The P-Block Elements - IV

GROUP 16 ELEMENTS

Oxidation states:

• They show -2, +2, +4, +6 oxidation states.
• Oxygen does not show +6 oxidation state due to absence of d – orbitals.
• Po does not show +6 oxidation state due to inert pair effect.
• The stability of -2 oxidation state decreases down the group due to increase in atomic size and decrease in electronegativity.
• Oxygen shows -2 oxidation state in general except in OF₂ and O₂F₂
• Thus, the stability of +6 oxidation state decreases and +4 oxidation state increases due to inert pair effect.

Ionisation enthalpy:

• Ionisation enthalpy of elements of group 16 is lower than group 15 due to half-filled p-orbitals in group 15 which is more stable.
• However, ionization enthalpy decreases down the group.

Electron gain enthalpy:

• Oxygen has less negative electron gain enthalpy than S because of small size of O.
• From S to Po electron gain enthalpy becomes less negative to Po because of increase in atomic size.

Melting and boiling point:

• It increases with increase in atomic number.
• Oxygen has much lower melting and boiling points than sulphur because oxygen is diatomic (O₂) and sulphur is octatomic (S₈).

Reactivity with hydrogen:

• All group 16 elements form hydrides.
• They possess bent shape.
• Bond angle: H₂O [373K] > H₂S [213K] < H₂Se [232K] < H₂Te [269K]

Acidic nature:

H₂O < H₂S < H₂Se < H₂Te

This is because the H-E bond length increases down the group. Therefore, the bond dissociation enthalpy decreases down the group.
Thermal stability:

H₂O < H₂S < H₂Se < H₂Te < H₂Po

This is because the H-E bond length increases down the group. Therefore, the bond dissociation enthalpy decreases down the group.
Reducing character:

H₂O < H₂S < H₂Se < H₂Te < H₂Po

This is because the H-E bond length increases down the group. Therefore, the bond dissociation enthalpy decreases down the group.
Reactivity with oxygen: EO₂ and EO3

• Reducing character of dioxides decreases down the group because oxygen has a
• strong positive field which attracts the hydroxyl group and removal of H+ becomes easy.
• Acidity also decreases down the group.
• SO₂ is a gas where as SeO₂ is solid. This is because SeO₂ has a chain polymeric structure whereas SO₂ forms discrete units.

Reactivity with halogens: EX_2, EX₄ and EX6

• The stability of halides decreases in the order F- > Cl- > Br- > I-.
• This is because E-X bond length increases with increase in size.
• Among hexa halides, fluorides are the most stable because of steric reasons.
• Dihalides are sp₃ hybridised and so, are tetrahedral in shape.
• Hexafluorides are only stable halides which are gaseous and have sp₃d₂ hybridisation and octahedral structure.
• H₂O is a liquid while H₂S is a gas. This is because strong hydrogen bonding is present in water. This is due to small size and high electronegativity of O.

Oxygen:
Preparation:

Oxides:
The compounds of oxygen and other elements are called oxides.
Types of oxides:
Acidic oxides: Non- metallic oxides are usually acidic in nature.

Basic oxides: Metallic oxides are mostly basic in nature. Basic oxides dissolve in water forming bases e.g.,

Amphoteric oxides: They show characteristics of both acidic as well as basic oxides.

Neutral oxides: These oxides are neither acidic nor basic. Example: CO, NO and N₂O
Ozone:

• Preparation:
• It is prepared by passing silent electric discharge through pure and dry oxygen 10 – 15 % oxygen is converted to ozone.

• Structure of Ozone:
• Ozone has angular structure. Both O = O bonds are of equal bond length due to resonance.

Sulphur:

• Sulphur exhibits allotropy:
• Yellow Rhombic ( a - sulphur)
• Monoclinic ( b  - sulphur)
  

• At 369 K both forms are stable. It is called transition temperature.
• Both of them have S₈ molecules.
• he ring is puckered and has a crown shape.
• Another allotrope of sulphur – cyclo S6 ring adopts a chair form.
• S₂ is formed at high temperature ( ∼1000 K).
• It is paramagnetic because of 2 unpaired electrons present in anti bonding p * orbitals like O₂.

Sulphuric acid:

Preparation:
By contact process

Exothermic reaction and therefore low temperature and high pressure are favourable.

It is dibasic acid or diprotic acid.
It is a strong dehydrating agent.
It is a moderately strong oxidizing agent.

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