Question

What is the atomic mass of calcium?

Answer 1

Hint : Carbon belongs to the family of alkaline earth metals and is the third member of the second group in the modern periodic table after Beryllium and Magnesium. Its electronic configuration can be used to determine the number of protons it contains.

Complete Step By Step Answer:
According to Dalton’s atomic theory, an atom is the smallest unit or building block of all matter.
Later it was observed that the atom could further be fragmented into smaller subatomic particles like electrons, protons and neutrons. These subatomic particles contribute to the total mass of an atom.
The structure of an atom is such that the tiny negatively charged electrons are present inside the orbitals and the dense nucleus at the centre contains protons and neutrons. The number of electrons and protons is equal in a neutral atom.
The atomic mass of an element is a measure of the amount of matter contained by the atom. It can be calculated using the sum of masses of total protons and neutrons present inside its nucleus. The number of protons is represented by atomic number $Z$ . Mass of electrons can be neglected.
Calcium has an atomic number of $20$ and the number of neutrons present inside calcium are also $20$ .
Each proton and neutron contributes a mass of $1\text{ amu}$ .
The total atomic mass in atomic mass units can be calculated by multiplying the mass of one neutron or proton with that of the total number of protons or neutrons.
 $\text{Atomic mass of Ca = (20 + 20)}\times \text{1amu}$
 $\text{Atomic mass of Ca}\simeq \text{40 amu}$
Hence the atomic mass of calcium is almost $\text{40 amu}$ .

Note :
The atomic mass unit is a relative unit of measuring atomic masses which considers the carbon atom to be the source of reference. The $C-12$ isotope is considered to be made up of exactly twelve atomic mass units, and the atomic mass of all other atoms are measured according to this scale.