How the periodic table is organized: periods and groups

0:00The periodic table shows every known element. How is it organized?

0:06An element's atomic number is the number of protons in the nucleus of its atoms. Sodium's is 11.

0:14The elements are placed in order of atomic number: 1, 2, 3, and so on.

0:25Today, 118 elements are known, from hydrogen to oganesson.

0:31The order is not by atomic mass, and not by name.

0:37Argon atoms are heavier than potassium atoms, yet argon comes first: it has 18 protons, potassium 19.

0:47Each row is called a period. There are seven, and this row is period 2.

0:53Each new period starts a new electron shell, a layer of electrons around the nucleus: lithium has two, sodium three, potassium four.

1:03Each column is called a group. There are 18, and this column is group 1.

1:09Lithium, sodium and potassium are in group 1, and each has one electron in its outer shell.

1:17That is why elements in the same group tend to react in similar ways.

1:22In 1869, Dmitri Mendeleev grouped elements that behave alike, and left gaps for ones not yet found.

1:30He predicted their properties, and gallium, found in 1875, and germanium, found in 1886, matched closely.

1:40So elements run in order of atomic number: rows are periods, and columns are groups.