Element Families and Periods

There are two ways to cut the periodic table, and both are here. Families group elements that behave alike, and they answer the question "why does this element do that?" Periods are the rows, and they answer a different question: "what happens as a shell fills?" Eleven of the first and seven of the second, eighteen hubs in total.

The distinction matters more than it sounds. A family hub is an argument about similarity — what the members share at the electron-configuration level, how the shared property changes down the column, and which member refuses to cooperate. A period hub is an argument about change: a row is a continuous walk from a metal that gives electrons away to a gas that will not take one, and the interesting thing about each row is where along that walk something unexpected happens.

The families are not all the same kind of object

It is worth knowing that "family" covers several different shapes, because the tidy colour blocks on a periodic table conceal that.

• Four are single columns — the alkali metals, the alkaline earth metals, the halogens and the noble gases. These are the classic groups, and their members really do line up vertically.
• Three are blocks — the transition metals, the lanthanides and the actinides. Each spans ten or fourteen columns, and members are related by which subshell they are filling rather than by sharing one.
• One is a diagonal band. The metalloids straddle the staircase between metals and nonmetals, and their membership is the least agreed-upon in chemistry.
• One is a region. The reactive nonmetals occupy the top-right corner rather than any column.
• One is a range of atomic numbers. The superheavy elements are everything from 104 upward, held together by how they are made rather than by where they sit.

Two of those definitions deliberately overlap. Elements 104 to 108 are counted both as transition metals and as superheavy elements, and each of those two hubs says so and hands the other the part of the story it owns. The halogens are a column within the reactive-nonmetal region, so their members appear in both, coloured by chemical character and hubbed by group. Overlap is not an error here; it is what happens when one set of elements is genuinely described by two different organising ideas.

Which one to start with

If an element's behaviour is what you are after — why it corrodes, why its compounds are that colour, why it is expensive — go to its family. The family hub carries the mechanism, and the element's own page carries the specifics.

If you are working through periodic trends, start with a period instead, and start with the third one. Period 3 is the row every syllabus builds its trends on, because its eight elements walk cleanly from metal to noble gas and its properties behave. Period 2 is the row to read next, because it is where the neat picture acquires its exceptions. The long rows are best read after both.

Three things about the set

The rows come in pairs. The opening row holds two elements because the first shell has room for two; after that, every row length occurs twice before the next one appears — two rows of eight, two of eighteen, two of thirty-two. That doubling is why the table has a wide middle and a detached foot.

Every element belongs to exactly one period and to at most one coloured family. The ten heaviest belong to period 7 and to the superheavy set, and to no family colour at all, because their chemistry has not been observed and a colour here is a claim.

Each hub's member list is computed from the element records themselves rather than maintained separately, so a hub and an element page cannot drift apart. The counts and ranges on the cards below are derived the same way.