Chemguide: Support for CIE A level Chemistry

Group 17 (previously Group 7)

Learning outcome 11.1.1

The statement only asks for chlorine, bromine and iodine, but I am including fluorine as well in case CIE ask you to predict its colour or state.

• Fluorine is a pale yellow gas.

• Chlorine is a greenish-yellow gas.

• Bromine is a dark red liquid, forming a reddish-brown gas even at room temperature.

• Iodine is a dark grey solid, forming a purple vapour on gentle heating.

The trend is for darker colours as you go down the Group.

You can consider melting points and boiling points as good indicators of volatility. If these are low, then the element will turn easily to a vapour - and that is all that is meant by volatility. If they are higher, then the element is less volatile.

That means that the elements become less volatile as you go down the Group.

We will have more to say about this in statement 11.1.3 below.

Learning outcome 11.1.2

This statement is about the strength of the bonds in the halogen molecules, X₂.

You will find this discussed (along with lots of other atomic properties) on the page about the atomic and physical properties of the halogens.

Everything you need for this statement is included in the section of the page headed "Bond enthalpies (bond energies or bond strengths)".

It isn't totally clear from the syllabus whether you need to know about the way fluorine breaks the overall pattern in the Group. But, taken literally, the statement asks for the trend in bond strengths of the halogen molecules. That definitely includes fluorine.

You won't need the section about the strengths of the hydrogen-halogen bonds until statement 11.2.3.

Learning outcome 11.1.3

This statement asks for an explanation of the volatility of the elements in terms of van der Waals dispersion forces (or as CIE describe them "instantaneous dipole - induced dipole forces").

You will find this discussed on the page about the atomic and physical properties of the halogens you have already looked at. It is part of the section on "Trends in Melting Point and Boiling Point".

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