The elements of group 1 of the periodic table, apart from hydrogen, are called the alkali metals. There are six of them: lithium, sodium, potassium, rubidium, caesium and francium.
Each has one electron in its outermost shell, and each readily gives up that electron to become a singly charged positive ion.
Francium is radioactive and only tiny amounts of it can be obtained, so here we compare the elements from lithium to caesium.
Going down the periodic table, each element has one more electron shell than the one above, so the atoms get larger. The atomic radius grows from about 152 pm for lithium to about 265 pm for caesium.
The larger the atom, the farther the outermost electron is from the nucleus, and the more weakly it is attracted. So the elements give up their outer electron more easily the further down the group they are.
The melting points of the alkali metals fall going down the periodic table. Lithium melts at about 181 °C, but rubidium melts at about 39 °C and caesium at about 28 °C.
This is because metallic bonding, the attraction between the positive ions and the free electrons, becomes weaker as the atoms get larger. Caesium, in which this bonding is weakest, turns liquid at a temperature below that of the human body.
All the alkali metals react with water, giving off hydrogen and forming a strongly basic solution.
The reaction becomes more violent from lithium to sodium to potassium. Rubidium and caesium react explosively when put into water.
The more easily an alkali metal gives up its outer electron, the more violently it reacts with water.
All the alkali metals give a colour in the flame test. Lithium colours the flame red, sodium yellow, potassium lilac, rubidium red-violet and caesium blue-violet.
Rubidium and caesium were discovered in the 19th century by splitting the light of a flame into a spectrum. From the deep red lines and the blue lines seen in their spectra, rubidium was named after the Latin word for deep red, and caesium after the Latin word for sky blue.