{Important Points to Remember}

- Atomic radii : \(\mathrm{Li}<\mathrm{Na}<\mathrm{K}<\mathrm{Rb}<\mathrm{Cs}\)

- Ionic radii : \(\mathrm{Li}^{+}<\mathrm{Na}^{+}<\mathrm{K}^{+}<\mathrm{Rb}^{+}<\mathrm{Cs}^{+}\)

- Electronegativity : \(\mathrm{Li}>\mathrm{Na}>\mathrm{K}>\mathrm{Rb}>\mathrm{Cs}\)

- First ionization potential : \(\mathrm{Li}>\mathrm{Na}>\mathrm{K}>\mathrm{Rb}>\mathrm{Cs}\)

- Melting point \(\mathrm{Li}>\mathrm{Na}>\mathrm{K}>\mathrm{Rb}>\mathrm{Cs}\)

- Colour of the flame : Li - Red, Na - Golden, K - Violet, Rb - Red, Cs - Blue, Ca - Brick red, Sr - Blood red, Ba-Apple green \(4 \mathrm{Rb}\) and \(\mathrm{Cs}\) show photoelectric effect.

- Stability of hydrides : \(\mathrm{LiH}>\mathrm{NaH}>\mathrm{KH}>\mathrm{RbH}>\mathrm{CsH}\)

- The finely divided BaSO4 is called blanc fire and used in paints.

- Pure \(\mathrm{Ca}\left(\mathrm{H}_{2} \mathrm{PO}_{4}\right)_{2}\) is used as american baking powder.

- Melting point:

For the same alkali metal, the melting points decrease in the order:

Fluoride \(>\) chloride \(>\) bromide \(>\) iodide

This is due to the decrease in lattice enthalpies as the size of the halide ion increases (size of cation is constant).

v \(\quad\) The stability of peroxides and superoxides increases in the order

\(\mathrm{O}_{2}<\mathrm{K}_{2} \mathrm{O}_{2}<\mathrm{Rb}_{2} \mathrm{O}_{2}<\mathrm{Cs}_{2} \mathrm{O}_{2} \mathrm{NaO}_{2}<\mathrm{KO}_{2}<\mathrm{RbO}_{2}<\mathrm{CsO}_{2}\)

B) Basic strength of oxides and hydroxides

\(\mathrm{BeO}<\mathrm{MgO}<\mathrm{CaO}<\mathrm{SrO}<\mathrm{BaO}\)

\(\mathrm{Be}(\mathrm{OH})_{2}<\mathrm{Mg}(\mathrm{OH})_{2}<\mathrm{Ca}(\mathrm{OH})_{2}<\mathrm{Sr}(\mathrm{OH})_{2}<\mathrm{Ba}(\mathrm{OH})_{2}\)

D Solubility of hydroxides

\(\mathrm{Be}(\mathrm{OH})_{2}<\mathrm{Mg}(\mathrm{OH})_{2}<\mathrm{Ca}(\mathrm{OH})_{2}<\mathrm{Sr}(\mathrm{OH})_{2}<\mathrm{Ba}(\mathrm{OH})_{2}\)

D Solubility of sulphates

\(\mathrm{BeSO}_{4}>\mathrm{MgSO}_{4}>\mathrm{CaSO}_{4}>\mathrm{SrSO}_{4}>\mathrm{BaSO}_{4}\)

H Solubility of carbonates

\(\mathrm{BeCO}_{3}>\mathrm{MgCO}_{3}>\mathrm{CaCO}_{3}>\mathrm{SrCO}_{3}>\mathrm{BaCO}_{3}\)

» \(\mathrm{CuBr}\) is more covalent than \(\mathrm{NaBr}\), although \(\mathrm{Cu}^{+}\)and \(\mathrm{Na}^{+}\)have the same charge +1 , and nearly the same size, i.e., \(\mathrm{Cu}^{+}(0.96 \stackrel{0}{\mathrm{~A}})\) and \(\mathrm{Na}+(1.02 \stackrel{0}{\mathrm{~A}})\). This is due to the fact that \(\mathrm{Cu}^{+}\)ion has a pseudo noble gas configuration, i.e., having \(18 e^{-9} \mathrm{~s}\) in the outermost shell.

D Peroxide \(\left(\mathrm{O}_{2}^{2-}\right)\) of the first group are colourless and diamagnetic while superoxide \(\left(\mathrm{O}_{2}^{-}\right)\)are coloured and paramagnetic.

H Except \(\mathrm{Cs}_{2} \mathrm{O}\), all other monoxide, e.g., \(\mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}, \mathrm{K}_{2} \mathrm{O}\) and \(\mathrm{Rb}_{2} \mathrm{O}\) have antiflourite structures, \(\mathrm{Cs}_{2} \mathrm{O}\) has anti \(\mathrm{CdCl}_{2}\) layer structure.

\(\mathrm{KO}_{2}\) and \(\mathrm{Na}_{2} \mathrm{O}_{2}\) are used as a source of oxygen (or for the purification of air) in confined space such as submarines, space shuttles and in emergency breathing instrument such as oxygen masks.