Write a note on the physical properties of alkali metal elements (group $1$).

(N/A) All the alkali metals are silvery-white,soft,and light metals.
Because of their large atomic size,these elements have low density,which generally increases down the group from $Li$ to $Cs$,although $K$ is lighter than $Na$.
The melting and boiling points of the alkali metals are low,indicating weak metallic bonding due to the presence of only a single valence electron.
The alkali metals and their salts impart a characteristic color to an oxidizing flame because the heat from the flame excites the outermost orbital electron to a higher energy level.
When the excited electron returns to the ground state,it emits radiation in the visible region as shown in the table below:

Metal	$Li$	$Na$	$K$	$Rb$	$Cs$
Color	Crimson red	Yellow	Violet	Red violet	Blue
$\lambda / nm$	$670.8$	$589.2$	$766.5$	$780.0$	$455.5$