ARPANSA - Australian Radiation Protection and Nuclear Safety Agency

Atoms can join together to form molecules, which in turn form most of the objects around you. Different elements, such as oxygen, carbon, and uranium contain different kinds of atoms. Atoms are the smallest unit of an element that behaves in the same way as the element.

An atom consists of an extremely small, positively charged nucleus surrounded by a cloud of negatively charged electrons. Although typically the nucleus is less than one ten-thousandth the size of the atom, the nucleus contains more that 99.9% of the mass of the atom! Nuclei consist of positively charged protons and electrically neutral neutrons held together by the so-called strong or nuclear force. This force is much stronger than the familiar electrostatic force that binds the electrons to the nucleus, but its range is limited to distances on the order of a few times 0.000 000 000 000 001 meters (Figure 1).

The number of protons in the nucleus, Z, is called the atomic number. Atoms with the same atomic number will be atoms of the same element. The number of neutrons in the nucleus is denoted by N. The mass number of the nucleus, A, is equal to Z + N (Figure 2). The mass of the nucleus in atomic mass units (amu) is usually slightly different from the mass number.

Atoms of the same element can have different numbers of neutrons. The various forms are called isotopes of that element. As an example hydrogen has three (3) isotopes: hydrogen-1 (hydrogen), hydrogen-2 (deuterium) and hydrogen-3 (tritium) (Figures 3).

In a neutral atom, the number of electrons orbiting the nucleus equals the number of protons in the nucleus. Since the electric charges of the proton and the electron are +1 and -1 respectively (in units of the proton charge), the net charge of the atom is zero.

At present, there are 112 known elements which range from the lightest, hydrogen, to the recently discovered and yet to-be-named element 112. All of the elements heavier than uranium are man made. Among the elements are approximately 270 stable isotopes, and more than 2000 unstable isotopes.

Atoms are often identified using the name of the element and the mass number: the total number of protons and neutrons in the nucleus e.g. lithium- 7 (Figure 4). When referred to in this way the atom is called a nuclide. Radioactive nuclides are called radionuclides.

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