What are groups and periods on the periodic table?

Groups: the 18 columns

Groups are numbered 1 to 18 from left to right. This is the numbering recommended by IUPAC, the international organization that sets chemistry naming rules. Many American textbooks also use an older system that numbers only the main groups, from 1A to 8A, so you may see group 17 called group 7A and group 18 called group 8A.

Groups 1, 2 and 13 to 18 are the main groups. Groups 3 to 12 in the middle of the table hold the transition metals.

Why elements in a group behave alike

Elements in the same main group have the same arrangement of outer electrons. Lithium ([He] 2s¹), sodium ([Ne] 3s¹) and potassium ([Ar] 4s¹) each have a single valence electron, which they lose easily to form +1 ions such as Na⁺. That is why all three are soft, shiny metals that react with water to produce hydrogen gas and a hydroxide.

The same idea works on the right side of the table. Fluorine, chlorine, bromine and iodine each have seven valence electrons (ns² np⁵) and gain one more to form −1 ions such as Cl⁻. As a result, each of them combines with sodium in the same 1:1 ratio: NaF, NaCl, NaBr and NaI.

Names of the main groups

Group 1, apart from hydrogen, holds the alkali metals: lithium, sodium, potassium, rubidium, cesium and francium. Group 2 holds the alkaline earth metals, from beryllium to radium. Groups 3 to 12 are the transition metals, which include everyday metals such as iron, copper, silver and gold.

Group 13 is the boron group and group 14 the carbon group. Group 15 is the nitrogen group, also called the pnictogens, and group 16 is the oxygen group, also called the chalcogens. Group 17 holds the halogens, such as fluorine, chlorine, bromine and iodine, and group 18 holds the noble gases, such as helium, neon, argon, krypton, xenon and radon.

Periods: the 7 rows

Periods are numbered 1 to 7 from top to bottom. The period number equals the outermost electron shell an atom uses: every element in period 3, from sodium to argon, has its valence electrons in the third shell (3s and 3p).

Periods have different lengths because each one fills a different set of subshells. Period 1 has only 2 elements (hydrogen and helium), periods 2 and 3 have 8 each, periods 4 and 5 have 18 each, and periods 6 and 7 have 32 each, counting the lanthanides and actinides printed below the main table.

What changes across a period

Moving from left to right across a period, each element has one more proton and one more electron than the one before. The extra electrons go into the same outer shell, while the growing positive charge of the nucleus pulls them in more tightly.

As a result, atoms generally get smaller, electronegativity and ionization energy rise, and the elements change from metals to nonmetals. Period 3 shows this clearly: sodium, magnesium and aluminum are metals, silicon is a metalloid, phosphorus, sulfur and chlorine are nonmetals, and argon is a noble gas. Electronegativity climbs from 0.93 for sodium to 3.16 for chlorine.

Finding an element’s group and period

You can work out an element’s position from its electron configuration. The highest shell number gives the period. For s-block elements, the group number equals the number of outer electrons; for p-block elements, add 10. Sulfur is [Ne] 3s² 3p⁴: its highest shell is 3, so it is in period 3, and its 6 outer electrons put it in group 16. Calcium is [Ar] 4s², so it is in period 4, group 2.

Helium is the one to watch. It has only 2 electrons (1s²) but sits in group 18 because, like the other noble gases, its outer shell is full.