If an element has 12 protons and 17 neutrons, how many electrons must it have?

The elements are electrically neutral, this is they have the same amount of positive as negative charge.

Electrons and protons have each the same amount of charge but different sign. Eletrons are negative and protons are positive.

Then to be neutral, the number of electrons must be equal to the number of protons. This mean that the element must have 12 electrons.

Answer: 12.

Number of electrons = Number of Protons

So, Number of electrons will be 12