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

The mass number ($A$) of an atom is the sum of the number of protons ($p$) and neutrons ($n$), i.e., $A=p + n$.

Step2: Substitute given values

Given $p = 11$ and $n=7$. Then $A=11 + 7$.

Step3: Calculate the result

$A = 18$.

Answer:

D. 18