Question

The mass number of an atom is slightly less than the actual atomic mass. Give reasons.

Answer 1

Hint: Mass number indicates total number of protons and neutrons present in the nucleus. Actual atomic mass is the weighted average of all existing atoms and isotopes per element. Isotopes have the same atomic number but different mass numbers.

Complete step by step solution:
-Atomic number indicates the number of protons present in the nucleus.
-Atomic number also indicates the number of electrons revolving outside the nucleus.
-In a neutral atom, the number of protons is equal to the number of electrons.
-The mass number represents the total number of protons and neutrons present in the nucleus.
-Actual atomic mass is calculated by taking an account of all isotopes and their relative abundance.
-Elements occur in nature as more than one isotope. For example, carbon has three isotopes,
Isotopes have atomic mass as 12,13,14 amu and relative abundances 98.892%, 1.108%, \[2\times {{10}^{-10}}\]% respectively.
-Average atomic mass of carbon will be
$= (0.988920)$ $(12)$ $+ (0.01108)$ $(13.0033)$ + ($2\times {{10}^{-10}}$) $(14.00317)$ $= 12.011 u$.
The atomic number of carbon is 6 which indicates the number of protons is 6 and the mass number of carbon is 12 as the number of neutrons is 6 and the number of protons is 6.
The mass number represents the total number of protons and neutrons present in the nucleus.
So in case of carbon, the mass number of an atom is slightly less than the actual atomic mass.
The mass number of an atom is slightly less than the actual atomic mass because of the presence of isotopes.

Note: Atomic mass of nitrogen is 14.0067 u and mass number of nitrogen is 14 as nitrogen has two stable isotopes and both are taken into account to calculate atomic mass. The mass number represents the total number of protons and neutrons present in the nucleus. Actual atomic mass is calculated by taking an account of all isotopes and their relative abundance.