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ChemistryEdu Logo Group 17 Elements | Chemical Properties#

In this article, we will discuss chemical properties of group 17 elements like oxidation state, reactivity and oxidizing nature.

  • All the halogens exhibit -1 oxidation state. However, chlorine, bromine and iodine can also exhibit +1, +3, +5 and +7 oxidation states.
  • The oxidation states of +4 and +6 occur in the oxides and oxoacids of chlorine and bromine.
  • The fluorine atom has no d-orbital in its valence shell and therefore cannot expand its octet. Being the most electronegative element, it exhibits -1 oxidation state.

Reactivity:#

  • All the halogens are highly reactive. They react with metals and non-metals to form halides.
  • The reactivity of halogens decreases down the group.

Oxidizing Nature:#

  • The ready acceptance of an electron is the reason for strong oxidizing nature of halogens.

  • F2 is the strongest halogen and it oxidizes other halide ions in solution or even in the solid phase.

  • In general, a halogen oxidizes halide ions of higher atomic number.

\[F_2 + 2X^- → 2F^- + X_2\]
\[[X = Cl,\ Br,\ I]\]
\[Cl_2 + 2X^- → 2Cl^- + X_2\]
\[[X = Br\ or\ I]\]
\[Br_2 + 2I^- → 2Br^- + I_2\]
  • The relative oxidizing power of halogens can also be illustrated by their reactions with water. Fluorine oxidizes water to oxygen whereas chlorine and bromine react with water to form corresponding hydrohalic and hydrohalous acids. The reaction of iodine with water is non-spontaneous. In fact, I- can be oxidized by oxygen in acidic medium, which just the reverse of the reaction observed with fluorine.
\[2F_{2(g)} + 2H_2O_{(l)} → 4H^+_{(aq)} + 4F^-_{(aq)} + O_{2(g)}\]
\[X_{2(g)} + H_2O_{(l)} → HX_{(aq)} + HOX_{(aq)}\]
\[[X = Cl\ or\ Br]\]
\[4I^-_{(aq)} + 4H^+_{(aq)} + O_{2(g)} → 2I_{2(s)} + 2H_2O_{(l)}\]

2. Anomalous Properties/Behaviour of Fluorine#

The anomalous behaviour of fluorine is due to its small size, highest electronegativity, low F-F bond dissociation enthalpy amd non-availability of d-orbitals in valence shell.

Some anomalous properties of fluorine are:

  • Ionization enthalpy, electronegativity and electrode potential are all higher for fluorine than expected.
  • Ionic and covalent radii, melting point, boiling point, enthalpy of bond dissociation and electron gain enthalpy are lower than expected.
  • It forms only one oxoacid (HOF) while othe halogens form a number of oxoacids.
  • Hydrogen fluoride is a liquid due to strong hydrogen bonding. Other hydrogen halides are gases.
  • Most of the reactions of fluorine are exothermic due to small and strong bond formed by it with other elements.

3. Reactivity towards hydrogen#

  • Halogens react with hydrogen to give hydrogen halides but affinity for hydrogen decreases from fluorine to iodine.
  • Acidic strength of hydrogen halides:
\[HF\ <\ HCl\ <\ HBr\ <\ HI\]
  • Bond dissociation enthalpy of hydrogen halides:
\[HF\ >\ HCl\ >\ HBr\ >\ HI\]
  • Stability of hydrogen halides:
\[HF\ >\ HCl\ >\ HBr\ >\ HI\]

Stability decreases due to decrease in bond dissociation enthalpy down the group.

4. Reactivity towards oxygen#

  • Fluorine forms two oxides: OF2 and O2F2.

Stability

\[OF_2 > O_2F_2\]

Both are fluorinating agents.

Example: O2F2 oxidizes plutonium to PuF6.

  • The higher oxides of halogens tend to be more stable than the lower ones.
\[Stability ∝ O.S\ of\ halogen\]

  • Chlorine Oxides: Cl2O, ClO2, Cl2O6, Cl2O7. The chlorine oxides are highly reactive oxidizing agents. ClO2 is used as bleaching agent.

  • Bromine Oxides: Br2O, BrO2, BrO3. Bromine oxides are very powerful oxidizing agents.

  • Iodine Oxides: I2O4, I2O5, I2O7. Iodine oxides aee insoluble solids. I2O5 is a very good oxidizing agent and used in the estimation of carbon monoxide.

5. Reactivity towards metals#

  • Halogens react with metals to form metal halides.
\[Mg_{(s)} + Br_{2(l)} → MgBr_{2(s)}\]
  • Ionic Character: MF > MCl > MBr > MI [M = Monovalent metal]
  • If a metal exhibits more than one oxidation state, halides in higher oxidation state is more covalent than halides in lower oxidation state.
\[Covalent\ character ∝ O.S\ of\ metal\]

Example: SnCl4, PbCl4, SbCl5, UF6 are more covalent than SnCl2, PbCl2, SbCl3 and UF4 respectively.

6. Reactivity of halogens towards other halogens#

  • Halogens combine amongst themselves to form a number of compounds known as interhalogens of the type XX', XX'3, XX'5, XX'7, where X is a large size halogen and X' is a small size halogen.

Questions#

Fluorine exhibits only -1 oxidation state whereas other halogens exhibit +1, +3, +5 and +7 oxidation states. Explain.

Fluorine is the most electronegative element and has absence of d-orbital. So, it cannot exhibit any positive oxidation state. Other halogens have d-orbitals and therefore can expand their octets and show +1, +3, +5 and +7 oxidation states also.

Considering the parameters such as bond dissociation enthalpy, electron gain enthalpy and hydration enthalpy, compare the oxidizing power of F2 and Cl2.

Bond dissociation enthalpy: F2 < Cl2, Electron gain enthalpy: F2 < Cl2, Hydration enthalpy: F2 > Cl2
The large value of hydration enthalpy for fluorine compensates the other two effects. Thus, fluorine is the stronger oxidizing agent than chlorine.

Why is ICl more reactive than I2?

I-Cl bond in ICl is weaker than I-I bond in I2. That is why ICl is more reactive than I2.

Give two examples to show anomalous behaviour of fluorine.

The two examples to show anomalous behaviour of fluorine are:

  • Fluorine forms only one oxoacid (HOF) while other halogens form a number of oxoacids.
  • HF is liquid due to strong hydrogen bonding while other halogen halides are gases.