atomic structure

Internal structure of an atom.

The nucleus
The core of the atom is the nucleus, a dense body only one ten-thousandth the diameter of the atom itself. The simplest nucleus, that of hydrogen, comprises a single stable positively charged particle, the proton. Nuclei of other elements contain more protons and additional particles, called neutrons, of about the same mass as the proton but with no electrical charge. Each element has its own characteristic nucleus with a unique number of protons, the atomic number. The number of neutrons may vary. Where atoms of a single element have different numbers of neutrons, they are called isotopes. Although some isotopes tend to be unstable and exhibit radioactivity, all those of a single element have identical chemical properties.

Electrons
The nucleus is surrounded by a number of moving electrons, each of which has a negative charge equal to the positive charge on a proton, but which has a mass of only 1/1,836 times as much. In a neutral atom, the nucleus is surrounded by the same number of electrons as it contains protons. According to quantum theory, the position of an electron is uncertain; it may be found at any point. However, it is more likely to be found in some places than others. The region of space in which an electron is most likely to be found is called an atomic orbital. The chemical properties of an element are determined by the ease with which its atoms can gain or lose electrons.

Orbitals
An atom may have a number of orbitals, each of which has a characteristic shape and size determined by mathematical functions. Each orbital is characterized by three numbers – called quantum numbers – which represent its energy (and hence size), angular momentum (and hence shape), and orientation. Each orbital can be occupied by one or (if their spins are aligned in opposite directions) two electrons. Electrons tend to be found in the orbitals with lowest energy. Those in high-energy orbitals may spontaneously jump into orbitals with lower energy, emitting a particle of light – a photon – in the process.

Attraction and repulsion
According to the theory of fundamental forces, atoms are held together by the electrical forces of attraction between each negative electron and the positive protons within the nucleus. The latter repel one another with enormous forces; a nucleus holds together only because an even stronger force, called the strong nuclear force, attracts the protons and neutrons to one another. The strong force acts over a very short range – the protons and neutrons must be in virtual contact with one another. If, therefore, a fragment of a complex nucleus, containing some protons, becomes only slightly loosened from the main group of neutrons and protons, the natural repulsion between the protons will cause this fragment to fly apart from the rest of the nucleus at high speed. It is by such fragmentation of atomic nuclei (nuclear fission) that nuclear energy is released.