Groups and periods

Lithium, sodium and potassium sit in one column, and all three have exactly one electron in their outer shell.

Drop a small piece of lithium into water and it fizzes gently. Sodium skates across the surface and may catch fire. Potassium bursts into a lilac flame almost at once. Three different metals, three versions of the same reaction, getting more violent each time. All three sit in group 1 of the table, one above the other. The column has gathered elements that behave alike, and the order inside the column tells you which one reacts most strongly.

A group shares its outer electrons

Chemistry happens at the outside of atoms. When two atoms meet, the electrons in the outermost shell, called valence electrons, are the ones that are given away, taken or shared. Elements in the same main group have the same number of valence electrons. Group 1 atoms have 1, group 2 have 2, and in the p block the count is the group number minus 10: group 13 has 3, group 17 has 7 and group 18 has 8. Helium is the exception, with a full first shell of 2.

Because the outer electrons match, the chemistry matches. Every group 1 metal forms a compound with chlorine in a one-to-one ratio: LiCl, NaCl, KCl. Every group 2 metal needs two chlorine atoms: MgCl₂, CaCl₂. Learn one member of a group well and you can make good guesses about the others. The model's legend lets you light up one family at a time so you can see where each one lives.

A period adds one shell

The period number tells you how many electron shells are in use. Sodium is in period 3, and its 11 electrons are arranged 2, 8, 1: three shells. Potassium, one row lower in period 4, has 19 electrons arranged 2, 8, 8, 1: four shells. Moving across a period, each element has one more proton and one more electron than the last, and the new electron usually goes into the same outer shell until that shell is full. Then the next period begins with a new shell.

This also explains why potassium reacts more violently than sodium. Its single outer electron is one shell further from the nucleus and is shielded by more inner electrons, so it is held more loosely and is easier to lose. Going down group 1, each step adds a shell, and each step makes the metal more reactive. Caesium, near the bottom, explodes in water.

Several groups have family names that you will meet again and again. Group 1 (without hydrogen) are the alkali metals, group 2 the alkaline earth metals, group 17 the halogens and group 18 the noble gases. Groups 3 to 12 together are the transition metals. Counting outer electrons there is less tidy, because electrons are being added to an inner d shell while the outer shell stays at one or two. That is why iron, cobalt and nickel, side by side in one period, are more alike than the members of a main group are with their horizontal neighbours.

Electron configuration on the card

The card in the model shows each element's electron configuration in short form. Sodium reads [Ne]3s¹. The part in brackets means "the same inner electrons as neon", and 3s¹ means one electron in the s orbital of the third shell. Chlorine is [Ne]3s² 3p⁵, which adds up to 7 electrons in the third shell. Every element in group 17 ends in s² p⁵, which is why they behave so much alike. You do not need to memorise these strings, but it is useful to see that the last part repeats down a column.