Question

an atom has 19 protons and 20 neutrons. what is its mass number? 19 20 39 38

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 values

Given $p = 19$ and $n=20$, then $A=19 + 20$.

Step3: Calculate result

$19+20 = 39$.

Answer:

39