what is the mass number of an atom that has 6 protons, 7 neutrons, and 6 electrons?\nseven\n13\nsix\n12

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: Identify values of $p$ and $n$

Given that $p = 6$ and $n=7$.

Step3: Calculate mass number

$A=6 + 7=13$.

Answer:

B. 13