General Properties of Group 17 Elements and
their Compounds
1. F Cl, Br, I and At are the elements of group
17. These are collectively known as halogens. At (astatine) is a radioactive element. Their general valence shell configuration is $ns^2np^5$.
2. Halogens have smallest atomic radii in their respective periods due to maximum effective nuclear charge. Atomic and ionic radii increases from fluorine to iodine.
3. Halogens have high ionisation enthalpy because they have very little tendency to lose electron.
4. Halogens have maximum negative electron gain enthalpy in the corresponding periods because they have only one electron less than stable noble gas configuration. On moving down the group, it becomes less negative. However, negative electron gain enthalpy of fluorine is less than that of chlorine. It is due to small size of fluorine atom and high electron density. As a result of this, there are strong interelectronic repulsions and the incoming electron (in 2p- orbital) does not experience much attraction.
5. Halogens are coloured due to absorption of radiations in visible region which results in excitation of valence shell electrons to higher energy level.
6. The decreasing order of bond dissociation enthalpies is $Cl—Cl>Br—Br>F—F>I—I.$ Smaller dissociation enthalpy of $F_2$ in comparison to $Cl_2$ shows that there is relatively large electron-electron repulsion between the lone pairs in $F_2$ molecule due to its small size.
7. All halogens exhibit $-1$ oxidation state. However, Cl, Br, I exhibits $+1, +3, +5$ and $+7$ oxidation states also. F exhibits only $-1$ oxidation state because it has no vacant d-orbitals in its valence shell.
17. These are collectively known as halogens. At (astatine) is a radioactive element. Their general valence shell configuration is $ns^2np^5$.
2. Halogens have smallest atomic radii in their respective periods due to maximum effective nuclear charge. Atomic and ionic radii increases from fluorine to iodine.
3. Halogens have high ionisation enthalpy because they have very little tendency to lose electron.
4. Halogens have maximum negative electron gain enthalpy in the corresponding periods because they have only one electron less than stable noble gas configuration. On moving down the group, it becomes less negative. However, negative electron gain enthalpy of fluorine is less than that of chlorine. It is due to small size of fluorine atom and high electron density. As a result of this, there are strong interelectronic repulsions and the incoming electron (in 2p- orbital) does not experience much attraction.
5. Halogens are coloured due to absorption of radiations in visible region which results in excitation of valence shell electrons to higher energy level.
6. The decreasing order of bond dissociation enthalpies is $Cl—Cl>Br—Br>F—F>I—I.$ Smaller dissociation enthalpy of $F_2$ in comparison to $Cl_2$ shows that there is relatively large electron-electron repulsion between the lone pairs in $F_2$ molecule due to its small size.
7. All halogens exhibit $-1$ oxidation state. However, Cl, Br, I exhibits $+1, +3, +5$ and $+7$ oxidation states also. F exhibits only $-1$ oxidation state because it has no vacant d-orbitals in its valence shell.
8. Halogens are strong oxidising agent, it is due to easy acceptance of an electron. $F_2$ is the strongest oxidising halogen and it oxidises other halide ions in solution or even in solid phase.
Their boiling points also increase from $HCl$ to $HI$. High boiling point of $HF$ is due to the intermolecular H-bonding.
10. $OF_2$ and $O_2F_2$ both are strong fluorinating agents and are called oxygen fluorides. $O_2F_2$ is used in removing $Pu$ as $PuF_2$ from spent nuclear fuel. Cl, Br and I form oxides in which halogens exist as $+1$
to $+7$ states.9. Halogens form $HX$ type hydrogen halides with hydrogen. Thermal stability and bond dissociation enthalpy decreases down the group from $HF$ to $HI$ but acidic strength increases from $HF$ to $HI$.
10. $OF_2$ and $O_2F_2$ both are strong fluorinating agents and are called oxygen fluorides. $O_2F_2$ is used in removing $Pu$ as $PuF_2$ from spent nuclear fuel. Cl, Br and I form oxides in which halogens exist as $+1$
11. Chlorine is one of the most important member of this family. Chlorine is a greenish yellow gas with pungent and suffocating odour. It is manufactured by Deacon’s process or by the electrolysis of brine (liberated at anode). It is a powerful bleaching agent and bleaching action is due to oxidation (in the presence of moisture). It is used in sterilising drinking water and in the preparation of poisonous gases such as phosgene $(COCI_2)$, tear gas $(CCI_3NO_2)$, mustard gas $(ClCH_2CH_2SCH_2CH_2Cl)$, etc.
12. Aqueous solution of hydrogen chloride is hydrochloric acid. A mixture of conc. $HCl$ and conc. $HNO_3$ (3:1 ratio) is known as aqua-regia which is used to dissolve noble metals such as Au, Pt, etc. When conc. $HCl$ reacts with finely powdered iron, ferrous chloride is formed. Liberation of $H_2$ prevents the formation of ferric chloride. Oxo-acids of halogen are important class of compound.
13. Fluorine forms only one oxoacid, i.e. $HOF$ due to high electronegativity and small size of F. The other halogens form several oxoacids such as $HOX, \ HOXO, \ HOXO_2 \ HOXO_3$.
(i) Order of acidic strength of oxoacids of the same halogen is
$HClO_4>HClO_3>HClO_2>HClO>$
(ii) Oxidising power of oxoacids of chlorine is
$HClO_4<HClO_3<HClO_2<HClO$
14. Halogens combine amongst themselves to form a number of compounds known as interhalogen compounds of the types $XX’, \ XX '_3,XX'_5$ and $XX' _7$ (here, $X$ is larger size halogen and X’ is smaller size halogen). These are more reactive than halogens except fluorine. On the basis of VSEPR theory, $XX' _3$, compounds have the bent T-shape, $XX' _5$ compounds have square pyramidal and $XX' _7 $ has pentagonal bipyramidal structure.
Interhalogen compounds can be used as non-aqueous solvents, fluorinating agents etc. $ClF_3$ and $BrF_3$ are used for the production of $UF_6$ in the enrichment of $^{235}U$.
Important Reactions of Halogen Compounds
Important Reactions of Halogen Compounds
For hydrochloric acid $(HCl)$
Interhalogen compounds
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