What is a simple definition for atomic number and atomic mass for a grade 8 student?

Answer 1

the atomic number (Z) (also known as the proton number) is the number of protons found in the nucleus of an atom. In an atom of neutral charge, the number of electrons also equals the atomic number.

atomic mass:

The average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally-occurring element.

Also Known As: Atomic Weight

Examples: The atomic mass of carbon is 12.011; the atomic mass of hydrogen is 1.0079

Answer 2

Each element has a certain number of protons and neutrons in the nucleus (middle part) and the same number of electrons as it has protons.

Atomic number is the number of protons (or electrons), e.g. oxygen = 8.

Atomic mass is the total of protons and neutrons, e.g. oxygen = 8+8 = 16.

The number of neutrons is NOT always equal to the number of protons, however.