- 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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- 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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