Group 17 Elements

1. The elements in Group 17 are : Fluorine, Chlorine, Bromine, Iodine, Astatine.
2. This elements are known as halogens.
3. This is because halogens are reactive non-metals.
4. Halogens exist as diatomic covalent molecules.

Electronegativity of halogens

1. The electronegativity of an element is a measurement of the strength of its atom in a molecule to pull electrons towards its nucleus.
2. Halogens are very electronegative.
3. Table below shows the electonegativities of halogens.

   Element	Fluorine	Chlorine	Bromine	Iodine
   Electronegativity (Pauling scale)	4.0	3.0	2.8	2.5

4. The electronegativity of halogens decreases when going down the group.
5. This is because the number if shells occupied with electrons in the atoms if halogens increases when going down the group.
6. This causes the outermost occupied shell to become further away from the nucleus and is screened by more inner shells containing electons
7. Hence, the strength of the nucleus to attract electrons becomes weaker.

 Physical properties

1. Table below shows some physical properties of Group 17 elements.

   Element	Fluorine	Chlorine	Bromine	Iodine
   Proton number	9	17	35	53
   Atomic radius (nm)	0.071	0.099	0.114	0.133

2. Table below shows the physical state and colours of various halogen.

   Halogen	Physical state and colour
   Fluorine	Pale yellow gas
   Chlorine	Greenish-yellow gas
   Bromine	Reddish-brown liquid
   Iodine	Purplish-black solid

3. All halogen have low melting and boiling points
4. This is because the halogen molecules are held together by weak van der Waals forces of attraction.
5. Hence, the small amount of heat energy is needed to overcome it during melting or boiling.
6. All halogens have low densities.
7. All halogens do not conduct electricity.
8. All halogens are weak conductors of heat.

Chemical Properties

1. Table below shows the electron arrangements of halogen.

   Element	Electron arrangement
   Fluorine	2.7
   Chlorine	2.8.7
   Bromine	2.8.18.7
   Iodine	2.8.18.18.7
   Astatine	2.8.18.32.18.7

2. All halogen exhibit similar chemical properties because all the atoms of halogens have 7 valence electrons.
3. The reactivity of halogens decreases when going down Group 17.
4. The reactivity of a halogen is measured by the ease of its atom to accept one electron so as to achieve a stable noble gas electron arrangement
5. When going down Group 17, the atomic size of halogen increases.
6. The outermost occupied shell becomes further away from the nucleus and is screened by more inner shells containing electrons.
7. This mean that the effective nuclear charge exerted on the outer valence shell decreases when going down the group.
8. Therefore, the strenght of the nucleus of a halogen atom to attract one more electron into the valence electron into the valence shell to achieve an octet electron arrangement decreases.