orbital

Meaning: An orbital is a wavefunction for an electron in an atom; its square gives the probability of finding the electron at a location. Quantum numbers label it: n sets energy and size, l sets shape (0 = s, 1 = p, 2 = d), and ml sets orientation. Each orbital holds at most two electrons, which must have opposite spins (Pauli exclusion principle). Orbitals of one subshell have equal energy in an atom, so electrons fill them singly with parallel spins first (Hund's rule).

Equation: Orbitals in a subshell = 2l + 1; orbitals in a shell = n². Here n is the principal quantum number and l is the angular momentum quantum number (0 to n − 1). Both are dimensionless integers, so there is no SI unit.

On the MCAT: Expect electron configurations, counting unpaired electrons to predict paramagnetism (unpaired electrons) or diamagnetism (all paired), and spotting forbidden quantum number sets such as l = n. Organic questions use hybrid orbitals (sp, sp², sp³) and sigma versus pi overlap.

Common confusion: An orbital is not an orbit. A Bohr orbit is a fixed path; an orbital is a probability distribution, and the familiar drawn shapes are boundaries enclosing most of that probability. Also keep orbitals and electrons apart: a p subshell has three orbitals but holds up to six electrons.