Group 17- General Trends

F, Cl, Br, I, At, Ts

Occurrence

Fluorine is present as insoluble fluorides (CaF2, Na3Al6, 3Ca3 (PO4)2.CaF2 and in small quantities in soil, river water plants, bones, teeth.

Sea water contains chlorides, bromides, iodides of Na, K, Mg, Ca; but mainly NaCl

Various seaweeds contain Iodine in their system, 0.5% , or Chile saltpetre contains 0.2% of NaI

Electronic Config.- ns2np5

Atomic/Ionic Radii- increases from fluorine to iodine and they are the smallest in their respective periods due to max. effective nuclear charge.

I.E

Due to small size, very high I.E. It decreases down the group.

E.G.E

Have maximum negative EGE due to only 1 electron reqd. for stable noble gas config.

Becomes less -ve DTG.

Exception: -ve EGE OF F is smaller than Cl due to very small size leading to more interionic repulsions in the small 2p orbitals and thus less attraction towards incoming electron

EN

Very high EN. It decreases down the group.

Physical Properties

F and Cl are gases, bromine is a liquid, iodine is a solid.
MP and BP regularly increases with atomic number.
All halogens are coloured due to absorption of radiations in the visible region of the EMR spectrum.
By absorbing different quanta of radiation, they display different colours. Eg. F2- yellow, Cl2- greenish yellow, Br2- red, I2- violet.
F and Cl react with water. Br and I are sparingly soluble in water but soluble in chloroform, CCl4, CS2, and HCs to give coloured sols.
the enthalpy of dissociation for F2 is smaller than Cl2 which is an anomaly, it is due to large e--e- repulsions among lone pairs in F2 molecule as they are closer to each other as compared to Cl2.

Chemical Properties

All halogens exhibit -1 oxidation state.
Cl, Br and I also exhibit +1, +3, +5, 7
These states are observed when they combine with small, highly EN atoms like F, O, i.e. in interhalogens, oxides, oxoacids.
+4 and +6 occur in oxides and oxoacids of Cl and Br.
highly reactive. react with metals/non. to form halides. reactivity decreases DTG.
F2 is the strongest oxidising halogen, In general, a halogen oxidises halide ions of higher atomic number.

F2 + 2X– → 2F– + X2 (X = Cl, Br or I)

Cl2 + 2X– → 2Cl– + X2 (X = Br or I)

Br2 + 2I– → 2Br– + I2

Fluorine oxidises water to oxygen whereas chlorine and bromine react with water to form corresponding hydrohalic and hypohalous acids
reaction of I with water is non-spontaneous, and I- can be oxidised by O2 in acidic medium; reverse of the reaction of F2 with H20
2F2(g) + 2H2O(l) ---> 4H+ (aq) + 4F- (aq) + O2 (g)
X2 + H20 ---> HX + HOX where X= Cl or Br
4I- + 4H+ + O2----> I2 + 2H20

Anomalous Behaviour of F

due to small size, highest electronegativity, low F-F bond dissociation enthalpy, and non availability of d orbitals in valence shell.

Most of the reactions of fluorine are exothermic (due to the small and strong bond formed by it with other elements).
It forms only one oxoacid
Hydrogen fluoride is a liquid (b.p. 293 K) due to strong hydrogen bonding. Hydrogen bond is formed in HF due to small size and high electronegativity of fluorine

1. Reactivity towards Hydrogen

They all react with hydrogen to give hydrogen halides.
affinity for hydrogen decreases DTG.
hydrogen halides dissolve in water to give hydrohalic acids. Acidic strength of these acids : HF < HCl < HBr < HI
stability of halides decreases DTG due to decrease in bond (H-X) diss enthalpy :: H–F > H–Cl > H–Br > H–I.

2. Reactivity towards oxygen

most oxides formed are unstable.
F forms OF2 and O2F2; OF2 is thermally stable at 298K; these form due to high EN of F than O; these are strong fluorinating agents.
O2F2 oxidises plutonium to PuF6 and the reaction is used in removing plutonium as PuF6 from spent nuclear fuel.
Cl, Br and I form oxides with oxidation states varying from +1 to +7.
Stability of oxides formed by halogens : I > Cl > Br
Higher stability of oxides of iodine is due to greater polarisability of bond between iodine and oxygen, multiple bond formation between chlorine and oxygen takes place due to availability of d–orbitals. This leads to increase in stability. Bromine lacks both the characteristics hence stability of oxides of bromine is least.
Chlorine oxides, Cl2O, ClO2 , Cl2O6 and Cl2O7 are highly reactive oxidising agents and tend to explode ; ClO2 is used as a bleaching agent for paper pulp and textiles and in water treatment.
Br2O, BrO2 , BrO3 are the least stable halogen oxides (middle row anomaly) and exist only at low temperatures. They are very powerful oxidising agents.
I2O4 , I2O5 , I2O7 are insoluble solids and decompose on heating. I2O5 is a very good oxidising agent and is used in the estimation of carbon monoxide

3. Reactivity towards metals

Halogens react with metals to form metal halides
The ionic character of the halides decreases in the order MF > MCl > MBr > MI (M= monovalent metal)
If a metal exhibits more than one oxidation state, the halides in higher oxidation state will be more covalent than the one in lower oxidation state

4. Reactivity of halogens towards other halogens

Halogens combine amongst themselves to form a number of compounds known as interhalogens of the types XX ′ , XX3 ′ , XX5 ′ and XX7 ′ where X is a larger size halogen and X′ is smaller size halogen.

	Created by Mahek Agarwal about 5 years ago

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