Group VII A Elements (Halogen Family)
The word halogen means salt former. Halogens belong to group VII-A or group 7 or group 17 of the periodic table.
This group consists of the elements which are given in the following .
Electronic Configuration of Group VII-A (Halogens)
Fluorine (F) (1s2, 2s2, 2p5 )
Chlorine (Cl) (1s2, 2s2, 2p6, 3s2, 3p5)
Bromine (Br) (1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p5 )
Iodine (I) (1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p6, 5s2, 4d10, 5p5)
Astatine (At) (1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p6, 5s2, 4d10, 5p6, 6s2, 4f14, 5d10, 6p5)
(i) Their general valence shell configuration of group 7 (Halogens) is ns2, np5.
(ii) All halogens are non-metals.
(iii) They exist as discrete diatomic molecules F2, Cl2, Br2, I2, At2.
(iv) Their atomic radii increase down the group.
(v) Their electronegativities are high and decrease down the group. Fluorine has the highest electronegativity which is four.
(vi) Electron affinity is the energy released or absorbed when an electron is added to a gaseous atom or ion. It is the measure of attraction between incoming electrons and the nucleus. Higher is the attraction higher will be the electron affinity.
Trend of electron affinity
F < Cl > B r > I > At
At < I < Br < F < Cl
Element F Cl Br I At
Electron Affinity (kJ/mol) 328 349 325 295 270
The trend is not regular. Fluorine is showing exceptionally low value than chlorine. Due to its very smaller atomic size valence shell is smaller so electrons present in it repel the incoming electron. This difference between the energy released due to attraction by the nucleus and energy absorbed during repulsion by the electrons is less and therefore, the electron affinity of fluorine is less than chlorine.
(vii) Their melting and boiling points increase down the group. Thus from top to bottom, they change from gas to solid. Astatine is radioactive and its half-life is 8.3hrs.
Element State M.P B.P Color
Fluorine gas -219.6oC -188.2oC pale yellow
Chlorine gas -101.0oC -34.7oC yellowish green
Bromine liquid -7.2oC 59oC dark red (vapors reddish brown)
Iodine solid 113.7oC 184oC dark crumbly (vapors violet)
Astatine solid 300oC 380oC black (vapors dark)
(viii) Bond Enthalpy is the energy required to break one mole of chemical bonds in gaseous molecules to form gaseous atoms.
Cl2(g) → 2Cl(g) ∆H = 242 kJ/mol
F2 has abnormally low bond enthalpy than chlorine due to its smaller atomic size. Due to the smaller size distance between nuclei of two atoms will be less and hence repulsion between nuclei will be more. As a result, bond will be weaker.
(ix) Halogens are good oxidizing agents; however, their oxidizing power decreases down the group.
Their oxidizing power depends upon
(a) Dissociation energy more is dissociation energy less is oxidizing power.
(b) Electron affinity: more is electron affinity more is oxidizing power.
(c) Reduction potential: more is reduction potential more is oxidizing power.
The dominating factor is reduction potential.
Order of oxidizing power is F2 > Cl2 > Br2 > I2
Standard reduction potential (V) 2.87 1.36 1.06 0.54
Halogens take electrons from other elements and thus oxidize them.
2Na + Cl2 → 2Na+ Cl–
e.g. (i) Fluorine and chlorine can oxidize colored dyes to colorless. Thus they are used as bleaching agents.
e.g. (ii) Chlorine water oxidizes KI to iodine and the solution turns brown due to the formation of iodine.
Cl2(g) + KI(aq) → KCl(aq) + I2(s)
In this reaction, Cl2 has oxidized I– to I2. Similarly, Cl2 can oxidize Br– to Br2.
(x) Reducing power of halide ions depends upon their sizes. Larger is the size of halide ion more is its reducing power. Larger ions can easily donate an electron and thus can easily reduce other substances.
Order of reducing power is I– > Br – > Cl– > F–
Ionic radius (nm) 0.216 0.195 0.181 0.136
e.g. (i) Br – ion reduces sulphate ion of sulphuric acid to SO2.
H2SO4 + 2H+ + 2Br – → Br2 + SO2 + 2H2O
e.g. (ii) I – ion is larger than Br – ion so it is stronger reducing agent than Br – ion. It reduces sulphate ion of sulphuric acid to S–2 ion.
H2SO4 + 8H+ + 8I – → 4I2 + H2S + 2H2O
(xi) Halogens have irritating odors and they attack the skin. Bromine causes burns that heal slowly
(xii) They have high ionization energies, electron affinities and electronegativities.
(xiii) Common oxidation state for halogens is -1, but they also show +1, +3, +5, and +7 oxidation states in their compounds. However, fluorine does not show a positive oxidation state as it has the highest electronegativity.
(xiv) They form ionic compounds with group I-A and group II-A elements.
(xv) Halogens directly react with hydrogen under different conditions to produce their hydrides (Hydrogen halides).
H2 + F2 → 2HF vigorous reaction
H2 + Cl2 → 2HCl in presence of sunlight
H2 + Br2 → 2HBr in presence of sunlight
H2 + I2 ⇋ 2HI in presence of sunlight
The order of reactivity of halogens towards this reaction is
F2 > Cl2 > Br2 > I2
As the size of halogen increases, the bond energy of H – X bond decreases and thus stability of halide decreases also the polarity of the bond decreases.
- The order of stability and polarity is HF > HCl > HBr > HI
- While order of reactivity of halogen acids is HI > HBr >HCl > HF
- The acidic strength of hydrogen halides increases down the group. As the stability of halogen acids decrease down the group accordingly their acidic strength increases down the group.
Order of acid strength HI > HBr > HCl > HF
Why hydrogen fluoride is a weaker acid than hydrochloric acid?
All halogens are highly soluble in water and are stronger acids. HF is also highly soluble in water but it is a weak acid.
- Bond enthalpies of hydrogen halides decrease down the group.
As down the group size of halogen increases so bond length increases and the hold of nuclei decreases on shared electrons so bond enthalpy decreases.
Also, the polarity of the molecule decreases down the group so the bond becomes weaker and thus bond enthalpy decrease.
Hydrogen halide H – F H – Cl H – Br H – I
Bond Enthalpy (kJ/mol) 568 432 366 298
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