The Alkali Metals
Group 1A, consisting of Li, Na, K, Rb, Cs is known as the alkali metals.
have 1 valence electron with a low ionization energy, and are all chemically very reactive.
Physically, they are all soft metals, easily cut with a knife, with low melting points and low
Li and Na are less dense than water.
Cs will melt in your hand.
Some typical chemical reactions are:
Reaction with halogens:
2M(s) + Cl
(g) → 2MCl(s)
where M represents any of the alkali metals.
is used to represent the halogens.
Reaction with water:
2M(s) + 2H
) → 2M
(aq) + 2OH
(aq) + H
These reactions can be very vigorous, sometimes even explosive.
When a small
piece of Na is added to H
O, the sodium floats on the water (it’s less dense than water), and
dances around on the surface, being propelled by the H
(g) being given off by the reaction.
The Na also turns into a ball, as the heat of the reaction melts the sodium (it has a relatively
low melting point).
Cs, the alkali metal with the lowest ionization energy, is the most reactive with
But the reason for this is not as easy as it seems.
Li, the alkali metal with the
highest ionization energy, is actually more easily oxidized to its aqueous ion than Cs, due
to the stronger hydration bond between the small lithium ion and water.
The low melting
point of Cs makes it react much faster with water than lithium due to a higher surface area
of contact with the water.
Lithium, the smallest alkali metal, reacts directly with nitrogen to form nitrides, as
does magnesium, the alkaline earth metal diagonally below it.
6Li + N
3Mg + N
Similarities in properties between an element and one below it diagonally to the
right, are called diagonal relationships.