What is a chemical element?

A piece of pure gold, the helium in a party balloon, a sparkling diamond: all of them are made of atoms. But gold atoms are different from helium atoms, and helium atoms are different from the atoms in a diamond. Atoms come in different kinds, and each kind is a chemical element. Let’s find out what makes each kind special.

What is an element?

Some substances are made of only one kind of atom. Pure gold contains only gold atoms, and the helium that makes balloons float contains only helium atoms. A diamond is made of only carbon atoms. A substance like this is called a chemical element, or an element for short.

Most things around you are not elements. They are made of two or more kinds of atoms, joined together. Water, for example, is made of hydrogen atoms and oxygen atoms, joined into tiny groups called molecules. A substance made of several elements joined like this is called a compound.

You can break water apart into hydrogen and oxygen, for example with electricity. But no chemical trick can break hydrogen or oxygen into anything simpler. That is what makes them elements: they are the simplest substances of all.

What makes each element different?

Every atom has a tiny nucleus in its middle, and the nucleus holds protons. The number of protons decides which element the atom belongs to:

Every atom with 1 proton is hydrogen.
Every atom with 2 protons is helium.
Every atom with 6 protons is carbon.
Every atom with 8 protons is oxygen.

This number is called the atomic number. Every element has its own, from 1 for hydrogen all the way up to 118. No two elements share a number, and no number is missing.

Count the protons, the balls marked with a plus. One means hydrogen, two helium, six carbon and eight oxygen.

An atom can gain or lose a few electrons. It can also have a few more or fewer neutrons. It still stays the same element, just as a bicycle with a basket or a bell is still a bicycle.

Atoms of one element with different numbers of neutrons are called isotopes. Atoms with too many or too few electrons are called ions.

Go deeper: Atomic number and mass number for grown-ups and the extra curious

The atomic number $Z$ is the number of protons in the nucleus. It alone defines the element.
The mass number $A$ is the number of protons and neutrons together. Carbon-12 has $A = 12$ (6 protons and 6 neutrons), and carbon-14 has $A = 14$ (6 protons and 8 neutrons).
The number of electrons can change without changing the element. An atom that loses or gains electrons becomes an ion, but it keeps its atomic number.
Only nuclear processes change $Z$ : radioactive decay, nuclear reactions inside stars, or collisions in particle accelerators. Chemistry cannot turn lead into gold, as the alchemists once hoped. Nuclear physics can: in 2025, scientists at CERN reported that their particle collider turns some lead nuclei into gold nuclei. From 2015 to 2018 it made about 86 billion of them: in all, only about 29 trillionths of a gram. Each one hit the collider’s walls and broke apart within a split second.

A short name for every element

Writing long names again and again would take ages. So every element has a short symbol of one or two letters. The first letter is always a capital, and the second is always small.

Element	Symbol
hydrogen	H
helium	He
carbon	C
oxygen	O
iron	Fe
gold	Au
sodium	Na

Some symbols look strange. Why is iron Fe, and why is gold Au? These symbols come from the elements’ Latin names: ferrum for iron, aurum for gold and natrium for sodium.

Scientists all over the world use the same symbols, whatever language they speak. Chemists in Slovakia and chemists in Japan both write Au for gold.

Every element gets a box like this one. The big letters are its symbol, and the small number tells how many protons each of its atoms has.

All the elements are arranged in a famous chart, the periodic table. It puts elements that behave alike next to each other.

How many elements are there?

Scientists know 118 elements. Of these, 94 are found in nature on Earth, though a few of them only in tiny traces. Scientists have made the rest, often only a handful of atoms at a time. The heaviest of them fall apart in less than a second, because they are radioactive.

The newest four got their names in 2016: nihonium, moscovium, tennessine and oganesson. Element 118, oganesson, is the heaviest element known so far.

Go deeper: How many are natural, exactly? for grown-ups and the extra curious

Elements 1 to 94 have all been found in nature. Six of them occur only in extremely tiny traces: technetium (43), promethium (61), astatine (85), francium (87), neptunium (93) and plutonium (94). Natural radioactive decay and rare natural nuclear reactions keep making small amounts.
Only 80 elements have at least one stable isotope, one that never decays. Every element heavier than lead (82) is radioactive, although some decay extremely slowly. Bismuth-209 has a half-life of about $2 \times 10^{19}$ years, more than a billion times the age of the universe.
IUPAC, the world body that looks after chemical names, announced on 30 December 2015 that the discoveries of elements 113, 115, 117 and 118 had been verified. That completed the seventh row of the periodic table. IUPAC approved their names in November 2016.

Which elements are the most common?

Some elements are everywhere, and others are very rare.

Where	The most common elements
the whole universe	hydrogen and helium
the Earth’s crust	oxygen and silicon
your body	oxygen, carbon and hydrogen

Hydrogen is by far the most common element in the universe: more than 9 of every 10 atoms are hydrogen atoms. Stars like our Sun are made mostly of hydrogen and helium.

In the rocks and sand under your feet, almost half of the weight is oxygen. You can’t see it, because it is locked up together with other elements, such as silicon.

About two-thirds of your body’s weight is oxygen, mostly in the water inside you. A child who weighs 30 kilograms carries almost 20 kilograms of oxygen!

Go deeper: The numbers for grown-ups and the extra curious

The universe (ordinary matter), by mass: hydrogen about 74%, helium about 24%, and all the other elements together only about 2%. Counted by atoms, it is about 92% hydrogen and 8% helium. These two were made in the first minutes after the Big Bang. Almost all heavier elements were made later, inside stars.
The Earth’s crust, by mass: oxygen 46%, silicon 28%, aluminium 8% and iron 6%, followed by calcium, sodium, magnesium and potassium.
The human body, by mass: oxygen 65%, carbon 18%, hydrogen 10%, nitrogen 3%, calcium 1.5% and phosphorus 1%. These six elements make up about 99% of your mass. Counted by atoms, hydrogen wins: about 62% of the atoms in your body are hydrogen atoms, because they are so light.

Elements join up

With only 118 elements, how can there be millions of different substances? Because elements combine. Their atoms hold on to each other with chemical bonds, and each new combination makes something new.

The new substance can be completely different from its elements! Sodium is a soft, shiny metal that fizzes wildly in water. Chlorine is a poisonous, yellowish-green gas. Joined together, they make table salt: the white crystals you sprinkle on your food.

When atoms let go of old partners and join new ones, we call it a chemical reaction.

Check yourself

An atom has 8 protons. Which element is it?

Show the answer

Oxygen — Every atom with 8 protons is an oxygen atom. The number of protons decides the element.