9.09.3 Group 7 - Halogens
Syllabus
The elements in Group 7 of the Periodic Table are known as the Halogens and have similar reactions because they all have seven electrons in their outer shell.
The halogens are non-metals and consist of molecules made of pairs of atoms.
Students should be able to describe the nature of the compounds formed when Chlorine, Bromine and Iodine react with metals and non-metals.
In Group 7, the further down the group an element is the higher its relative molecular mass, melting point and boiling point.
In Group 7, the reactivity of the elements decreases going down the group.
A more reactive halogen can displace a less reactive Halogen from an aqueous solution of its salt.
Students should be able to:
explain how properties of the elements in Grp 7 depend on the outer shell of electrons
predict properties from given trends down the group.
What does this mean?
What are Halogens
You don't need to learn why Group 7 elements are called Halogens (it means 'salt-makers).
You do need to know which Halogens are Solids, which are Liquids and which are Gases.
You also need to know their colours.
States
Trend: the Halogens range from gas to solid.
Most people recall that Chlorine is a poisonous gas bubbled through water to kill germs.
So Bromine should be a liquid at Room Temperature.
And Iodine should be a solid.
So we can predict that Astatine will be solid, and Fluorine will be a gas.
Colours
Trend: The colours get darker down the group
Chlorine: Chloro- means Green (as in Chlorophyll)
Bromine is Brown (or Orange)
Iodine is Grey (as a solid)/ Dark Purple as a gas
So Astatine should be darker than Grey = Black
And Fluorine should be lighter than Green = Pale Yellow
Melting Points
Cl2 -101oC
Br2 -7oC
I2 114oC
There's a difference of almost exactly 100 oC between elements so you can predict a melting point of Astatine to be around 100oC above Iodine's - somewhere over 200oC (actualy 300oC).
And a melting point of Fluorine of around 100oC lower than Chlorine's - somewhere around -200oC (actually -219oC)
Predictions don't have to be right - just sensible.
Boiling Points
Cl -34oC
Br +59oC
I +184oC
The increase from Cl to Br is 93oC
The next increase from Br to I is 125oC
So it's reasonable to predict that Astatine's boiling point will be at least 150oC above Iodine's.
Prediction: somewhere around 335oC (actually 380oC)
It's also reasonable to think that Fluorine's will be about 80oC lower than Chlorine's.
Prediction: somewhere around -115oC (actually -188oC)
Trend in Reactivity
Group 7 elements (Halogens) are almost the opposite of Group 1 elements (Alkali Metals).
Halogens = Non-metal Alkali Metals= Metal
Halogens need to gain 1 electron each, Alkali Metals need to lose 1 electron each.
Halogens become less reactive down the group, Alkali Metals become more reactive.
At GCSE we don't need to understand electron configurations beyond 2.8.8.2
But we should know that Group 7 elements end with a 7 and that elements have more shells the further down the group they are.
Each of these elements needs to gain one electron, which they attract by having a positively charged nucleus.
Fluorine has the fewest shells so the electron gets much closer to the nucleus than it would in Astatine.
So Fluorine is better at attracting electrons.
And hence it reacts most quickly.
Displacement
When a halogen atom gains an electron it becomes a Halide ion.
But Halogens (like most gas elements) exist as pairs (diatomic molecules )
So an equation for a Halogen (X2) becoming a Halide (X-) by gaining an electron (e-) would be:
X2 + 2 e- --> 2X-
Smaller Halogens should be able to attract electrons away from Halides that are bigger.
So Fluorine should be able to take electrons from Chloride, Bromide and Iodide ions.
And Chlorine should be able to take electrons from Bromide and Iodide ions.
While Bromine should be able to take electrons from Iodide ions.
So, if small Halogens remove electrons from other Halide ions the ions will lose the electrons.
In other words, the Halide will be Oxidised.
And the Halogens gain the electrons so they will be Reduced
So Fluorine can oxidise Chloride, Bromide and Iodide ions.
And Chlorine can oxidise Bromide and Iodide ions.
While Bromine can only oxidise from Iodide ions.
We say that reactive Halogens displace less reactive Halides from compounds
You could be asked to complete a table like the one below and say what you would observe:
Obviously, Fluorine atoms can't take electrons from (oxidise) Fluoride ions, or Chlorine from Chloride.
So we can ignore these combinations.
And large halogens can't take electrons from (oxidise/displace) smaller halide ions.
So, all other combinations will involve a colour change, the colour of the Halogen will disappear and be replaced by a new colour.
You could just remember that Chlorine displaces everything (except itself), Bromine only displaces Iodides, Iodine displaces nothing.
Halogen displacement reactions video
Halogen displacement Powerpoint
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Extension - Astatine
Like Francium, Astatine can be proven to exist but is so radioactive that it isn't really possible to study.
What do we actually know about Astatine?