PREPARED BY  MR. AMULYA KUMAR SAMAL  PGT(CHEMISTRY)  KENDRIYA VIDYALAYA,KHANAPARA  GUWAHATI  PHONE NO   MR. AMULYA KUMAR SAMAL  PGT(CHEMISTRY)  KENDRIYA VIDYALAYA,KHANAPARA  GUWAHATI  PHONE NO  Group 16 Elements

OBJECTIVES:- After studying this unit, the students will be able to : 1.appreciate general trends in the chemistry of elements of groups 16 2.describe the properties of group Give answer the reasoning question. After studying this unit, the students will be able to : 1.appreciate general trends in the chemistry of elements of groups 16 2.describe the properties of group Give answer the reasoning question.

The p-block elements  The p-block elements are placed in groups 13 to 18 of the periodic table.  Their valence shell electronic configuration is ns 2 np 1–6 (except He which has 1s 2 configuration)

Group 16 Elements OOxygen(O) PPhosphorus(S) SSelenium(Se) TTellurium(Te) PPolonium(Po) Metalloids Typical Metal Non Metals

GROUP-16

SymbolAtomic numberElectronic configuration O 8 [He]2s 2 2p 4 S16 [Ne] 3s 2 3p 4 Se34 [Ar]3d 10 4s 2 4p 4 Te52 [Kr]4d 10 5s 2 5p 4 Po84 [Xe]4f 14 5d 10 6s 2 6p 4

Trends in properties  Electronic Configuration The s orbital in these elements is completely filled and p orbitals are four electrons. The valence shell electronic configuration of these elements is ns 2 np 4.

 Atomic and Ionic Radii O S Se Te Po considerable increase small increase due to the presence of completely filled d and/or f orbitals in heavier members.sss

 Ionisation Enthalpy Δ i H O S Se G-15 Po Δ i H Decreases. Te G-16 Δ i H decreases Because of the extra stable half-filled p orbitals electronic configuration of group-15 Due to gradual increase in atomic size Δ iH 1 < Δ iH 2 < Δ iH 3

 Electronegativity O S Se Po E.N Decreases. Te Due to gradual increase in atomic size

Physical Properties Oxygen and sulphur are non-metals,selenium and tellurium metalloids, whereas polonium is a metal The melting and boiling points increase with an increase in atomic number down the group. The large difference between the melting and boiling points of oxygen and sulphur may be explained on the basis of their atomicity; oxygen exists as diatomic molecule (O2) whereas sulphur exists as polyatomic molecule (S8) The melting and boiling points increase with an increase in atomic number down the group. The large difference between the melting and boiling points of oxygen and sulphur may be explained on the basis of their atomicity; oxygen exists as diatomic molecule (O2) whereas sulphur exists as polyatomic molecule (S8)

Chemical Properties Oxidation states and trends in chemical reactivity The common o.s of these elements are –2, +4 and 6. Group-15 Tendency to exhibit –2 o.s decreases due to increase in size and metallic character. The stability of +6 o.s decreases and that of +4 state increases (due to inert pair effect) down the group.

The stability of + 6 oxidation state decreases down the group and stability of + 4 oxidation state increase (inert pair effect). Bonding in +4 and +6 oxidation states are primarily covalent. Since electronegativity of oxygen is very high, it shows only negative oxidation state as –2 exceptin the case of OF2 where its oxidation state is + 2. Other elements of the group exhibit + 2, + 4, + 6 oxidation states but + 4 and + 6 are more common. Sulphur,selenium and tellurium usually show + 4 oxidation state in their compounds with oxygen and + 6 with fluorine.

Anomalous properties of oxygen Reasons: 1.Its small size 2.high electronegativity 3.high ionisation enthalpy 4.non-availability of d orbitals.

Some anomalous properties of oxygen 1.State: oxygen is a gas whereas the others elements are solids and have allotropic forms. 2.Atomicity: oxygen exists as diatomic molecule (O2) whereas sulphur exists as polyatomic molecule (S8).

3.Ability to form pπ -pπ multiple bonds: oxygen has unique ability to form pπ -pπ multiple bonds with itself and with other elements having small size and high electronegativity (e.g., C, O). Heavier elements of this group do not form pπ -pπ bonds as their atomic orbitals are so large and diffuse that they cannot have effective overlapping.

4) Reactivity towards hydrogen: Their acidic character increases from H 2 O to H 2 Te. The increase in acidic character can be explained in terms of decrease in bond (H–E) dissociation enthalpy down the group.

H 2 O < H 2 S < H 2 Se < H 2 Te (Acid Strength and Reducing Character) --- BDE H 2 Te < H 2 Se < H 2 S < H 2 O (Thermal Stability) ---BDE H 2 O < H 2 Te < H 2 Se < H 2 S (Volatility) --- H-Bond and Van der waal’s force H 2 S < H 2 Se < H 2 Te < H 2 O (Boiling Point) -- H-Bond and Vander waal’s force

Self Assesment EXAMINATION DECODED Important question with answer Q.1# Thermal stability of water is much higher than that of H 2 S. Ans:- Due to high bond dissociation enthalpy of O—H than S—H bond. (2)Elements of group 16 generally show lower value of first ionization enthalpy compared to the corresponding periods of group 15. Ans: Due to extra stable half-filled p orbitals electronic configurations of Group 15 elements, larger amount of energy is required to remove electrons compared to Group 16 elements. (3) Tendency to show –2 oxidation states diminishes from Sulphur to polonium in group 16. Ans:The outer electronic configuration of group 16 elements is ns 2 np 4. These elements therefore have the tendency to gain two electrons to complete octet. Since elctronegativity and I.E. decrease on going down the group, tendency to show –2 oxidation state diminishes.

(4) Oxygen generally exhibit oxidation state of –2 only whereas other members of the family exhibit +2, +4, +6 oxidation states also. Ans:Oxygen is a electronegative element thus exhibit oxidation state of –2..Other members of the family have d orbitals and therefore, can expand their octets and show + 2, + 4, + 6 oxidation states also. (5)There is large difference between the melting point of Oxygen & Sulphur. Ans:The large difference between the melting and boiling points of oxygen and sulphur may be explained on the basis of their atomicity; oxygen exists as diatomic molecule (O 2 ) whereas sulphur exists as polyatomic molecule (S 8 ).

(6) SF 6 is known but SH 6 is not known. (6) SF 6 is known but SH 6 is not known. Ans: Because of very high ectronegativity if fluorine, sulphur exhibits its maximum oxidation stateof +6 in SF 6. SH 6 is not formed because hydrogen is feebly electronegative. (7) SF 6 is not easily hydrolysed. Ans: Since sulphur is hexa-covalent it cannot expand its covalency.. Sulphur hexafluoride, SF 6 is exceptionally stable for steric reasons.