By Michael E Carpenter, Updated Aug 30, 2022
Electrons are negatively charged subatomic particles that occupy discrete energy levels—often visualized as shells—surrounding the atomic nucleus. A shell must be filled before an electron can move to a higher energy level. The capacity of each shell differs, and the actual electron distributions deviate from simple circular orbits.
The first shell can hold up to two electrons; hydrogen (1 e⁻) and helium (2 e⁻) have only this shell. The second shell accommodates eight electrons, the third 18, and the fourth 32.
Within each shell, sub‑shells—denoted s, p, d, and f—represent finer energy divisions. The s sub‑shell holds two electrons; p holds six; d holds ten; f holds fourteen. Each successive sub‑shell can hold four more electrons than the previous one.
The electron configuration of an atom is written as a sequence of shell number, sub‑shell letter, and electron count. For example, boron (5 e⁻) is described as 1s² 2s² 2p¹, indicating two electrons in the first shell’s s sub‑shell, two in the second shell’s s sub‑shell, and one in the second shell’s p sub‑shell.
The probability density shapes differ among sub‑shells. s sub‑shells are spherical; p sub‑shells resemble dumbbells. Each p orbital can host two electrons, so a full p sub‑shell contains three such orbitals, totaling six electrons.
Electrons do not follow fixed circular paths; instead, they exist as a probability cloud. In an s sub‑level, the two electrons occupy a spherical region, but they can be found anywhere within that volume at any instant. Quantum mechanics allows the electron to exist beyond the classical boundary, creating a diffuse cloud of probability that applies to all sub‑shells.
