Question

Atomic mass of an element is the same as the mass of a single atom of that element. (State whether the given statement is true or false).
A. True
B. False

Answer 1

Hint:Atomic mass is the sum of masses of the protons and neutrons present in an atom. It varies for different elements. A single element however, may exist in one or more forms, which have a varying value of atomic mass.

Formulas used:
${M_{avg}} = \dfrac{{\Sigma {M_i}{x_i}}}{{100}}$
Where ${M_{avg}}$ is the average atomic mass, ${M_i}$ is the mass of a single isotope, ${x_i}$ is the abundance expressed in percentage and $\Sigma $ represents summation.

Complete step by step solution:
The mass of a single atom is computed by summing up the individual masses of each particle in that atom, that is, the sum of masses of protons and neutrons in that atom. Thus, different elements have different atomic masses.
However, atomic mass of an element cannot be the same as the mass of a single atom of that element, due to the fact that an element may have several isotopes. Isotopes are atoms of the same element having different mass numbers. Thus, they have the same atomic numbers (same number of protons) but a varying number of neutrons. A lot of elements are known to have isotopes. Thus, we cannot take the atomic mass of an element as the mass of any one atom of that element. Rather, we can compute the average atomic mass of that element. Average atomic mass of an element is calculated by summing up the masses of each isotope of that element, each isotope being multiplied with their corresponding relative abundances. For example,
Lithium exists in nature in the form of two isotopes, $Li - 6$ and $Li - 7$ with atomic masses $6.0151\mu $ and $7.016\mu $ respectively. Their relative abundances are $7.59\% $ and $92.41\% $.
Therefore, the average atomic mass of lithium is given as:
${M_{avg}} = \dfrac{{\Sigma {M_i}{x_i}}}{{100}}$
Where ${M_{avg}}$ is the average atomic mass, ${M_i}$ is the mass of a single isotope, ${x_i}$ is the abundance expressed in percentage and $\Sigma $ represents summation.
Hence, substituting the values, we get:
${M_{avg}} = \dfrac{{(6.0151 \times 7.59) + (7.016 \times 92.41)}}{{100}}$
$ \Rightarrow {M_{avg}} = \dfrac{{45.654609 + 648.34856}}{{100}} = 6.939\mu $
Therefore, the average atomic mass of lithium is $6.939\mu $, which is different from the atomic mass of a single atom of lithium
Hence, the given statement is false. Thus, the correct option to be marked is B.

Note:
Masses of electrons are not taken into account while computing the mass of an atom, since electrons have negligible mass. Note that the atomic masses of each element we see in the periodic table are actually the average atomic masses of all its isotopes. Atomic masses can be computed using spectrometric methods.