To determine the number of neutrons in an atom what must be done

The mass number is the sum of protons and neutrons in the nucleus. The atomic number is the number of protons. Therefore, you can subtract the atomic number from the mass number to find the number of neutrons. For example, Carbon's atomic number/number of protons is 6 and the mass number is 12.011.

Chemistry? I’m only in 8th grade science

Okay well gravitational energy is matter forced by gravity making energy though the kinetic energy then the electric energy produces electricity from the kinetic energy and the kinetic energy gets its energy from the gravitational energy. was that the answer you wanted or something else?  

☆ionisation energy increases across a period from na -> ar☆ionisation energy decreases down a group from li -> fr☆ionic radius decreases across a period from na -> cl☆ionic radius increases down a groupfrom f -> ii hope it clarifies your doubts