Question

1. an atom has an atomic number of 15 and a mass number of 31. how many neutrons does it have?

Explanation:

Step1: Recall neutron - proton - mass number relation

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 proton number and mass number

The atomic number is equal to the number of protons. Given atomic number = 15, so $p = 15$, and mass number $A=31$.

Step3: Calculate number of neutrons

We can re - arrange the formula $A=p + n$ to find $n=A - p$. Substitute $A = 31$ and $p = 15$ into the formula: $n=31−15$.
$n = 16$

Answer:

16