- 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.
- The mass of the atom of a particular isotope relative to hydrogen 1 (or to one twelfth the mass of carbon 12), generally very close to the whole number represented by the sum of the protons and neutrons in the atomic nucleus of the isotope; it is not to be confused with the atomic weight of an element, which may include a number of isotopes in natural proportion.
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What is mass number equal to?
1 Answer
The total number of nucleons - protons and neutrons - in the nucleus of an atom is called the Mass Number.
For example, the most common isotope of fluorine has an atomic number of 9 and a mass number of 19.
The atomic number tells us there are 9 protons in the nucleus (and also 9 electrons in the shells surrounding the nucleus).
The mass number tells us the nucleus contains 19 particles in total. Since 9 of these are protons, the other 10 are neutrons. We call it the mass number because virtually all the mass in an atom comes from the protons and neutrons - electrons weigh about 1/2000th of the mass of a proton or neutron.
Mass Numbers Game
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