Question

How many valence electrons does an ion with $15$ protons have?

Answer 1

This element is phosphorus and it has five valence electrons.
Explanation:
We can determine this two ways. The element with $15$ protons, meaning it has atomic number $15$, is phosphorus. Phosphorus is in the group $15/VA$. Based on its group number, phosphorus has $5$ valence electrons, as do all of the elements in that group.
Another way we can answer this question is to write the electron configuration for a neutral phosphorus atom, which would have $15$ electrons: ${\text{1s}}^2{\text{2s}}^2{\text{2p}}^6{\text{3s}}^2{\text{3p}}^3$The highest energy (outermost) s and p sublevels make up the valence shell. So according to the electron configuration, there are $5$ electrons in the valence shell.