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Atoms are made up of protons and neutrons. All atoms with the same number of protons are part of the same element. However, if the number of neutrons vary, then they are different isotopes. Isotopes can be thought of as individual varieties of a given element.

Overview And Examples

Chemistry is based on the electrons around an atom. So if the atom is short one electron from filling an electron shell, then that atom badly wants to combine with other atoms to share an electron to fill that shell. This causes atoms to chemically combine forming molecules. The number of electrons (with an electric charge of negative one each) in an atom, must balance (in the long term) the number of protons (with an electric charge of positive one each) in the atom. So in the periodic table, it makes sense to organize the elements by the number of protons. Those protons (and the electrons filling shells around them), determine the chemistry of each element.

Some atoms are made up of a single stable isotope, while others have several. Examples:

Helium has two stable isotopes: Helium 4 (made up of two protons and two neutrons), and Helium 3 (Made up of 2 protons and 1 electron).

A few things to note. ANYTHING with two protons is Helium, that is what Helium means.

If you add the number of protons and neutrons together, you get the atomic mass number for that isotope.

Let us look at Hydrogen. The most common form of Hydrogen is Hydrogen 1, with one proton and zero neutrons. A hydrogen isotope with 1 proton and 1 neutron (Hydrogen 2) is called deuterium. A radioactive isotope with one proton and two neutrons (Hydrogen 3) is called tritium.

Isotopes are important in two ways.

-- First the variation and how common isotopes are effect the average atomic mass of each element. For example, Chlorine (Cl) has two common (non-radioactive) isotopes: Chlorine 35 with an atomic mass of 35, and Chlorine 37 with an atomic mass of 37. Cl35 is the most common isotope making up 75.8% of the natural Cl, while Cl37 makes up 24.2% of natural Chlorine.

However average the atomic masses (and abundances) of Chlorine: ((35 * 0.758) + (37 * 0.242) ) / 2 = 35.45. This was very mysterious to physicists when they were creating the periodic table and measuring the mass of the the elements. Most elements had integer atomic masses, how could Chlorine have one half way between two integers??? Now that we know about isotopes this mystery is explained.

-- The second reason why isotopes are important has to do with radioactive decay and nuclear power. This will be discussed below.


Like charges repel. So if two electrons (both with –1 electric charge) are close together, they will repel and unless stopped, will shoot away from each other. The closer they are to each other, the more the repelling force.

Protons each have a +1 positive charge, but inside the nuclei of an atom, they are much, much closer together, so the electromagnetic force trying to force them apart is gigantic. Why do protons stay together inside atomic nuclei? The answer is the strong nuclear force. Protons and neutrons exchange 'gluons' (beyond the scope of this discussion), which holds them together. Whereas the electromagnetic force has an infinite range (tho it is very weak far away), the strong nuclear force has a very short range, it only works at the scale of atomic nuclei. (And large nuclei are pushing that distance.) Small atoms like to have approximately equal number of protons and neutrons. As we go to larger atoms, they want more neutrons than protons to stay stable.

It turns out that nucleons (protons and neutrons) also like to form shells. (The number of protons or neutrons in each shell are different from the number of electrons needed to fill electron shells for reasons we won't go into here.) Filled shells are especially stable and unlikely to radioactively decay.

Small atoms like hydrogen can gain energy by adding more nucleons. So by fusioning light elements we can generate power. Huge atoms can gain energy by splitting apart, so by fissioning heavy elements we can also generate power. Iron is in the middle ground, it can't gain energy by adding or subtracting from it.

Let us look at Uranium. All Uranium isotopes have exactly 92 protons, but the number of neutrons will be higher than that, and can vary. U235 (made up of 92 protons and 143 neutrons) is unstable, it can (fairly) easily split in half releasing a lot of energy. U238 (92 protons and 146 neutrons) however, is very stable, it is barely radioactive at all.

In a nuclear power plant, we will bombard heavy isotopes with neutrons trying to change them into the rare isotopes which will fission into two halves which release a lot of heat energy. Ideally they will generate enough extra neutrons to keep the reaction going. There are only a handful of isotopes that are likely to fission, and only 3 are important to nuclear power: Uranium 233, Uranium 235, and Plutonium 239.