1.2 : 原子の電子構造

An atom is made of three subatomic particles – electrons, protons, and neutrons. Positively charged protons and electrically neutral neutrons constitute the atomic nucleus, where all of the positive charge and most of the mass of an atom reside.

Electrons are the negatively charged, low-mass constituents located in the empty space around the nucleus. An atom is electrically neutral when it has as many electrons as protons.

Electrons exhibit a dual nature — particle and wave. Their location in space and their energy are complementary properties. Therefore, for an electron with a well-defined energy, its position is uncertain.

The quantum mechanical model of an atom predicts that the probability of finding an electron of a given energy in a three dimensional region in space can be determined from the square of the wavefunction expressing the electron's wavelike nature, also called the electron probability density.

A function describing the probable spatial location of an electron of specific energy in an atom is called an atomic orbital.

Each orbital corresponds to a unique combination of three quantum numbers. The principal quantum number, n, groups orbitals into energy levels or shells.

The angular momentum, or azimuthal, quantum number, l, groups orbitals within shells into subshells. Each value of l corresponds to either an s, p, d, or f orbital of varying shape.

Some orbital shapes may have multiple possible orientations, each associated with a magnetic quantum number, m_l. The number of orientations for a given subshell is equal to two-l plus one.

The fourth quantum number — the spin quantum number, m_s — is associated with the electrons occupying an orbital and has only two possible values: ‘plus one-half’ and ‘minus one-half’. This ensures that no two electrons in an atom have the same values for all four quantum numbers.

Overlaying the orbitals on top of one another produces a roughly spherical shape, like a ‘cloud’ of electrons surrounding the nucleus. This is why atoms are generally represented as spheres.