How to read the periodic table

The periodic table looks crowded at first, but it follows a few simple rules. Once you know them, you can tell a lot about an element just from where it sits.

1. Read one tile

Every tile shows at least two things. The atomic number is the number of protons in one atom of the element. It is what makes hydrogen (1) different from helium (2). The symbol is a one or two letter short form, such as O for oxygen or Fe for iron. Many tables also print the atomic mass, the average mass of the atoms in atomic mass units.

2. Follow the order

Elements are arranged in order of atomic number, left to right and top to bottom, like words on a page. There are no gaps in the numbering from 1 to 118.

3. Rows are periods

The seven rows are called periods. The period number tells you how many electron shells the atoms have. Sodium is in period 3, so its electrons occupy three shells. Going across a period, atoms gain one proton and one electron at each step, and the elements change from metals on the left to nonmetals on the right.

4. Columns are groups

The eighteen columns are called groups. Elements in a group have the same number of electrons in their outer shell, so they behave alike. Group 1 metals all react with water. Group 17 elements all form salts with metals. Group 18 gases almost never react.

5. Find the metals and nonmetals

Metals fill the left and the middle. Nonmetals are at the upper right, plus hydrogen at the top left. Between them runs a stair-step line of metalloids, which are partly like each.

6. The two rows at the bottom

The lanthanides and actinides belong in periods 6 and 7, after lanthanum's and actinium's positions. They are printed below the table only to stop it becoming too wide.

7. Blocks

The table is also divided into four blocks, named after the kind of electron sub-shell 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 (the two bottom rows).

Try it

Open chlorine. It is in period 3 and group 17. So it has three electron shells, seven outer electrons, and is a halogen: a reactive nonmetal that forms salts. All of that follows from its position.