chemical element

A chemical element is a kind of atom, and the thing that defines the kind is one number: how many Protons sit in the Atomic nucleus. That count is the atomic number, written Z. Every atom with Z = 6 is carbon. Nothing else about the atom gets a vote.

That sounds obvious now. It was not. For 250 years after Robert Boyle, an element was defined by what you could not do to it — a pure substance that no chemical reaction could break into anything simpler — and elements were told apart by their atomic weights, which chemists could measure but not explain. The definition was a confession of ignorance dressed as a rule.

Henry Moseley replaced it in 1913. Using X-ray spectroscopy he measured the nuclear charge of each element from aluminium to gold and showed that this integer, not the atomic weight, is what puts elements in order. It fixed the places where weight and chemistry disagreed — tellurium is heavier than iodine but belongs before it — and it turned "how many elements are there?" into a question with countable answers, because you can now point at a gap and say: number 43 is missing.

How many there are

count	which
118	recognised by IUPAC, atomic numbers 1 through 118
94	occur naturally on Earth (elements 1–94)
24	synthetic only — everything past plutonium
6	natural but in extreme trace amounts: technetium, promethium, astatine, francium, neptunium, plutonium
80	elements with at least one stable isotope

The seventh row closed on 28 November 2016, when IUPAC approved the names nihonium (Nh, 113), moscovium (Mc, 115), tennessine (Ts, 117) and oganesson (Og, 118). Jan Reedijk, then president of IUPAC's Inorganic Chemistry Division, put it this way:

For now, we can all cherish our periodic table completed down to the seventh row.

Element is not the same as isotope

Change the Neutron count and you get a different Isotope, not a different element. Carbon-12, carbon-13 and carbon-14 all have six protons, so all three are carbon and all three behave like carbon in a reaction, because chemistry is run by the six Electrons that the six protons hold. This is exactly why atomic number, and not mass, is the identifying property: 54 of the 94 natural elements have more than one stable isotope, and only 26 are monoisotopic.

Hydrogen is the loud exception to "isotopes act alike." Doubling the mass of the lightest atom changes reaction rates measurably — the kinetic isotope effect — which is why deuterium gets its own symbol.

Where elements come from

The 94 natural elements were made by at least four different processes, and the periodic table is partly a map of which one built what.

• Big Bang nucleosynthesis — in the first few minutes, giving roughly 75% ¹H and 25% ⁴He by mass, plus a little deuterium and traces of lithium and beryllium. Nothing heavier than boron came out of it.
• Stellar fusion — everything from carbon up to iron, built inside stars over their lifetimes. Iron is where the profit stops; see Nuclear binding energy.
• Explosive nucleosynthesis — supernovae and Neutron star mergers make the elements past iron, including uranium.
• Cosmic ray spallation — lithium, beryllium and boron, chipped off carbon, nitrogen and oxygen nuclei in flight. The Big Bang skipped them, and stars destroy them, so they are rare despite being light.

The most common nuclides in the Solar System

Mass fraction, parts per million.

nuclide	mass fraction (ppm)	atom fraction (ppm)
hydrogen-1	705,700	909,964
helium-4	275,200	88,714
oxygen-16	9,592	774
carbon-12	3,032	326
neon-20	1,548	100
iron-56	1,169	27
nitrogen-14	1,105	102
silicon-28	653	30
magnesium-24	513	28

Two elements are 98% of the mass of everything. The rest of chemistry — all of biology, all of geology — happens in the remainder.

Naming one

IUPAC does not just record elements, it referees them. It sets the criteria a discovery claim must meet, assigns priority between competing labs, invites the winners to propose a name, and enforces the rules the name must follow. Until that finishes, an element carries a placeholder built from its digits. Element 113 was ununtrium before it was nihonium.

Related: Isotope · periodic table · hydrogen · uranium · Atom · Atomic nucleus