electrons

Meaning: An electron is a negatively charged subatomic particle (charge −1.60 × 10⁻¹⁹ C) with about 1/1836 the mass of a proton. Electrons occupy quantized orbitals around the nucleus, filled according to the Aufbau principle, the Pauli exclusion principle and Hund's rule. Valence electrons, those in the outermost shell, take part in bonding. Gaining electrons makes an anion; losing them makes a cation.

Equation: F = k q₁q₂ / r², where F is the electric force between two charges (N), k is Coulomb's constant, 8.99 × 10⁹ N·m²/C², q₁ and q₂ are the charges (C), and r is their separation (m). It explains why electrons are held by the nucleus and why closer electrons are held more tightly.

On the MCAT: Expect trends in atomic radius and ionization energy explained by effective nuclear charge and shielding, electron configurations of ions, and redox, where oxidation is loss of electrons and electrons flow from anode to cathode.

Common confusion: Students assume that because 4s fills before 3d, the 3d electrons leave first. Transition metals actually lose their outer s electrons before d electrons, so Fe²⁺ is [Ar] 3d⁶, not [Ar] 3d⁴4s².