What is the atomic number, mass number, and atomic mass?  How are they related?

An atom is composed of protons, electrons, and neutrons.  Protons, or positively-charged subatomic particles, are located in the nucleus of an atom along with neutrons.  Negatively-charged particles, called electrons surround the nucleus.

The number of protons usually equal the number of electrons in an atom.  The atomic number refers to the number of protons (and thus the number of electrons in a neutral atom).

The mass number takes into account the number of neutrons.  When the atomic number is added to the number of neutrons, we get the mass number.  The same element can have different mass numbers, but not different atomic numbers.

Take for example, potassium.  Potassium’s atomic number is 19.  However, potassium can have mass numbers of 39, 40, or 41.  Potassium-39, Potassium-40, and Potassium-41 are all different isotopes of the element potassium.  Each isotope has 19 protons and 19 electrons, but the number of neutrons vary.  Potassium-39 has 20 neutrons, Potassium-40 has 21 neutrons and Potassium-41 has 22 neutrons.  Therefore, each isotope of an element will have a different mass number but the same atomic number.

Atomic mass is the weighted average mass of these isotopes.  The term, “weighted average mass” means that our calculation will take into account the relative abundance (or percent abundance) of each isotope.  For example lets look at Element Z.  In nature, half of Element Z is in the form of an isotope having a mass of 2 amu.  The other half of Element Z is in the form of an isotope having a mass of 4 amu.  What would be the weighted atomic mass of Element Z?  The answer would be 3 amu.

Check out this forum link, for some practice problems calculating atomic mass.


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