A neutral beryllium atom has four protons and five neutrons. How many electrons does it have?

Four electrons

Explanation:

Neutrons are neutral (have no charge), while protons have a positive charge and electrons have a negative charge with the same magnitude as the proton's charge. For an atom to be neutral, the positive and negative charges must cancel, so the number of electrons must equal the number of protons.

4.5 electron because you have to add the proton and the electron and then divide by two.