The word “atom” means “indivisible” and was first coined by the ancient Greeks over two thousand years ago, as a term for the smallest particles in the Universe.
Today the term “atom” refers to particles that, although incredibly small, are not actually indivisible. Rather, they are made up of subatomic particles called the electron, the proton and the neutron.
Protons have an electric charge of +1 (in units of the size of the charge of an electron) and electrons have an electric charge of -1. Neutrons have no electric charge. Atoms contain equal numbers of protons and electrons, giving them an overall electric charge of 0 (i.e. they are neutral).
Protons and neutrons have roughly the same mass, whereas in comparison, electrons are almost massless.
Atoms are defined in terms of an atomic number, which is usually denoted by the letter “Z”, and a mass number, which is usually denoted by the letter “A”.
The atomic number equals the number of protons in an atom. All atoms with the same atomic number are the same chemical element.
The mass number is equal to the number of protons and neutrons in an atom.
For a given atomic number, atoms may exist with varying mass numbers, due to different numbers of neutrons in the atom. Atoms with different mass numbers but the same atomic number are called isotopes.
The smallest atom is the hydrogen atom. This is formed of one proton and one electron. It does not contain any neutrons. Therefore, the hydrogen atom has an atomic number of 1 and a mass number also of 1.
The next smallest atom is helium. This is formed of two protons and two electrons. The most common form of helium also contains two neutrons, giving it a mass number of 4. This is known as the helium-4 isotope. Helium can also be found in a form containing just one neutron. This is known as the helium-3 isotope and is much rarer than the helium-4 isotope.
In total there are 92 elements that can be found in existence naturally. They have atomic numbers from 1 (hydrogen) to 92 (uranium). Larger atoms have been produced in experiments, but these are unstable and quickly decay and break down into smaller atoms.
The basic model of an atom’s structure is called the Bohr model, after the Danish physicist Niels Bohr who developed it. In the Bohr model, atoms contain a nucleus at their center, which is where the protons and neutrons are located. The electrons orbit the nucleus in discrete “shells” or “energy levels”.
The shells are filled up with electrons in order from the closest to the nucleus, to the furthest. A fixed number of electrons are permitted in each shell. For example, in the shell closest to the nucleus, a maximum of two electrons can be in orbit. The next shell can contain eight electrons.
In fact, scientists now know that the Bohr model is an oversimplification of atomic structure. However, it is still a very good approximation of the structure of electronic orbitals.
Atoms can undergo both chemical and nuclear reactions.
In chemical reactions, electrons in the outer shells are exchanged or shared with the outer shells of other atoms. In this way, atoms can form bonds with other atoms to make structures such as molecules, crystals and metals.
In nuclear reactions, as the name suggests, it is the nucleus of the atom which takes part in the reactions. There are two kinds of nuclear reaction: fusion and fission.
Fusion is the joining together of two small nuclei, such as hydrogen, to form a larger nuclei, such as helium or carbon. Fusion is the reaction which keeps all stars, including the sun, burning.
Fission is the opposite of fusion. In fission reactions, large nuclei, such are uranium, break up into smaller nuclei. This is the type of reaction that takes place in a nuclear reactor to provide nuclear energy.
In summary, atoms are formed of a nucleus containing protons and neutrons, and electrons which orbit the nucleus in shells. Atoms are defined by their atomic number, which is the number of protons, and their mass number, which is the number of protons plus the number of neutrons. Despite their name, atoms are not indivisible and can undergo various interactions, including breaking up into smaller atoms in a reaction known as nuclear fission.