elements

Hydrogen makes up approximately 74% of all ordinary matter in the universe by mass — about 92% of all atoms. Helium accounts for roughly 24% of ordinary mass. Everything else — every carbon atom in every living cell, every silicon atom in every rock, every iron atom in every star's core — adds up to approximately 2% of the universe's total atomic mass. The dominance of hydrogen and helium is not a coincidence or a geological quirk. It is a direct record of the conditions that existed in the first three minutes after the Big Bang. Understanding why the cosmos is almost entirely these two lightest elements tells you how the universe was born and how it will eventually end.

Astatine is usually called the rarest natural element, with estimates of its total presence in Earth's crust running from under one gram to about 28 grams at any moment. It is rare because it decays within hours, not because it formed in small amounts. Rarity has several distinct causes, from radioactive decay to planet formation to production cost, and each one reveals a different piece of nuclear physics and geochemistry.

How the periodic table organizes the elements: Mendeleev's predictions, atomic number, electron shells, trends, gold's color, and superheavy oganesson.

The periodic table is the most powerful organizing tool in science, and it predicted elements that had not been discovered yet.

How Dmitri Mendeleev organized elements and predicted undiscovered elements

Nucleosynthesis is the process by which atomic nuclei are built from simpler components. Hydrogen and helium were made in the first three minutes after the Big Bang. Every heavier element was forged inside stars or in their violent deaths. Much of the gold and platinum is made in neutron star mergers — cosmic events detected as gravitational waves.