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

The atomic number of an atom tells us how many protons are in the nucleus of that atom. Why is that important? 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. Each element has a different number of protons in the nuclei of atoms of that element; so each element has a different atomic number.

Hydrogen atoms have 1 proton, and thus an atomic number of 1. Carbon has 6 protons and an atomic number of 6; oxygen has 8 protons and thus and atomic number of 8. The atomic number of uranium is 92!

Scientists also use the concept of "atomic mass". 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 atomic mass of an atom is essentially a count of the number of neutrons plus the number of protons. Common carbon has 6 protons and 6 neutrons in each carbon atom, so its atomic mass is 12 ( = 6 + 6). Sometimes scientists use the letter "Z" to stand for atomic number and the letter "A" to stand for atomic mass.

Most elements have different "versions" with varying numbers of neutrons. The different versions are called isotopes. Carbon, for example, has isotopes with 7 neutrons and with 8, along with the standard 6-neutron variety. Scientists specify which isotope they are talking about by including the atomic mass in the name. Normal carbon is thus carbon-12, while the less common varieties are written as carbon-13 and carbon-14. Remember, however, that the different isotopes of carbon behave almost identically in most chemical reactions, for they share the same atomic number.

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

The nucleus of an atom has protons and neutrons in it. Each element (like carbon or oxygen or gold) has a different number of protons in its atoms. Scientists have a special name for the number of protons in an atom. They call it the "atomic number".

Why is the atomic number important? Normal atoms have the same number of electrons as protons. The number of electrons is what makes each element behave a certain way in chemical reactions. So the atomic number, which is the number of protons and thus of electrons, is what makes one element different from another.

Hydrogen atoms have 1 proton, and thus an atomic number of 1. Carbon has 6 protons and an atomic number of 6; oxygen has 8 protons and thus and atomic number of 8. The atomic number of uranium is 92!

Atoms of the same element and same atomic number can have different numbers of neutrons. All carbon atoms have 6 protons. Most carbon atoms also have 6 neutrons, but some carbon atoms have 7 or even 8 neutrons. Scientists call these different kinds of carbon atoms "isotopes" of carbon.

Scientists also talk about the "atomic mass" of an atom. The nucleus of an atom contains nearly all (more than 99%) of an atom's mass. Neutrons and protons have almost exactly the same mass. So, to calculate atomic mass, we just add up the number of protons plus the number of neutrons. A carbon atom with 6 protons and 8 neutrons has an atomic mass of 14 ( = 6 + 8). Sometimes 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 number". The carbon atom (14C) nucleus on the top has 6 protons. It has an atomic number of 6. Tritium (3H), an isotope of hydrogen, is shown on the bottom. It has 1 proton in its nucleus. Tritium has an atomic mass of 1. Click on image for full size (46 Kb) Original artwork by Windows to the Universe staff (Randy Russell).

Every atom has a nucleus. The nucleus has protons and neutrons in it. Scientists have a special name for the number of protons in an atom. They call it the "atomic number".

There are almost 100 different elements, like carbon and oxygen and gold. Each element has a different atomic number. Hydrogen atoms have 1 proton, so they have an atomic number of 1. Carbon has 6 protons and an atomic number of 6; oxygen has 8 protons and thus and atomic number of 8. The atomic number of uranium is 92!

Atoms of one kind of element always have the same number of protons. They often have different numbers of neutrons, though. Most carbon atoms have 6 protons and 6 neutrons. A few carbon atoms have 6 protons and 8 neutrons. The different kinds of atoms of the same element are called "isotopes".

Sometimes scientists talk about the "atomic mass" of an atom. If we add up the number of protons plus the number of neutrons we get the atomic mass. The atomic mass of the isotope of carbon that has 8 neutrons is 14 ( = 6 protons + 8 neutrons). Sometimes scientists use the letter "Z" to stand for atomic number and the letter "A" to stand for atomic mass.