# Consider a hypothetical atom with an atomic number of 4 and a net electronic charge of + 1. It has 4 protons. How many neutrons does this atom have?

We know that the atom is $\text{beryllium}$ if $Z = 4$ (why?). The number of neutrons is likely $5$.
$Z \text{, the atomic number}$, is the number of protons, massive positively charged particles present in the nucleus. $Z$ defines the element, and if $Z = 4$, the element beryllium is specified.
While beryllium has several isotopes, ""^9Be is the most common, and certainly overwhelmingly abundant. Even at A level, however, (or even at 1st year undergraduate level), it would not be appropriate to ask a student which is the most stable beryllium nuclide.