Question

What is the mass of aluminium?
(A) $\text{24 u}$
(B) $\text{26 u}$
(C) $\text{27 u}$
(D) $\text{30 u}$

Answer 1

Hint: The atomic mass is the mass of a single atom of a chemical element is the sum of the atomic number of the element and the number of neutrons present in its nucleus. Atomic mass is expressed in the atomic mass unit.

Complete Step by step answer: We know that the atomic number of aluminium is $\text{13}$. Thus, the number of electrons around the nucleus of an aluminium atom is $\text{13}$.
The number of electrons is always equal to the number of protons so that the atom remains electrically neutral. Thus, the number of protons in the nucleus of an aluminium atom is $\text{13}$.
The number of neutrons in the nucleus of the aluminium atom is $\text{14}$.
The mass of an atom of any element is the sum of the atomic number of the element and the number of neutrons. Thus,
$\text{Mass of aluminium}=\text{Atomic number of aluminium}+\text{Number of neutrons in the aluminium atom}$
Substitute $\text{13}$ for the atomic number, $\text{14}$ for the number of neutrons. Thus,
$\text{Mass of aluminium}=13+14=27\text{ u}$.
Thus, the mass of aluminium is $27\text{ u}$.

Thus, the correct option is (C) $27\text{ u}$.

Additional Information: The atomic mass and the atomic number of energy elements are different. The atomic mass is the sum of the number of electrons and the number of neutrons. The atomic number is equal to the number of protons or the number of electrons.

Note: Remember that the number of neutrons in the nucleus of aluminium atoms is $\text{13}$ and not $\text{14}$. There are two naturally occurring isotopes of aluminium: ${}^{27}\text{Al}$ and ${}^{26}\text{Al}$. The isotopic abundance of ${}^{27}\text{Al}$ is $99.9\mathrm{\%}$. The ${}^{27}\text{Al}$ isotope has $\text{14}$ neutrons in its nucleus.