Semiconductor Technology from A to Z

An atom [Gr. átomos: indivisible] is the smallest building block of matter that cannot be further divided by chemical means. There are various types of atoms, each composed of a specific number of electrons, protons, and neutrons. Positively charged protons and uncharged neutrons form the atomic nucleus, which is orbited by negatively charged electrons at certain distances. An atom occurring in nature is electrically neutral, meaning it contains exactly as many protons as electrons. The number of neutrons, however, can vary.

According to the Bohr atomic model, the electrons are located on so-called shells, which represent different energy levels and are arranged concentrically around the atomic nucleus. There are a maximum of seven shells, which can hold varying numbers of electrons; the electrons in the outermost shell are also referred to as valence electrons.

Highly simplified representation of a neon atom

By exchanging electrons with other atoms, atoms can achieve a more stable state, which is why different types of bonds form between atoms.

Mass

The mass of an atom is determined mainly by the atomic nucleus, since the mass of protons and neutrons, at 1.67·10^-27 kg, is roughly 1800 times greater than that of the electrons in the electron shell (9.11·10^-31 kg).

Building blocks of the atom

Dimensions

The diameter of the electron shell is 0.1–0.5 nm (0.0000000001 m, or 1 ångström); the diameter of the atomic nucleus is smaller by a further factor of 100,000. To illustrate this, imagine a pinhead at the center of a football field representing the atomic nucleus. The distance to the corner flags would then correspond to the distance at which the electrons orbit the nucleus.

Density

Within the nucleus of an atom, protons and neutrons are packed extremely tightly. If Earth were compressed to the same density, its radius would shrink from its original 6,700,000 m to just 100 m.

Atoms of major importance in the semiconductor industry