What is the periodic table?

Ordered by atomic number

Every element has its own atomic number. Hydrogen has 1 proton, so its atomic number is 1; helium has 2, carbon has 6, iron has 26, and oganesson, the heaviest element named so far, has 118. Reading each row from left to right, the atomic number goes up by one from box to box, except where the lanthanides and actinides have been moved to the rows below.

A neutral atom has as many electrons as protons, so the atomic number also gives the number of electrons. Those electrons, especially the outermost ones, decide how an element reacts, so ordering by atomic number makes properties repeat at regular intervals. That repeating pattern is what “periodic” means.

Who made the periodic table?

Several chemists tried to sort the elements during the 1800s, but the Russian chemist Dmitri Mendeleev published the version that made the table famous in 1869. He arranged the 63 elements known at the time by atomic mass and grouped elements with similar properties together.

Mendeleev left gaps where his pattern said an element should exist and predicted the properties of those missing elements. Gallium (discovered in 1875), scandium (1879) and germanium (1886) later filled three of the gaps, with properties close to his predictions.

In 1913 the English physicist Henry Moseley showed that atomic number, not atomic mass, is the true basis of the order. That explained why a few pairs, such as tellurium (atomic mass 127.6) and iodine (126.9), have to be placed out of mass order to land in the right groups.

How to read an element box

Each square on the table describes one element. Most tables show at least four things: the atomic number, the one- or two-letter chemical symbol, the element’s name and its atomic mass.

Take iron as an example: atomic number 26, symbol Fe, atomic mass 55.845. The atomic mass is a weighted average of the masses of the element’s natural isotopes, so it is usually not a whole number. The same number gives the mass of one mole of the element in grams, which is why iron’s molar mass is 55.845 g/mol.

A symbol always starts with a capital letter, and a second letter, if there is one, is lowercase. Co is cobalt, while CO is carbon monoxide, a compound of carbon and oxygen.

Groups and periods

The 18 vertical columns are called groups. Elements in the same group generally have the same number of outer electrons, so they react in similar ways: the alkali metals of group 1 all react with water, and the noble gases of group 18 hardly react with anything.

The 7 horizontal rows are called periods. The period number tells you how many electron shells an atom uses; sodium, in period 3, has electrons in three shells. Moving across a period, each element has one more proton and one more electron than the one before, and the elements change step by step from metals on the left to nonmetals on the right.

Metals, nonmetals and the two rows at the bottom

A zigzag “staircase” line on the right side of the table separates metals from nonmetals. More than 80 of the elements are metals, and they fill the left side and the middle. Nonmetals such as carbon, nitrogen, oxygen, the halogens and the noble gases sit at the upper right; hydrogen is also a nonmetal, even though it is placed at the top of group 1. Metalloids such as silicon lie along the staircase and share properties of both.

The two separate rows under the main table are the lanthanides (elements 57–71) and the actinides (elements 89–103). They belong in periods 6 and 7 but are printed below so the table stays a convenient width.

Why the periodic table is useful

Once you know an element’s position, you can predict how it behaves. Sodium is in group 1, so it loses one electron to form Na⁺; chlorine is in group 17, so it gains one to form Cl⁻. One of each balances out, which is why table salt is NaCl. The same reasoning helps you predict formulas, charges and bond types without memorizing every fact.