quantum theory

Heisenberg uncertainty principle

it is impossible to know exactly both the velocity and the position of a particle at the same time

Schrödinger Quantum Mechanical Model of the Atom

Nucleus in the middle, locations of the electrons are based on wave equations to give a region of probability for finding an electron with known energy

Aufbau Principle

the rule that electrons occupy the orbitals of lowest energy first

Pauli Exclusion Principle

An atomic orbital may describe at most two electrons, each with opposite spin direction

Hund's Rule

orbitals of equal energy are each occupied by one electron before any orbital is occupied by a second electron, and all electrons in singly occupied orbitals must have the same spin