The sum of the protons and the neutrons in an atom equals the ?

Mass number

Explanation:

The mass number equals to the number of protons and neutrons in an atom.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.

it takes on an electronical energy