Group 7 elements (halogens) - reactivity

Group 7 elements (halogens) were first discovered from 1776 onwards.

Some of the physical properties are shown below.

Element / Fluorine / Chlorine / Bromine / Iodine / Astatine
Boiling pointoC / -188 / -35 / 58 / 183 / ?
Melting point oC / -220 / -101 / -7 / 114 / ?
Density g/cm3 / 1.7g/l / 3.21 g/l / 7.79 g/l / 4.93 / ?
Atomic mass / 19 / 35.5 / 79.9 / 126.9 / (210)
Diameter of atom (nano-metres) / 0.142 / 0.198 / 0.228 / 0.266 / ?

As the atomic mass (and the size of the atom) of the Group 7 elements (halogens) increases the:

  • boiling point
  • melting point
  • densityall increase.

The reason why these halogens are reactive is that they all have one electron missing in their outer shell.

This means that the atoms of the halogens are not "very happy" and they will do all they can to gain one more electron such that all the electrons in the outer shells have partners.
In order of reactivity fluorine is the most reactive followed by chlorine, then bromine, iodine and astatine is the least reactive. /

Electronic configurations.

Element / Atomic number / Electrons in 1st shell / Electrons in 2nd shell / Electrons in 3rd shell / Electrons in 4th shell / Electrons in 5th shell / Electrons in 6th shell
Fluorine / 9 / 2 / 7
Chlorine / 17 / 2 / 8 / 7
Bromine / 35 / 2 / 8 / 18 / 7
Iodine / 53 / 2 / 8 / 18 / 18 / 7
Astatine / 85 / 2 / 8 / 18 / 32 / 18 / 7

Electronic configurations (diagrams) for fluorine and chlorine

are shown below.

NOTE

As the atom becomes bigger and bigger the ability to attract an electron into the outer shell becomes less and less.

So for fluorine (size 0.142 nm)

  • the outer electron shell is very close to the attracting nucleus.
  • an electron is easily attracted.

Fluorine ISvery reactive.

So for Iodine (size 0.266 nm)

  • the outer electron shell is a long way away from the attracting nucleus.
  • it is very hard to attract electrons to the outer shell..

Iodine is NOT very reactive.