a) KH, CaH₂, ------ GaH₃, GeH₄, AsH₃, H₂Se, HBr
b) Trend in the formula:

c) First 2 members: KH & CaH₂. KH is alkali hydride (Grey powder) CaH₂ is an alkaline earth hydride.
KH reacts vigorously with water liberating H₂ gas.
\(\mathrm{LiH}_{(\mathrm{s})}+\mathrm{H}_{2} \mathrm{O}_{(h)} \rightarrow \mathrm{Li}(\mathrm{OH})_{a q}+\mathrm{H}_{2}\)
CaH₂ reacts with water liberating H₂ gas
\(\mathrm{CaH}_{2}+2 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_{2}+2 \mathrm{H}_{2}\)

1. Deuterium is used in heavy water moderated fission reactors, usually as liquid D₂O to slow neutrons
2. To prepare D₂O (heavy water)
3. To prepare deuterated halides
4. To prepare compounds like deutero methane, deutero ammonia etc..
5. To manufacture deuterium lamps with quartz or UV glass bulb.

When compounds containing hydrogen are treated with D₂O, hydrogen undergoes an exchange for deuterium
\( 2 \mathrm{NaOH}+\mathrm{D}_{2} \mathrm{O} \rightarrow 2 \mathrm{NaOD}+\mathrm{HOD} \)
\(\mathrm{HCl}+\mathrm{D}_{2} \mathrm{O} \rightarrow \mathrm{DCl}+\mathrm{HOD} \)
\(\mathrm{NH}_{4} \mathrm{Cl}+4 \mathrm{D}_{2} \mathrm{O} \rightarrow \mathrm{ND}_{4} \mathrm{Cl}+4 \mathrm{HOD}\)
These exchange reactions are useful in determining the number of ionic hydrogens present in a given compound.
For example, when D₂O is treated with of hypo-phosphorus acid only one hydrogen atom is exchanged with deuterium. It indicates that, it is a monobasic acid.
\(\mathrm{H}_{3} \mathrm{PO}_{2}+\mathrm{D}_{2} \mathrm{O} \rightarrow \mathrm{H}_{2} \mathrm{DPO}_{2}+\mathrm{HDO}\)

Elements having same atomic number but different mass numbers are called isotopes. These are variant, of an element which differ in neutron numbers, which contain equal number of protons. They differ in relative atomic mass but not in chemical properties.
The isotopes of hydrogen are:
1. Protium \(\left(\mathrm{H}^{1} \text { or } \mathrm{H}\right)\) [Predominant form]
2. Deuterium (or) Heavy hydrogen \(\left({ }_{1} \mathrm{H}^{2} \text { or }{ }_{1} \mathrm{D}^{2}\right)\)
3. Tritium \(\left({ }_{1} \mathrm{H}^{3} \text { or }{ }_{1} \mathrm{T}^{3}\right)\)

a) Oxidation is performed in acidic medium:
Eg : H₂O₂ Oxidises FeSO₄ to Fe (SO₄)₃
H₂O₂ + 2H⁺+ 2e^- ➝ 2H₂O (E⁰ = +1.77 V)
2FeSO₄ + H₂SO₄ + H₂O₂ ➝ Fe₂(SO₄)₃ + 2H₂O
b) Reduction is performed in basic medium:
Eg : H₂O₂ reduces KMnO₄ to MnO₂
HO₂^- + OH^- ➝ H₂O + 2e^- (E⁰ = +0.08 V)
\(2 \mathrm{KMnO}_{4}+3 \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow 2 \mathrm{MnO}_{2}+2 \mathrm{KOH}+2 \mathrm{H}_{2} \mathrm{O}+3 \mathrm{O}_{2}\)