Thereisadifference betweenthe meaningsof the chemistrytermsatomic massandmass number. Oneisthe average weight of an element and the otheristhe totalnumberof nucleonsin the atom'snucleus. ... Themass number isa countof thetotalnumberof protons and neutronsin an atom'snucleus.

There is a difference between the meanings of the chemistry termsatomic massandmass number. One is the average weight of an element and the other is the total number of nucleons in the atom's nucleus.

Atomic mass is also known asatomic weight.Atomic massis theweighted average massof an atom of an element basedon the relative natural abundanceof that element's isotopes.The mass number is a count of thetotal numberof protons and neutrons in an atom'snucleus.

Key Takeaways: Atomic Mass Versus Mass Number

The mass number is the sum of the number of protons and neutrons in an atom. It is a whole number.The atomic mass is the average number of protons and neutrons for all natural isotopes of an element. It is a decimal number.Atomic mass value sometimes change over time in publications as scientists revise the natural isotope abundance of elements.

Atomic Mass and Mass Number Example

Hydrogen has threenatural isotopes:1H,2H, and3H. Each isotope has a different mass number.

1H has 1 proton. Its mass number is 1.2H has 1 proton and 1 neutron. Itsmass numberis 2.3H has 1 proton and 2neutrons. Its mass number is 3. 99.98% of all hydrogen is1H 0.018% of all hydrogen is2H 0.002% of all hydrogen is3H Together, they give avalue of atomic massof hydrogen equal to 1.0079 g/mol.

Atomic Number and Mass Number

Be careful you don't confuse atomic number and mass number. While the mass number is the sum of the protons and neutrons in an atom, the atomic number is only the number of protons. The atomic number is the value found associated with an element on the periodic table because it is the key to the element's identity. The only time the atomic number and mass number are the same is when you are dealing with the protium isotope of hydrogen, which consists of a single proton. When considering elements in general, remember the atomic number never changes, but because there may be multiple isotopes, the mass number may change.