Atomic Structure & History

Average Atomic Mass

The weighted average mass of all isotopes of an element based on their natural abundance. It's what is typically reported as the atomic mass on the periodic table.

Isotopes

Variants of an element with the same number of protons but different numbers of neutrons. They have different mass numbers but the same chemical behavior.

Wave Properties & Atomic Spectra

Electrons exhibit wave-like behavior, and atomic spectra arise due to electrons absorbing or emitting energy when they jump between quantized energy levels (orbitals).

Electron Configurations

A notation that shows the arrangement of electrons in an atom's orbitals according to energy levels and sublevels (s, p, d, f). This describes how electrons fill up the atomic orbitals.

Longhand (Standard) Notation

Writing out the full electron configuration with each orbital and number of electrons explicitly shown, e.g., 1s² 2s² 2p⁶ 3s¹ for sodium.

Orbital (Diagram) Notation

A visual representation using boxes for orbitals and arrows for electrons, showing electron spin and pairing within orbitals.

Noble Gas (Shorthand) Notation

A simplified electron configuration using the noble gas core to represent filled inner shells, followed by the valence electrons explicitly, e.g., [Ne] 3s¹ for sodium.

Valence Electrons

Electrons in the outermost shell or highest energy level that participate in chemical bonding. Determined from electron configurations by counting electrons in the highest principal quantum number orbitals.

Nuclear Chemistry & Half-Life

Nuclear chemistry studies reactions and changes in the atomic nucleus. Half-life is the time required for half of a radioactive sample to decay [general chemistry knowledge].