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 point oC
-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
 density
all 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 2nd
shell
7
Electrons
in 3rd
shell
9
Electrons
in 1st
shell
2
Fluorine
Electrons
in 4th
shell
Electrons
in 5th
shell
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
Electrons
in 6th
shell
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 IS very 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.