Vary in appearance: fluorine; very pale yellow gas, chlorine; greenish gas, bromine; red brown liquid, Iodine; black solid.
They become darker and more dense down the group.
Both melting and boiling points increase as you go down the group.
Electronegativity decreases down the group.
A number of properties of fluorine are untypical
It stems from the fact the F-F bond is unexpectedly weak.
Because the fluorine atom is so small, there is repultion between the non-bonding electrons as they are so close together.
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Size of Atoms.
The atoms become bigger when going down the group.
Each element has one extra filled main level of electrons
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Electronegativity.
The measure of the ability of an atom to attract electrons or,electron density; towards itself within a covalent bond
Depends on the attraction between the nucleus and bonding electrons in the outer shell.
This depends on a balance between the number of protons in the nucleus and the distance between the nucleus and the bonding electrons, and the shielding effect of inner shells of electrons.
In the ion version of the halogen, the shared electons in the bond get further away from the nucleus as the atoms get larger going down the group.
This makes the shared electrons further from the halogen nucleus and increases the sheilding by more inner shells of electrons
these factors are more important than increasing nuclear charge and therefore the electronegativity decreases down the group.
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Melting and Boiling Points.
Increase going down the group.
Because larger atoms have more electrons, making van der Waals forces between molecules stronger.
The lower the boiling point, the more volatile the element; eg. Cl, is a gas at room temperature so it is more volatile than I which is a solid.
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