This picture illustrates the idea of "atomic mass". The carbon atom (14C) nucleus on the top has 6 protons plus 8 neutrons, giving it an atomic mass of 14. Tritium (3H), an isotope of hydrogen, is shown on the bottom. It has 1 proton plus 2 neutrons in its nucleus, giving it an atomic mass of 3. Click on image for full size (46 Kb) Original artwork by Windows to the Universe staff (Randy Russell).

"Atomic mass" is a term physicists use to describe the size (mass) of an atom of a specific type. Since the nucleus of an atom contains nearly all (more than 99%) of an atom's mass, "atomic mass" is more-or-less a description of the mass in the nucleus. The nucleus of an atom contains protons and (in most cases) neutrons. Protons and neutrons have nearly identical masses. "Atomic mass" is essentially a count of the number of neutrons plus the number of protons in the nucleus of a particular type of atom. For example, a simple hydrogen atom with just a single proton has an atomic mass of 1; a "normal" carbon atom with 6 neutrons and 6 protons has an atomic mass of 12.

Sometimes you may see atomic masses expressed as fractions, such as 15.9994 for oxygen. What gives? Surely there isn't a fraction of a proton or neutron hiding inside the oxygen atom's nucleus! The atomic mass for an element is expressed as the weighted average across all isotopes of that element, based on its abundance on Earth. For example, 92% of silicon in Earth's crust is the isotope ²⁸Si, which has 14 protons plus 14 neutrons. However, 5% of silicon atoms have 15 neutrons and another 3% have 16 neutrons, giving them atomic masses of 29 and 30 respectively. The atomic mass of silicon is said to be, therefore, 28.086, when one factors in the masses and relative abundances of different isotopes of silicon.

The behavior of an atom depends more on its atomic number than on its atomic mass. All isotopes of nitrogen behave pretty much the same in chemical reactions, for example. That is because the chemical properties of an element are determined by the number of electrons in its atoms, and the number of electrons equals the number of protons in "normal", neutral atoms. Scientists use the letter "Z" to stand for atomic number and the letter "A" to stand for atomic mass.

This picture explains the idea of "atomic mass". The carbon atom (14C) nucleus on the top has 6 protons plus 8 neutrons. It has an atomic mass of 14. Tritium (3H), an isotope of hydrogen, is shown on the bottom. It has 1 proton plus 2 neutrons in its nucleus. Tritium has an atomic mass of 3. Click on image for full size (46 Kb) Original artwork by Windows to the Universe staff (Randy Russell).

One way scientists measure the size of something is by its mass. Scientists can even measure very, very tiny things like atoms. One measure of the size of an atom is its "atomic mass". Almost all of the mass of an atom (more than 99%) is in its nucleus, so "atomic mass" is pretty much a measure of the size of the nucleus of an atom.

The nucleus of an atom is made up of protons and neutrons. Protons and neutrons are almost exactly the same size. If you add up the number of protons and neutrons in the nucleus of an atom, you get that atom's atomic mass. A simple hydrogen atom has just one proton and zero neutrons. Its atomic mass is 1. The most common kind of carbon atom has 6 neutrons and 6 protons. It has an atomic mass of 12.

All atoms of a certain element have the same number of protons. Oxygen atoms always have 8 protons; carbon atoms all have 6 protons. Most atoms come in different types called isotopes. Isotopes have different numbers of neutrons. The most common isotope of carbon has 6 neutrons and 6 protons. Its atomic mass is 12. A rare, radioactive isotope of carbon has 8 neutrons. Its atomic mass is 14 ( = 6 protons + 8 neutrons).

In chemistry, the number of protons in an atom is more important than the number of neutrons. Scientists call the number of protons the "atomic number". Normal atoms have the same number of electrons as protons. The number of electrons is the main thing that controls how atoms behave in chemical reactions. Scientists use the letter "Z" to stand for atomic number and the letter "A" to stand for atomic mass.

This picture explains the idea of "atomic mass". The carbon atom (14C) nucleus on the top has 6 protons plus 8 neutrons. It has an atomic mass of 14. Tritium (3H), an isotope of hydrogen, is shown on the bottom. It has 1 proton plus 2 neutrons in its nucleus. Tritium has an atomic mass of 3. Click on image for full size (46 Kb) Original artwork by Windows to the Universe staff (Randy Russell).

One way scientists measure the size of something is by its mass. Mass is sort of like weight. Scientists can even measure very, very tiny things like atoms. One measure of the size of an atom is its "atomic mass". Almost all of the mass of an atom (more than 99%) is in its nucleus. "Atomic mass" is pretty much a measure of the size of the nucleus of an atom.

The nucleus of an atom is made up of protons and neutrons. Protons and neutrons are almost exactly the same size. If you add up the number of protons and neutrons in the nucleus of an atom, you get that atom's atomic mass. A hydrogen atom has just one proton and zero neutrons. Its atomic mass is 1. Normal carbon atoms have 6 neutrons and 6 protons. They have an atomic mass of 12.

All atoms of an element have the same number of protons. Oxygen atoms always have 8 protons. Carbon atoms all have 6 protons. Scientists call the number of protons the "atomic number". Scientists use the letter "Z" to stand for atomic number and the letter "A" to stand for atomic mass.

Most atoms come in different types called isotopes. Isotopes have different numbers of neutrons. Normal carbon atoms have 6 neutrons and 6 protons. Normal carbon atoms have an atomic mass of 12. A rare, radioactive isotope of carbon has 8 neutrons. Its atomic mass is 14 ( = 6 protons + 8 neutrons).