How the table is organised

Element 1 is hydrogen and element 118 is oganesson, named in 2016. Every number in between is filled.

A supermarket puts milk next to yoghurt and pasta next to rice, so you can find things without reading every label. The periodic table does the same job for the chemical elements. Each element sits in a fixed square, and the position of that square tells you a great deal about how the element behaves before you look up a single fact about it.

Ordered by atomic number

The elements are lined up by atomic number, which is the number of protons in the nucleus of each atom. Hydrogen has 1 proton, helium 2, lithium 3, and so on, without a single gap, up to oganesson with 118. You read the table like a page of text: left to right along a row, then down to the start of the next row. When a row ends, the next element starts a new row on the left.

Why break the line into rows at all? Because the properties of the elements repeat. Lithium, sodium and potassium are all soft, shiny metals that react with water. They turn up at regular intervals in the number sequence, so the table is cut into rows of the right length to stack them in one column. That repeating pattern gives the table its name: periodic means "repeating at intervals".

Rows, columns and blocks

The 7 rows are called periods and the 18 columns are called groups. The rows are not all the same length. Period 1 holds only 2 elements, periods 2 and 3 hold 8 each, periods 4 and 5 hold 18, and periods 6 and 7 hold 32. Those numbers are not arbitrary. They match the number of electrons that can fill each new layer of the atom, which you will meet in the next unit.

Chemists also split the table into four blocks named after the kind of orbital that is being filled: the s block (groups 1 and 2, plus helium), the p block (groups 13 to 18), the d block (groups 3 to 12) and the f block. Periods 6 and 7 each contain 14 extra f-block elements. If they were printed in place, the table would be 32 columns wide and would not fit on a page, so most tables lift the lanthanides (57 to 71) and actinides (89 to 103) out and print them as two rows underneath. The 3D model does the same, and the two grey tiles in group 3 mark where they belong.

What a single square tells you

Click any tile in the model and a card opens. The big letters are the symbol, one or two letters, the first always a capital: C for carbon, Ca for calcium, Cl for chlorine. Some symbols come from Latin names, so sodium is Na (natrium), potassium is K (kalium) and iron is Fe (ferrum). The small number in the corner is the atomic number. The number at the bottom is the standard atomic weight, the average mass of the element as it is found on Earth, measured on a scale where one twelfth of a carbon-12 atom is exactly 1.

Elements that are made only in laboratories, or that have no stable form, have no standard atomic weight. For those the model shows a mass number in square brackets, such as [97] for technetium. The weights in the model are the 2024 abridged values published by the IUPAC Commission on Isotopic Abundances and Atomic Weights. Hydrogen is 1.0080, carbon 12.011 and oxygen 15.999.