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2
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- Concepts:
- halogen, diatomic, energetic
stability, valence electrons,
- Reaction, displacement reaction, halide ion
- Skills:
- Describe and explain their trend in color, state, and reactivity with
other halide ions
- Predict properties of other elements in the group given data where
appropriate
- Identify uses for each halogen
- Describe and explain the change in reactivity of non-metals in period
3.
- Identify trends in other non-metal groups given information about the
elements concerned
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3
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- All the elements in Group 7 are nonmetals except for astatine, which is
a radioactive metalloid.
- These elements are called halogens, which means “salt-former.”
- All of the halogens form salts with sodium and with the other alkali
metals.
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- The Halogens are a very colourful group of elements.
- The Halogens do not “hang about” as single atoms.
- Each Halogen element goes about in pairs of its atoms:
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- Fluorine is written as: F2
- All of the Halogens are DIATOMIC
- = 2 (goes about in pairs)
- A molecule of a halogen is made up of two atoms, and therefore their
formulae are:
- Fluorine, F2 Chlorine, Cl2 Bromine, Br2 Iodine, I2
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- Why are the Halogens DIATOMIC ?
- Fluorine, like all the Halogens
is in GROUP 7
- Every fluorine atom has 7 valence electrons in its outer shell.
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- What can the TWO fluorine atoms
possibly do to fill both their outer shells and become energetically stable ?
- The outer shells of each atom have cross over each other.
- Each atom shares its valence electron with the other atom.
- (co-operation)
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- Electronic
configuration: 2, (7 +1 = 8) 2, (7 +1 = 8)
outer shell full outer shell full
energetically stable energetically stable
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- BOTH fluorine atoms have BONDED together in order to fill their outer
shells and become energetically stable.
- The 2 atoms shared: CO-operated with their VALENT (valence)
electrons.
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- To displace:
- to kick something out and take its place.
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- Cartman, Stan, Kyle and Kenny are good friends…well most of the
time…except when it comes to the question of who gets to play with
piggy.
- Cook, of course tries to help settle any argument or console any kid.
- Piggy remains in a small pen waiting for one or more of the kids to come
and play with it.
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- And today, Stan comes along to play with it.
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- And as everyone knows, Cartman gets what he wants.
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- