Sun January 22, 2012 By:

What is the difference between atomic mass and atomic mass number ?

Expert Reply
Sun January 22, 2012
Atomic mass is also known as atomic weight. Atomic mass is the weighted average mass of an atom of an element based on the relative natural abundance of that element's isotopes.

Mass number is a count of the total number of protons and neutrons in an atom's nucleus.

Hydrogen has three natural 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. Its mass number is 2. 3H has 1 proton and 2 neutrons. Its mass number is 3. 99.98% of all hydrogen is 1H 0.018% of all hydrogen is 2H 0.002% of all hydrogen is 3H Together, they give a value of atomic mass of hydrogen equal to 1.0079 g/mol.
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