The Periodic Table

One chart organizes every known element and predicts how each behaves, which is frankly rude of it to be so tidy.

By Greg Goodwin · Published November 4, 2026

Cheat Sheet

Size: 118 elements, with the seventh row completed when IUPAC approved names for elements 113, 115, 117, and 118 on 28 November 2016.
Layout: 18 groups (columns) and 7 periods (rows), with lanthanides (57 to 71) and actinides (89 to 103) shown as separate rows below.
Newest names: nihonium (Nh, 113), moscovium (Mc, 115), tennessine (Ts, 117), and oganesson (Og, 118).
Naming rule: new elements honor a place or a scientist; endings are -ium for groups 1 to 16, -ine for group 17, and -on for group 18.
Founding year: Mendeleev proposed the periodic law in 1869, and Lothar Meyer independently reached a similar conclusion.
Not fixed: IUPAC revised the standard atomic weights of gadolinium, lutetium, and zirconium in 2024; zirconium went from 91.224 to 91.222.

The 60-Second Version

The periodic table is a map of every known chemical element, arranged by atomic number, the count of protons in each atom's nucleus. What makes it more than a list is the pattern: when elements are lined up in order, their properties repeat at regular intervals, so elements in the same column behave alike. Sodium and potassium, for example, sit in the same column and are both soft, reactive metals, while helium and neon sit in another and barely react at all. That regularity means the table works like a cheat sheet, letting chemists guess how an element will bond, what it will form, and where it sits relative to its neighbors without memorizing each one. The table has 118 elements, filled in over about a century and a half of discovery and laboratory creation, and chemists still revise parts of it as measurements improve.

The Long Version

Before Mendeleev

By the early nineteenth century, chemists knew a growing pile of elements and had no good way to organize them. According to Britannica, J.W. Döbereiner showed in 1817 that the combining weight of strontium lies midway between those of calcium and barium, and later found other such triads, including chlorine, bromine, and iodine. In 1862 A.-E.-B. de Chancourtois plotted atomic weights on a cylinder, and in 1864 J.A.R. Newlands proposed his law of octaves, named by analogy with the seven intervals of the musical scale. Each attempt caught part of a real pattern, but none became the standard.

Mendeleev's Bet

In 1869, after an extensive comparison of the properties and atomic weights of the elements, Dmitri Mendeleev proposed the periodic law: elements arranged by atomic weight show a periodic change of properties. Lothar Meyer had independently reached a similar conclusion. Mendeleev's famous move was to leave gaps and predict the elements that would fill them. Sources credit his predicted eka-aluminium with matching gallium, discovered in 1875, and eka-silicon with matching germanium, discovered in 1886. Seeing predicted properties turn up in newly found elements was the strongest argument the table had.

Atomic Number Settles the Order

Mendeleev ordered the elements by atomic weight, and that occasionally clashed with chemical behavior. Britannica notes that it was not actually recognized until the second decade of the twentieth century that the order is that of the atomic numbers, the whole numbers equal to the positive charge of each nucleus. Henry Moseley's X-ray work, usually dated to 1913, helped clarify the point. Once the table was understood as ordered by proton count, the pattern had an explanation in the electron structure of atoms, and the gaps that remained could be counted exactly.

Reading the Grid

The modern table has 18 groups, the vertical columns, and seven periods, the horizontal rows. Elements in a group share chemical traits because they have similar outer-electron arrangements. Two rows of elements are normally pulled out and shown below the main body to keep the table a manageable width: the lanthanides, elements 57 through 71, and the actinides, elements 89 through 103. Hydrogen sits at the top left as element 1, helium as element 2. The table works as a lookup for a lot of practical chemistry, from guessing whether an element is a metal to predicting what charge its ions will carry.

The Seventh Row and Beyond

On 28 November 2016, IUPAC approved the names nihonium (Nh), moscovium (Mc), tennessine (Ts), and oganesson (Og) for elements 113, 115, 117, and 118. The president of IUPAC's Inorganic Chemistry Division remarked that the table was now completed down to the seventh row. Oganesson honors Yuri Oganessian, born in 1933. The table is not frozen: researchers are searching for elements beyond the seventh row. At Berkeley Lab, a team made two atoms of livermorium (element 116) with a titanium-50 beam in 2024, planning to apply the method toward element 120. As far as we can tell, no element 119 or 120 has been confirmed yet. Standard atomic weights also get updated as measurements improve.

Why People Care

The table is the shared vocabulary of chemistry, physics, materials, and medicine, and it shows up in school, lab, and pub-quiz alike. Knowing how to read it lets you guess at an element without looking anything up.

Glossary

Atomic number: The number of protons in an atom's nucleus, which defines the element and sets its position on the table.
Group: A vertical column of elements that share similar chemical behavior.
Period: A horizontal row of the table; there are seven of them.
Periodic law: The observation that element properties repeat in a regular pattern when elements are ordered by atomic number.
Superheavy element: A very heavy element, made one atom at a time in particle accelerators, at the far end of the table.

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