The sum of the protons and neutrons in an atom equals the? nucleus number
atomic number
atomic mass
mass number

mass number

Explanation:

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

So;

Mass number = number of protons + number of neutrons.

2h2o2--> 2h2o + o2

this reaction is of the spontaneous decomposition of hydrogen peroxide down into water and oxygen.

add 2 molecules of hydrogen peroxide and 2 molecules of water. since oxygen is naturally diatomic, the total number of atoms of each element is now the same on both sides of the equation so it is balanced.

this may not be entirely correct but let me know.

"the periodic table is arranged in order of atomic number."