Heavy water is toxic when taken in large quantities. Heavy water is not radioactive. The deuterium in it is stable; it does not decay. Nobody will be in danger at all from radiation. It is heavier than plain water. D₂O performs little different from H₂O in chemical reactions. One has to drink a lot of D₂O to kill him.

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
\(\mathrm{CaH}_{2}+2 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_{2}+2 \mathrm{H}_{2}\)

(i) The order : NH₃ < H₂O < HF
(ii) Strength of H bond depends upon the atomic size and electronegativity of the other atom to which H atom is covalently bonded. 
Since electronegativity of F > O > N, the strength of H-bond is in the order H-F......H > H - O......H > H-N......H.
H-bonds are much weaker than covalent bonds.

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.
\(\mathrm{H}_{3} \mathrm{PO}_{2}+\mathrm{D}_{2} \mathrm{O} \rightarrow \mathrm{H}_{2} \mathrm{DPO}_{2}+\mathrm{HDO}\)

1. Heavy water is widely used as moderator in nuclear reactors as it can lower the energies of fast neutrons
2. It is commonly used as a tracer to study organic reaction mechanisms and mechanism of metabolic reactions
3. It is also used as a coolant in nuclear reactors as it absorbs the heat generated.

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)
\(2 \mathrm{KMnO}_{4}+3 \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow 2 \mathrm{MnO}_{2}+2 \mathrm{KOH}+2 \mathrm{H}_{2} \mathrm{O}+3 \mathrm{O}_{2}\)

(i) 2KMnO₄ + 3H₂O₂ ➝ 2MnO₂ + 2KOH + 2H₂O + 3O₂ (Redox reaction)
(ii) CrCl₃ + 6H₂O ➝ [Cr Cl₂ (H₂O)CI. 2H₂O [Hydration reaction]
(iii) CaO + H₂O ➝ Ca(OH)₂ (Hydration reaction)

(i) By treatment with catalyst like Pt or Fe.
(ii) By passing an electric discharge
(iii) By heating to 800°C or more.
(iv) By mixing with paramagnetic molecules like O₂,NO,NO₂·
(v) By mixing with nascent hydrogen or atomic hydrogen.