What are the physical properties of alkali metals?

Alkali metals are soft, shiny, highly reactive, and have low melting and boiling points.

Alkali metals, which include lithium, sodium, potassium, rubidium, caesium, and francium, are found in Group 1 of the Periodic Table. They are characterised by their softness, which is so pronounced that some of them can be cut with a knife. This softness is due to the weak metallic bonding in their structures, which also contributes to their low densities.

These metals have a shiny appearance when freshly cut, but they tarnish rapidly in air due to their high reactivity. This reactivity is a result of their single electron in the outermost energy level, which they are eager to lose in order to achieve a stable electron configuration. This makes them highly reactive, especially with water and oxygen. For example, sodium reacts vigorously with water to produce sodium hydroxide and hydrogen gas.

Alkali metals also have low melting and boiling points, which decrease down the group. This is because the strength of the metallic bonds decreases as the size of the atoms increases. For instance, lithium, the smallest alkali metal, has a melting point of 180.5 degrees Celsius, while francium, the largest, melts at a mere 27 degrees Celsius.

Furthermore, alkali metals are good conductors of heat and electricity. This is due to the 'sea of electrons' in their metallic structure, which allows for the easy flow of electric current. They also have distinctive flame colours when heated, which is a result of the energy absorbed by the electrons being released as light. For example, sodium gives a yellow flame, while potassium gives a lilac flame.