Download GROUP VII ELEMENTS

GROUP VIIA ELEMENTS
(HALOGENS)
Objectives
• At the end of this session students have to
know the following;a.) what are group VIIA elements?
b.) what physical properties of group VIIA
elements?
c.) what are the chemical properties of group
VIIA elements?
d.) what are the pharmaceutical importance of
group VIIA elements?
Introduction
• Group VIIA elements are also called Halogens
which means “salt former”
• These are elements which have seven(7)
electrons in their outer most shell.
• They include fluorine(F), chlorine(Cl),
bromine(Br), iodine(I), astatine(At).
Group VII elements and their configuration
Element
Fluorine(F)
Chlorine(Cl)
Bromine(Br)
Iodine(I)
Astatine(At)
Electronic
Atomic number configuration
9
17
35
53
85
[He]2s2 2p5
[Ne]3s23p5
[Ar]3d104s24p5
[Kr]4d105s25p5
[Xe]4f145d106s26p5
Atomic weight
(gmol-1 )
19.998
35.457
79.904
126.904
210
Physical properties of halogens
• Physical state and appearance
Fluorine and Chlorine exist as gases, bromine as a
liquid and iodine and astatine are solid.
The strength of intermolecular forces increases
down the group.
This is the reason that there is a gradual change
from gaseous state to solid state as we go down the
group.
Halogens are colored compounds.
Physical properties cont…..
Halogen
Fluorine(F)
Chlorine(Cl)
Bromine(Br)
Iodine(I)
Astatine(At)
State
Colour
Gas
Pale yellow-brown
Gas
Pale green
Liquid
Reddish-brown
Solid
Violet
Solid
Black
Physical properties cont…..
• Atomic size
Atomic size increases down the group as the
number of shell increases.
Halogen
Fluorine(F)
Chlorine(Cl)
Bromine(Br)
Iodine(I)
Astatine(At)
Covalent radius
Ionic radius(X-)
71
133
99
181
114
196
133
220
150
Physical Properties Cont…………
• Density
Density increases down the group. This is
because down the group the molecules
become larger and inter molecular force
becomes strong
Element
Fluorine(F)
Chlorine(Cl)
Bromine(Br)
Iodine(I)
Astatine(At)
Density(gcm-3 )
0.0017
0.0032
3.1028
4.933
8.91-8.95
Physical properties cont………
• Hydration energy
Hydration energy decreases down the group
as is inversely related to ionic radius.
Elements
Fluorine(F)
Chlorine(Cl)
Bromine(Br)
Iodine(I)
Astatine(At)
Hydration energy(KJmol-1)
-540
-361
-330
-285
-
Physical properties cont…….
• Boiling points
Boiling points increases down the group down the
group as they exist as diatomic molecules the forces
between the molecules are weak van der waals’
forces hence the strength of this intermolecular
forces increases down the group.
Physical properties cont…….
Elements
Fluorine(F)
Chlorine(Cl)
Bromine(Br)
Iodine(I)
Boiling point)
85.01
239.18
331.93
457.5
609.8(estimated)
Astatine(At)
Physical properties cont……..
• Melting point
Melting point increases down the group the
reason for this is the same as in boiling point.
Elements
Fluorine(F)
Chlorine(Cl)
Bromine(Br)
Iodine(I)
Astatine(At)
Melting point(K)
53.53
171.6
265.8
386.85
575.15
Physical properties cont…….
• Ionization energy
Halogens have very high ionization energy and
decreases down the group. This is because
down the group increases down the group.
Elements
Fluorine(F)
Chlorine(Cl)
Bromine(Br)
Iodine(I)
Astatine(At)
First ionization energy
(KJmol-1)
1681
1251
1140
1008
890
Physical properties cont…….
• Electronegativity
Halogens have very high electronegativity.
This is due to their small atomic size and high
effective nuclear charge.
Electronegativity decreases down the group
from fluorine to astatine
Physical properties cont….
