What is the periodic table? How the elements are arranged

The periodic table is a chart of all the known chemical elements, the basic substances everything is made of, listed in order of atomic number, the number of protons in each atom's nucleus. It is laid out in rows and columns so that elements in the same column behave alike. That pattern comes from how each atom's electrons are arranged, and it lets chemists predict how an element will react before they test it.

In short

• Each element has its own atomic number, the count of protons in its nucleus, and the table runs from 1 (hydrogen) to 118 (oganesson).
• Columns, called groups, hold elements with the same number of outer electrons, which is why they react alike. Rows, called periods, add one shell of electrons at a time.
• Dmitri Mendeleev's 1869 table left gaps for elements nobody had found yet, and predicted their properties closely.

What is an element?

An element is a substance made of only one kind of atom. Gold, oxygen and carbon are elements; water is not, because each water molecule is two hydrogen atoms joined to one oxygen atom. What makes an atom one element and not another is the number of protons in its nucleus. Every atom with 6 protons is carbon, and every atom with 79 is gold. Atoms of one element can have different numbers of neutrons, and those versions are called isotopes, but the proton count never changes without the atom becoming a different element.

The International Union of Pure and Applied Chemistry (IUPAC) is the body that confirms new elements and keeps the official table. Its latest release, dated 4 May 2022 and still the latest in November 2026, runs to element 118. The last four were formally named on 28 November 2016: nihonium, moscovium, tennessine and oganesson, after Japan, Moscow, Tennessee and the physicist Yuri Oganessian, as IUPAC announced. Labs in Russia, Japan and the United States have been trying to make elements 119 and 120, and any claim has to be checked by IUPAC and the physicists' union, IUPAP, before it joins the table.

How do you read one square of the periodic table?

Each square holds the same few facts. Take sodium:

What's in the square	For sodium	What it tells you
Atomic number	11	11 protons, and in a neutral atom 11 electrons
Symbol	Na	From its Latin name, natrium; most symbols come from English or Latin names
Name	Sodium
Atomic weight	22.990	The average mass of its atoms, in atomic mass units, which counts protons and neutrons together
Position	Period 3, group 1	Three shells of electrons, with one electron in the outermost

Now the position does real work. Sodium's 11 electrons sit in shells of 2, 8 and 1. That lone outer electron is easy to lose, so sodium gives it away readily, which is why the metal reacts so violently with water and is never found pure in nature. Chlorine, element 17, has shells of 2, 8 and 7: one short of a full outer shell, so it grabs an electron readily. Put them together and sodium hands its electron to chlorine, and the result is sodium chloride, table salt. You can read that whole reaction from where the two elements sit in the table.

Why do elements in the same column behave alike?

The columns line up atoms with the same number of electrons in their outer shell, and the outer electrons are what chemistry is about: they are what atoms share, give or take when they bond. The rules for how electrons fill the shells come from quantum mechanics, which was worked out in the 1920s, long after the table was built.

A few groups have names worth knowing:

• Group 1, the alkali metals (lithium, sodium, potassium): one outer electron, soft, and highly reactive.
• Group 17, the halogens (fluorine, chlorine, bromine): one electron short of full, reactive, and keen to form salts with metals.
• Group 18, the noble gases (helium, neon, argon): full outer shells, so they barely react at all, which is why neon is safe in a glowing sign.

Moving along a row, each element has one more proton and one more electron than the last, and the properties change step by step from reactive metals on the left to nonmetals and noble gases on the right. The two rows set below the main table, the lanthanides and actinides, belong inside periods 6 and 7; they are printed separately so the table fits on a page.

How did Mendeleev build the table?

In 1869, the Russian chemist Dmitri Mendeleev arranged the 63 elements then known by atomic weight and noticed that similar properties came back at regular intervals, which is what "periodic" means. Others had seen parts of the pattern. What set Mendeleev apart was that he trusted it enough to leave gaps where no known element fit, and in 1871 he predicted what the missing elements would be like.

His boldest prediction was an element he called eka-silicon, one place below silicon. It was found in 1886 and named germanium. Here is his forecast against the real thing, from the record of Mendeleev's predicted elements:

Property	Mendeleev's prediction (1871)	Germanium as measured
Atomic weight	72	72.63
Density	5.5 g/cm³	5.32 g/cm³
Density of its oxide	4.7 g/cm³	4.23 g/cm³
Boiling point of its chloride	Under 100 °C	86.5 °C

Gallium, found in 1875, and scandium, whose oxide was isolated in 1879, filled two more of his gaps. A theory that predicts what nobody has yet seen, and turns out right, is the kind of evidence science values most.

Mendeleev's ordering by weight had a few awkward spots, where he had to swap elements to keep the families together. The reason came in 1913, when the English physicist Henry Moseley used X-rays to show that each element has a whole-number charge in its nucleus, its atomic number, as his biography records. Ordered by atomic number, the swaps disappear. Moseley was killed at Gallipoli in 1915, aged 27.

Where do the heaviest elements come from?

Elements 1 to 94 occur in nature, at least in traces. The rest exist only because people made them, by smashing lighter nuclei together in accelerators or in nuclear reactors. The heaviest last a fraction of a second before they break apart. That instability matters outside the lab too: very heavy nuclei like uranium and plutonium can split and release energy, which is the principle behind nuclear power and nuclear weapons. At the other end of the table, the lightest elements can fuse into heavier ones, which is how stars shine and what nuclear fusion research is trying to copy.

Check yourself

• Without looking back, explain why sodium and chlorine react so readily with each other, using only their positions in the table.
• What does an element's atomic number count, and why does it decide which element it is?
• Why was Mendeleev's prediction of germanium such strong evidence for his table?

Sources

• Periodic Table of Elements — International Union of Pure and Applied Chemistry
• IUPAC Announces the Names of the Elements 113, 115, 117, and 118 — IUPAC, 2016
• Periodic Table of Elements — PubChem, U.S. National Library of Medicine
• Mendeleev's predicted elements — Wikipedia
• Henry Moseley — Wikipedia