Recall that atomic nuclei are comprised of positively charged protons and uncharged neutrons. The chemical properties of an atom are determined solely by the number of protons, because chemical bonds involve an exchange or sharing of electrons, and the amount of positive charge in the nucleus determines how many electrons the atom attracts. Neutrons have no charge and therefore make no difference to an atom’s chemical properties.Neutrons can make a minor difference to the speed of a chemical reaction, as explained in the fine print section below. An atom of a given chemical element can have any number of neutrons. A hydrogen atom, for example, may have no neutrons at all; or it may have one neutron, or two neutrons. Any atom with exactly one proton is a hydrogen atom, regardless of how many neutrons it has.
Remember, the number of neutrons does not affect an atom’s chemical properties, so different isotopes behave the same way in chemical bonds. Moreover, the nucleus itself is unaffected by chemical reactions, which only involve the exchange or sharing of electrons.
Atoms that have the same number of protons but different numbers of neutrons are called different isotopes of the same element. Protium, deuterium, and tritium are three isotopes of hydrogen. Protium has no neutrons, deuterium has one neutron, and tritium has two. Since the neutrons don’t affect an atom’s chemical properties, all isotopes of hydrogen can form the same chemical bonds: they can all combine with oxygen to form water, for example. Although all isotopes of an element can form exactly the same chemical bonds, chemical reactions may happen more slowly with heavier isotopes, because heavier atoms move around more slowly at any given temperature. In that way, neutrons can have a minor effect on chemical behavior.
The mass number of an atom is the total number of protons and neutrons in its nucleus. (In contrast, the atomic number is just the number of protons.) Different isotopes of an element always have the same atomic number but different mass numbers. Isotopes are often named by placing the mass number with a hyphen at the end of the element’s name.
All isotopes of hydrogen have atomic number 1, since they all have exactly one proton; but they have different mass numbers. Hydrogen-1 (also called protium) has mass number 1, since it has one proton and no neutrons; hydrogen-2 (deuterium) has mass number 2, since it has one proton and one neutron; hydrogen-3 (tritium) has mass number 3 because it has 1 proton and 2 neutrons.
Similarly, all atoms of carbon have exactly 6 protons, since that is the atomic number of the element carbon. But there are different isotopes of carbon, with different mass numbers. For example, carbon-12 has 6 protons and 6 neutrons; carbon-14 has 6 protons and 8 neutrons.
Isotopes are symbolized by writing the mass number in superscript on the left side of the chemical element symbol. For example, carbon-14 is symbolized 14C. Sometimes the atomic number is also included, written in subscript on the left side, like this: 146C. Here are a few more examples:
Hydrogen-1 (protium) is symbolized 1H or 11H.
Hydrogen-2 (deuterium) is symbolized 2H or 21H.
Hydrogen-3 (tritium) is symbolized 3H or 31H.
Lithium-7 is symbolized 7Li or 73Li.
Some isotopes occur in nature much more frequently than others, partly due to the process of radioactive decay (which will be discussed on the next page). A sample of any particular element found in nature usually contains more than one isotope of that element, but the proportions of each isotope are usually about the same, and for this reason the average mass of the atoms in a sample tends to be more or less constant regardless of where the sample was found. On the periodic table, the average mass of each element (as it occurs in nature) is shown in the upper left corner. The units are called atomic mass units (symbolized “u”), or sometimes daltons (symbolized “Da”) in honor of John Dalton. One atomic mass unit (or dalton) is approximately equal to the mass of a proton or neutron.
The average atomic mass of hydrogen (as found in nature) is 1.00794 u, which means that the vast majority of hydrogen atoms are protium (hydrogen-1). Heavier isotopes of hydrogen are relatively uncommon.
The average mass of lithium is 6.941 u. This suggests that the most common isotope of lithium is lithium-7, which consists of 3 protons and 4 neutrons. But since the average mass is slightly less than 7 u, a lighter isotope must also be common. (Indeed, as it turns out, lithium-7 and lithium-6 are the two most common isotopes. About 92% of naturally-occurring lithium is lithium-7; nearly all of the remaining 8% is lithium-6.)