Elements
Fluorine(F)
Chlorine(Cl)
Bromine(Br)
Iodine(I)
Astatine(At)
Electronegativity
4.0
3.0
2.8
2.5
2.2
Physical properties cont….
• Electron affinity
Since the atomic size increases down the
group, electron affinity generally decreases
down the group(At<I<Br<F<Cl). However,
fluorine has a lower electron affinity than
chlorine. This can be explained by the small
size of fluorine, compared to chlorine.
Physical properties cont….
Elements
Fluorine(F)
Chlorine(Cl)
Bromine(Br)
Iodine(I)
Astatine(At)
Electron affinity(KJmol-1)
-328.0
-349.0
-324.6
-295
-270
Physical properties cont….
• Fluorine has unexpectedly smaller electron
affinity than chlorine due to the following
reasons;• Fluorine atom is very small. So, the electron
charge density on fluorine atom is large.
Therefore, the electron experiences more
repulsive force from the electron cloud of the
F atom that comes to the atom.
Physical properties cont….
• As a result the net energy released during the
reaction, F(g)+eF- (g), becomes slightly
lesser, and the electron affinity of fluorine
becomes smaller than that of chlorine.
Physical properties cont….
• Dissociation energy
The energy change when 1 mole of halogen
molecules are broken into atoms in the
gaseous phase is called the dissociation
energy, or atomization energy.
Thus, the dissociation energy of X2 is equal to
the energy change in the reaction.
X2(g)
2X(g)
Physical properties cont….
• The dissociation energy decreases while going
from chlorine to iodine. However, fluorine has
dissociation energy much lower than of chlorine.
• The lower dissociation energy of F2 molecule in
comparison to Cl2 molecule is due to its small
atomic size. F atom is very small. So F-F bond
length is small in F2 molecule. As a result, the
charge density and the repulsion between the
repulsion non-bonding electrons on fluorine
atoms is very high as compared to that in Cl2,Br2
and I2 molecules.
Physical properties cont….
• This makes the F atoms in F2 molecules repel
each other, and make the dissociation of F2
molecules into F atoms.
Elements
Fluorine(F)
Chlorine(Cl)
Bromine(Br)
Iodine(I)
Astatine(At)
Dissociation energy(KJmol-1)
159
243
193
151
Physical properties cont….
• Oxidation state
Halogen attain stable configuration of the nearest
noble gas by acquiring one electron.
This may be done either by gain one electron or
by sharing of one electron pair with some other
atom.
Fluorine is the most electronegative element and
shows only -1 oxidation state.
Other halogens show both the negative and
positive and positive oxidation states which are as
follows;-
Physical properties cont…
Elements
Fluorine(F)
Chlorine(Cl)
Bromine(Br)
Iodine(I)
Astatine(At)
Oxidation state
-1
-1,+1,+3,+5,+7
-1,+1,+3,+5
-1,+1,+3,+5,+7
Fluorine does not show higher or variable oxidation states
because it does not have d-orbitals in its valence shell.
Due to the availability of the d-orbitals can be unpaired by
promoting them to have vacant d-orbitals. This opens the
possibilities of variable oxidation states
Chemical properties of halogens
• Halogens have seven valence electrons
because halogens have one electron missing,
they form negative ions and are highly
reactive. They can gain an electron by reacting
with atoms of other elements. Flourine is one
of the most reactive elements in existence.
• The following chemical properties are as
follows;-
Chemical properties….
• Oxides
-Halogens reacts with water to form halogen
oxides.
-Fluorine is the only element which forms
neutral oxides,F2O. This is because in this
compound fluorine is more electronegative
than oxygen.
- Other oxides of halogens are acidic.
Chemical properties….
• For chlorine(Cl)
Cl2O + H2O
Cl2O + H2O
Cl2O7 + H2O
• For bromine(Br)
Br2O + H2O
2BrO3 + H2O
2HOCl
HClO4 + HClO3
HClO4
2HOBr
HBrO3 + HBrO4
Chemical properties….
