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ChemistryEdu Logo Group 17 Elements | Physical and Atomic Properties#

Introduction#

  • The elements of group 17 are Fluorine(F), Chlorine(Cl), Bromine(Br), Iodine(I) and Astatine(At).
  • Group 17 elements are known as halogens.
  • Astatine is a radioactive element.

Atomic Properties of Group 17 Elements#

1. Electronic Configuration#

The general valence shell electronic configuration of group 17 elements is ns2np5.

2. Atomic and Ionic Radii#

  • The halogens have the smallest radii in their respective periods due to maximum effective nuclear charge.
  • Atomic and ionic radii increase from Fluorine to Iodine due to increasing number of quantum shells.

3. Ionization Enthalpy#

  • Group 17 elements have little tendency to lose electron. Thus, they have very high ionization enthalpy.
  • Due to increase in atomic size, ionization enthalpy decreases down the group.

4. Electron Gain Enthalpy#

  • Halogens have maximum negative electron gain enthalpy in the corresponding periods. This is due to the fact that the atoms of these elements have only one electron less than stable noble gas configurations.
  • Electron gain enthalpy becomes less negative down the group.

Exception

The electron gain enthalpy of fluorine is less negative than chlorine. It is due to small size of fluorine. As a result, there are strong interelectronic repulsions in the relatively small 2p orbitals of fluorine and thus, the incoming electron does not experience much attraction.

5. Electronegativity#

  • Group 17 elements have very high electronegativity because of their strong tendency to attract shared bond pair towards themselves.
  • Fluorine is the most electronegative element in the periodic table.
  • Electronegativity decreases down the group.

Physical Properties of Group 17 Elements#

1. Physical State#

  • Fluorine and chlorine are gases, bromine is a liquid and iodine is a solid.

2. Colour#

  • All group 17 elements are coloured. This is due to absorption of radiations in visible region which results in excitation of outer electrons to higher energy level. By absorbing different quanta of radiation, they display different colours.

3. Melting and Boiling Points#

  • The melting and boiling points of group 17 elements increase steadily down the group.

4. Enthalpy of dissociation#

  • The X-X bond dissociation enthalpy order is: \(Cl-Cl\ >\ Br-Br\ > I-I\)

Exception

The bond dissociation enthalpy of F2 is lower than Cl2 and Br2. This is due to relatively large electron-electron repulsion among the lone pairs in F2 molecule where they are much closer to each other than in case of Cl2 and Br2.

So, bond dissociation order is: \(Cl_2\ >\ Br_2\ >\ F_2\ >\ I_2\)

Questions#

Halogens have maximum negative electron gain enthalpy in the respective periods of the periodic table. Why?

Halogens have the smallest size in their respective periods and therefore have high effective nuclear charge. As a consequence, they readily accept one electron to acquire noble gas electronic configuration.

Although electron gain enthalpy of fluorine is less negative compared to chlorine, fluorine is a stronger oxidizing agent than chlorine. Why?

Fluorine is a better oxidizing agent than chlorine because fluorine has low enthalpy of dissociation of F-F bond. Also, the hydration enthalpy of F- ion is high.