Families in columns

About this video

On the wall of almost every chemistry classroom hangs the same chart: the periodic table, a wide grid of boxes that is higher at the two ends than in the middle. Why that odd shape?

Every element has an atomic number: the number of protons in its atoms. Hydrogen is 1, helium 2, lithium 3, and so on up to 118. The periodic table puts the elements in that order, and you read it like a page of an English book: along each row from left to right, then down to the start of the next row.

The clever part is where each row stops. The rows are cut so that elements that behave alike end up in the same column, like a family. The first column (after hydrogen) holds soft, shiny metals such as lithium, sodium and potassium, which fizz when they touch water. The last column holds gases such as helium, neon and argon, which hardly react with anything; that is why helium is safe in party balloons. Know one member of a family, and you can guess how the others behave.

The table was first drawn in 1869 by the Russian chemist Dmitri Mendeleev. Where no known element fitted, he left a gap, and said it belonged to an element nobody had found yet. In 1875 one of them was found, a metal now called gallium, and it matched his description closely.

Transcript

1. 0:00Why does the periodic table have this odd shape?
2. 0:07The elements go in order of atomic number, the number of protons. Hydrogen one, helium two, and on.
3. 0:18Each row stops so that elements that behave alike sit in one column, like a family.
4. 0:28In 1869 Mendeleev left gaps for elements nobody had found. In 1875, gallium filled one.
5. 0:38Order them by protons, and the families line up.