Isotope

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The atoms of a chemical element can exist in different forms. These are called isotopes. They have the same number of protons (and electrons), but different numbers of neutrons. We can say that different isotopes of the same element have different masses. Mass is another name for weight. Because they have different numbers of neutrons, they do not all weigh the same.

Different isotopes of the same element have the same atomic number. They have the same number of protons. The atomic number is decided by the number of protons. Isotopes have different mass numbers, though, because they have different numbers of neutrons.

The word isotope, meaning at the same place, comes from the fact that isotopes are located at the same place on the periodic table.

In a neutral atom, the number of electrons equals the number of protons. Isotopes of the same element also have the same number of electrons and the electronic structure. Because how an atom acts is decided by its electronic structure, isotopes are almost the same chemically, but different physically to their original atoms.

Heavier isotopes react slower than lighter isotopes of the same element. This "mass effect" is large for protium (1H) and deuterium (the common name of 2H), because deuterium has twice the mass of protium. For heavier elements, the relative mass difference between isotopes is much less, and the mass effect is usually not much.

Atomic nuclei are protons and neutrons held together by the strong nuclear force. Because protons have a positive charge, they are not attracted to one another. Neutrons are electrically neutral, and they let positive charges remain apart. This makes the nucleus more stable. So, we need neutrons for more than two protons to be in a nucleus. As there are more protons, more neutrons are needed to have a stable nucleus. But, if there are too many neurtrons, the nucleus becomes unstable.

In Nature only a few elements have just a single isotopes. For example, fluorine ¹⁹F is the only isotope of fluorine. On the other hand, a few elements have many isotopes. For example, xenon has 30 isotopes. Hydrogen has two common isotopes. A hydrogen atom with an extra neutron (atomic mass of 2) is called deuterium and another neutron (atomic mass of 3) creates tritium.

Also, some isotopes are radioactive. These are called radioactive isotopes. Others are not radioactive. These are called stable isotopes. Usually, the heavier the isotope, the more likely it is to be radioactive. This is why both isotopes of hydrogen are not radioactive since they are so light, but all the isotopes of uranium are radioactive, since they are so much heavier. This is because the forces inside the nucleus of the atom have a tough time holding together all the particles with so many protons and neutrons inside.