An atom is the smallest unit into which matter can be divided without charged particles being released. It also is the smallest unit of matter with the characteristic properties of an element.
During the 5th century BC, Greek philosophers proposed the notion that atoms were hard, indivisible fundamental particles of nature. This view held sway until about 1900, when modern scientific experimentation and mathematical deduction finally gave rise to a more accurate conception of the building blocks of matter.
Today, it is known that an atom consists largely of empty space and various constituent particles. Near the centre of the atom is a dense core, or nucleus, comprising protons and neutrons. These so-called nucleons are minute but extremely massive; as a result, the nucleus constitutes more than 99.9 percent of the mass of the entire atom, though it occupies only about 10-14 of the volume. The nucleons cling together to form the nucleus because of an attractive force that they exert on one another when in close proximity. This force is called the nuclear, or strong, force. The nucleus is positively charged, since the protons each carry one unit of positive electric charge while the neutrons carry none at all. As tiny as they are, both the protons and the neutrons are made up of still-smaller particles called quarks. Each nucleon consists of three of these apparently fundamental particles.
The nucleus is surrounded by a diffuse cloud of electrons, particles with a negative electric charge and almost no mass. Because opposite electric charges attract, the negatively charged electrons are bound to the positively charged nucleus. In neutral atoms, the number of electrons equals the number of positive charges on the nucleus (i.e., the number of constituent protons), but any atom may have more or fewer electrons than positive charges and thus be negatively or positively charged as a whole. Such charged atoms are called ions.
Within each atom or ion, the electrons are not at rest but seem to move in complicated orbits around the nucleus. The size of an atom and its response to other atoms, particles, and electromagnetic radiation are determined by the arrangement of the orbiting electrons. Atoms combine with each other to form larger structures, such as molecules, by transferring or sharing electrons. The attraction between electrons that are attached to one atomic nucleus and the nuclei of nearby atoms (known as the van der Waals forces) explains much of the mechanism of the formation of liquids and solids.
The most significant characteristic of an atom is its atomic number, which is the number of protons in its nucleus. The great importance of this property stems from the observation that all atoms with the same atomic number have nearly identical chemical properties and thus constitute a given element. Not all the atoms of an element have the same number of neutrons in their nuclei. Atoms with the same atomic number but a different number of neutrons are isotopes of that element. Isotopes have identical chemical properties, but they can have very different nuclear properties. These include mass, tendency to become radioactive in nuclear reactions (i.e., radioactivity), and magnetic properties.
Excerpt from the Encyclopedia Britannica without permission.