In many classrooms hangs a big chart full of colourful boxes, each with one or two letters. It is the periodic table. Every chemical element known to science has a box in it, all 118 of them.

It is a map of every building block that stuff can be made of. But why does it have such a strange shape? Let’s find out.

One long line of elements

Start with the lightest element, hydrogen, whose atoms have 1 proton. Next comes helium with 2 protons, then lithium with 3 and beryllium with 4. Each element has one more proton than the one before. The line goes all the way to oganesson, with 118 protons.

This order is the backbone of the periodic table. The number in each box is the element’s atomic number: how many protons its atoms have.

Why is the line cut into rows?

Here comes the surprise. As you walk along the line, similar elements keep coming back:

  • Lithium (3), sodium (11) and potassium (19) are soft, shiny metals that fizz in water.
  • Helium (2), neon (10) and argon (18) are gases that almost never join with anything.

The pattern repeats again and again. So the long line is cut into rows, and each new row starts under the one before. Then similar elements end up in the same column! That is what “periodic” means: coming back again and again, like the seasons of the year.

The first 20 elements of the periodic table The first 20 elements in four rows, each box with its atomic number and symbol. Row 1: H and He. Row 2: Li, Be, B, C, N, O, F, Ne. Row 3: Na, Mg, Al, Si, P, S, Cl, Ar. Row 4: K and Ca. Numbers above the eight columns show how many outer electrons the atoms have, from 1 to 8. The alkali metals Li, Na and K share the first column. The halogens F and Cl share the seventh column. The noble gases He, Ne and Ar share the last column. outer electrons 123456781234 1 H 2 He 3 Li 4 Be 5 B 6 C 7 N 8 O 9 F 10 Ne 11 Na 12 Mg 13 Al 14 Si 15 P 16 S 17 Cl 18 Ar 19 K 20 Ca alkali metals halogens noble gases
The first 20 elements. Atoms in one column have the same number of outer electrons, so they behave alike. Helium is the exception: its only shell is full with just 2.

Rows are called periods

Each row is called a period, and there are 7 of them.

Why does a new row start where it does? It has to do with the electrons around the nucleus. They sit in layers called electron shells, and each new row begins when atoms start filling a new shell. So the atoms in row 1 have one shell, and the atoms in row 3 have three.

Columns are called groups

Each column is called a group, and there are 18 of them. Elements in the same group usually have the same number of electrons in their outer shell. These outer electrons decide how an atom behaves, so the elements of one group behave like members of one family. Some families have their own names:

  • Alkali metals fill the first column, except for hydrogen. They are metals soft enough to cut with a knife. Each atom has one outer electron and gives it away very easily. In water they fizz, sodium dashes around, and potassium even bursts into a lilac flame!
  • Halogens fill the second column from the right: fluorine, chlorine, bromine and iodine. They are eager to grab one more electron. Chlorine helps keep swimming pools clean. The name “halogen” means “salt-maker” in Greek.
  • Noble gases fill the last column: helium, neon, argon and more. Their outer shell is already full, so they hardly ever react with anything. Helium makes balloons float, and neon glows red-orange in shop signs.
Go deeper: Why the table has this shape for grown-ups and the extra curious

Each electron shell is split into smaller parts called subshells, named s, p, d and f. They hold 2, 6, 10 and 14 electrons. The table’s shape follows the order in which they fill:

  • the two tall columns on the left are the s-block
  • the six columns on the right are the p-block
  • the ten columns in the middle are the d-block, the transition metals
  • the two separate rows at the bottom are the f-block, the lanthanides and actinides

That is why the seven rows hold 2, 8, 8, 18, 18, 32 and 32 elements. The rule “same group, same number of outer electrons” works best for the s- and p-blocks. In the d-block, electrons fill an inner shell, so neighbouring elements often behave alike across a row too.

Two elements sit in slightly odd places. Hydrogen has one outer electron like the alkali metals, but it is a gas and not a metal. Helium has only 2 outer electrons, yet it sits above the noble gases, because its only shell is full, just like theirs.

Metals and nonmetals

The periodic table has one more pattern. Metals are on the left and in the middle, and nonmetals are on the right.

  • Metals are shiny and bendy, and they carry electricity and heat well. They are good conductors. Iron, copper, gold and aluminium are metals. More than three out of every four elements are metals!
  • Nonmetals are very different. Many are gases, like oxygen and nitrogen. Others are brittle solids, like sulfur. Most of them do not carry electricity well. The carbon in pencil lead is a surprise: it does carry electricity!
  • Along a zigzag line between the two sit a few in-between elements, such as silicon. Computer chips are made from silicon.
The shape of the whole periodic table All 118 elements as small coloured boxes in 7 rows and 18 columns, with two more rows of 15 boxes printed below. Metals fill most of the table: the left side, the middle, the lower right and both bottom rows. Nonmetals sit at the top right, and a zigzag of in-between elements separates them from the metals. The noble gases fill the last column. Two dashed boxes in rows 6 and 7 show where the two bottom rows really belong. 1234567 metals in-between nonmetals noble gases
The whole periodic table. Metals fill most of it, nonmetals sit at the top right, and the noble gases fill the last column. The two rows at the bottom really belong inside rows 6 and 7.

Why are two rows at the bottom?

Two rows float below the rest of the table. They really belong inside rows 6 and 7, where the dashed boxes are. But they would make the table very wide, too wide for a page. So they are printed underneath instead.

All the elements in these two rows are metals. The first row holds metals used in super-strong magnets, like the tiny ones in headphones. The second row holds uranium, the fuel of nuclear power stations.

Who made the first periodic table?

In 1869, a Russian chemist named Dmitri Mendeleev wrote each of the 63 elements known at the time on its own card. On every card, he noted how heavy the element’s atoms were and how it behaved. Then he moved the cards around, a bit like in a card game, until a pattern appeared.

His cleverest idea was to leave gaps. Where no known element fitted, he left an empty box. He said that elements must be missing there, not yet found, and he even described what they would be like.

A few years later, the missing elements turned up: gallium in 1875, scandium in 1879 and germanium in 1886. They matched his descriptions very closely. Element 101 is called mendelevium in his honour.

Go deeper: How good were Mendeleev’s guesses? for grown-ups and the extra curious
  • He called the missing element below aluminium “eka-aluminium” (eka means “one” in Sanskrit). He predicted an atomic mass of about 68 and a density of about 6.0 g/cm³. Gallium, found in 1875, has an atomic mass of 69.7 and a density of 5.91 g/cm³.
  • His “eka-silicon” became germanium. He predicted a mass of 72 and a density of 5.5 g/cm³, and germanium has 72.6 and 5.32 g/cm³.
  • Mendeleev put the elements in order of their atomic weight. In 1913, Henry Moseley measured the X-rays that elements give off and showed that the true order is by the charge of the nucleus: the atomic number. This settled pairs that seemed swapped. Tellurium, for example, is heavier than iodine, but it has fewer protons, so it comes first.
  • Mendeleev’s table had no noble gases at all. Argon was only discovered in 1894, and the noble gases then fitted neatly into a new column.

Check yourself

Sodium and potassium are in the same column. What does that tell you?

Show the answer

They behave in similar ways — Sodium and potassium both have one outer electron, so they behave alike. Both are soft metals that fizz in water.

Check yourself

Why did Mendeleev leave gaps in his table?

Show the answer

He guessed that some elements had not been found yet — He guessed that some elements were still missing and even described them. Gallium, scandium and germanium later filled the gaps.