• For iodine
I2O4 + H2O
HIO2 + HIO3
I2 O5 + H2O
2HIO3
I2 O7 + H2O
HIO4
-The oxides of iodine are solids at room
temperature and are salt like.
Chemical properties….
• Hydrides of halogen
-All the halogens form hydrides by direct
combination of the elements.
X2(g) + H2(g)
2 HX
Chlorine reacts with hydrogen gas to form
hydrochloric acid.
Cl2(g) + H2(g)
2HCl
Iodine reacts with hydrogen gas to form
hydrochloric acid
I2(g) + H2(g)
2HI
Chemical properties….
• The hydrogen halides formed are covalently
bonded, and when pure are colourless gases
at room temperature. All the hydrogen halides
are freely soluble in water and react according
to the general equation.
•
HX + H2O
H3O+ + X-
Chemical properties….
Hydrogen fluoride and hydrogen chloride are
very stable to heat. They do not split up into
hydrogen and fluorine or chlorine again if
heated at normal laboratory temperature.
Hydrogen bromide splits slightly into hydrogen
and bromine on heating as well as hydrogen
iodide splits to an even greater extent.
Chemical properties….
• Justification of decreasing the oxidizing power of
hydrogen halides on decreasing the halogen
group.
• This can be justified by considering the reaction
between hydrogen halides and concentrated
sulphuric acid which is good oxidizing agent.
HF + Conc.H2SO4
No reaction
HCl + Conc.H2SO4
No reaction
Chemical properties….
• So HF and HCl are not strong reducing agent
enough to be oxidized by concentrated
sulphuric acid.
HBr + Conc.H2SO4
Br2 + SO2 + 2 H2O
• Hence Conc.H2SO4 is reduced by HBr whereby
oxidation state of sulphur is reduced from
+6(in H2SO4 ) to +4(in SO2) and HBr become
oxidized.
Chemical properties….
In this case, Conc.H2SO4 is more reduced by HI
than HBr does as the oxidation state of
sulphur is much more reduced from +6 in
H2SO4 to -2 in H2S. HI has been oxidized to I2
8HI + Conc.H2SO4
4I2 + H2S + H2O
ELEMENTS OF PHARMACEUTICAL
IMPORTANCE
• Fluorine
Despite its toxicity, fluoride can be found in
many everyday products, including
toothpaste, vitamin supplements, baby
formulas, and even public water. Many dental
products contain fluoride in order to prevent
tooth decay, but overconsumption of fluoride
can be fatal
Pharmaceutical importance cont…
• Chlorine
Chlorine accounts for about 0.15 percent of human
body weight. Chlorine is primarily used in the
production of hydrochloric acid, which is secreted
from the parietal cells in the stomach and is used
in maintaining the acidic environment for pepsin.
It plays a vital role in maintaining the proper acidbase balance of body fluids. It is neutralized in
the intestine by sodium bicarbonate.
Pharmaceutical importance cont…
• Chlorine also reacts with sodium to create
sodium chloride, more commonly known as
table salt.
• Both chlorine and bromine are used as
disinfectants for drinking water, swimming
pools, fresh wounds, spas, dishes, and
surfaces. They kill bacteria and other
potentially harmful microorganisms through a
process known as sterilization
Pharmaceutical importance cont…
• Chlorine and bromine are also used in
bleaching. Sodium hypochlorite, which is
produced from chlorine, is the active
ingredient of most fabric bleaches. Chlorinederived bleaches are also used in the
production of some paper products.
Pharmaceutical importance cont…
• Iodine
• Iodine is an essential mineral for the body. It is
used in the thyroid gland but can also be found in
breast tissue, salivary glands, and adrenal glands
• Without iodine, thyroid hormones cannot be
produced, which leads to a condition called
hypothyroidism. Without treatment, the thyroid
gland will swell and produce a visible goiter.
Pharmaceutical importance cont…
• Children with hypothyroidism may develop
mental retardation. In women,
hypothyroidism can lead to infertility,
miscarriages, and breast and ovarian cancer.