Question

one isotope of sodium has 11 protons and 7 neutrons. what is the mass number of this isotope?
○ 4
○ 7
○ 11
○ 18

Explanation:

Step1: Recall mass number formula

Mass number ($A$) is the sum of protons ($p$) and neutrons ($n$), so $A = p + n$.

Step2: Substitute values

Given $p = 11$ and $n = 7$, then $A = 11 + 7 = 18$.

Answer:

18 (corresponding to the option "18")