The sum of the protons and neutrons in an atom equals the
a.
atomic number
c.
atomic mass unit
b.
nucleus number
d.
mass number

the anser is mass number. i took the test and got it correct

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.

distance, luminosity, brightness, radius, chemical composition, and temperature