The document discusses several properties of alkali metals. It states that the hydration enthalpy of ions, which is the heat released when ions dissolve in water, is always negative and decreases with increasing ionic size from Li+ to Cs+. It also notes that density increases down the group, though K is lighter than Na. Different alkali metals and their salts impart different colors to flames due to electron excitation and emission. Detection methods include flame tests, flame photometry, and atomic absorption spectroscopy. Chemical reactivity increases down the group as ionization enthalpy decreases.

1. •The heat energy released when new bonds are
made between the ions and water molecules is
known as the hydration enthalpy of the ion.
•The hydration enthalpy is the enthalpy change
when 1 mole of gaseous ions dissolve in
sufficient water to give an infinitely dilute
solution.
•Hydration enthalpies are always negative.
• The hydration enthalpies of alkali
metal ions decrease with increase in
ionic sizes.
• Li+ > Na+ > K+ > Rb+ > Cs+
• Li+ has maximum degree of hydration
and for this reason lithium salts are
mostly hydrated, e.g., LiCl· 2H2O

3. Physical Properties
• density increases down the group from Li to
Cs. However, potassium is lighter than sodium.

4. • The alkali metals and their salts impart
characteristic colour to an oxidizing
flame.
• This is because the heat from the flame
excites the outermost orbital electron to
a higher energy level. When the excited
electron comes back to the ground state,
there is emission of radiation in the
visible region of the spectrum

5. • Detection of Alkali metals,
i. It can be detected by the respective flame tests
ii. can be determined by flame photometry or
atomic absorption spectroscopy.
• These elements when irradiated with light, the
light energy absorbed may be sufficient to make
an atom lose electron.
This property makes caesium and potassium useful
as electrodes in photoelectric cells.

6. Chemical Properties
• The alkali metals are
highly reactive due to
their large size and low
ionization enthalpy.
• The reactivity of these
metals increases down
the group.