Question

What is the mass number of an atom of potassium that has $18$ neutrons?

Answer 1

Mass number of an atom is the total number of protons and neutrons present in an atom and it is represented by the letter $A$. It was discovered by Rutherford who showed that the vast majority of the mass of an atom is concentrated in the nucleus, which is composed of protons and neutrons.

Complete step by step answer:
We can find the mass number of an element by adding the number of protons and neutrons present in the atom.
Potassium has the atomic number $19$ on the periodic table.
We know that the atomic number of an element is equal to the number of protons present.
Given, the number of neutrons in potassium$=$18.
$\therefore$Mass number$=$Protons$+$neutrons$= 19 + 18 = 37amu$.
**Therefore, the mass number of an atom of potassium that has $18$ neutrons is $37amu$.

Note:**
The mass number of an element is written as a superscript to the left of the element’s symbol. The most common example of an isotope of carbon is carbon$- 12$or$^{12}C$, which has $6$ protons and $6$ neutrons. The full isotope of the element would have the atomic number $(Z)$ which is written as a subscript to the left bottom of the element's symbol.