The___ of an element is the number of element.*
__ of an atom of that
atomic mass, electrons in the nucleus
mass number, neutrons in the nucleus
mass number, protons plus neutrons in the outermost energy level
atomic number, protons in the nucleus

atomic mass, neutrons in the nucleus

Explanation: because I remember from when I took honors chemistry last year we learned about this and it’s called the atomic mass when looking at an atom and the neutrons in the nucleus are effected by it.

it is the most commonly accepted one

the answer is a! (2.9 x 10^-18 j) hope i could someone. : )