# What is the isotopic composition of an element?

Dec 10, 2015

The identity of an element is determined by the number of PROTONS it contains in its nucleus. Nuclei of the same element may, however contain different number of NEUTRONS.

#### Explanation:

You know that the atomic nucleus contains a certain number of PROTONS, massive positively charged particles, and NEUTRONS, massive NEUTRALLY charged particles.

The number of positively charged particles gives the identity of the nucleus; this is $Z$, the atomic number. $Z = 1$, hydrogen; $Z = 2$, helium; $Z = 6$, carbon; $Z = 79$, gold.

Take carbon as an exemplar. Most carbon nuclei (and ALL contain 6 PROTONS in their nuclei, WHY?) have 6 neutrons in their nuclei as well. A mole (a large number) of ""^12C nuclei would thus have a mass of $12.00$ $g$ precisely. But some carbon nuclei have 7 neutrons in their nucleus (""^13C), a smaller few have 8 neutrons (""^14C) in their nucleus. These are still carbon nuclei, by definition, because $Z$ $=$ $6$ still and have identical chemistry, but they necessarily have different masses because of the different number of neutrons. The weighted average of the masses (the mass multiplied by the isotopic abundances) sums to $12.011$ $g$; this is the number you see on the Periodic Table, and the number you use when you calculate the molecular mass of an organic molecule.

As atoms get bigger, they can support different numbers of isotopes. Actinides and lanthanides have a lot of different isotopes, some of which may be used in nuclear reactions.

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