In the previous section we learned that atoms of the same element always have the same number of protons. But, we also learned that two atoms of the same element can have different numbers of neutrons than each other. When an atom is of the same element as another atom, but has a different number of neutrons, it is called an isotope. Remember that the atomic number of an atom is its number of protons. This never changes. However, the atomic weight of an atom is made up of the number of protons added to the number of neutrons. Because atoms of the same element can have differing numbers of neutrons, they can have different atomic weights. Let us look at Potassium (K) for example:

isotopes KFrom our calculations shown in the above illustration, we know that potassium has 19 protons (its atomic number) and 20 neutrons. But why do we have the 0.098 tagged to the atomic weight?

The answer: isotopes.

Isotopes are atoms of the same element — in this case potassium — that have different numbers of neutrons. The word isotope is made up of two Greek words. “Isos” which means “same,” and “topos” which means “place.” So the word means “same place” and refers to the fact that, despite their differences, these atoms with varying numbers of neutrons occupy the same place on the periodic table. Every element on the periodic table comes in a variety of isotopes. In the case of potassium, scientists have calculated how common each isotope is, and have come up with the following results:

isotopes K 2

Therefore, if 93.3 out of 100 potassium atoms have 20 neutrons and 6.7 have 22 neutrons and finally 0.012 have 21 neutrons, we can perform a basic math calculation to get the average of these. The average is 20.098 neutrons and therefore this makes the average weight of a potassium atom 39.098 (20.098 neutrons plus 19 protons). And this is the atomic weight number you will see at the bottom of potassium (K) on the periodic chart.

Atomic Mass Units (AMU)

An amu is an atomic mass unit. Each proton and each neutron in a nucleus are worth one amu each. So the potassium isotope with exactly 20 neutrons and 19 protons has a total amu of 39. The average number of neutrons in potassium is 20.098 so the amu listed on the periodic chart is 39.098. As you may have noticed, 39.098 is also called the atomic weight of potassium. Therefore the atomic weight of an element is measured in atomic mass units (amu).

Before moving onto the next tutorial, we suggest you try the sample questions for this tutorial. Have fun!

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