).

EXPLANATION:As we know that the mass of an atom is too small to be determined practically. However, certain instruments enable us to determine the ratio of the atomic masses of various elements to that of carbon-12 atoms. This ratio is known as the relative atomic mass of the element.

Based on carbon-12 standard, the mass of an atom of carbon is 12 units and l/12th of it comes to be1 unit. When we compare atomic masses of other elements with atomic mass of carbon- 12 atom, they are expressed as relative atomic masses of those elements.

Atomic mass unit: One atomic mass unit is 1/12th the mass of one atom of carbon-12th.The unit for relative atomic masses is called atomic mass unit, with symbol ‘amu’.  When this atomic mass unit is expressed in grams, it is:

1 amu = 1.66 x 10-24g.

For example:

Mass of a Proton = 1.0073 amu or 1.672 x 10-24g

Mass of a Neutron = 1.0087 amu or 1.674 x 10-24g

Mass of an Electron = 5.486 x 10–4 amu or 9.106 x 10-28g.

Use and need of atomic mass unit: The mass of an atom is too small to be determined practically, so they are compared with C-12 isotope. This enable us to determine the mass of an atom.

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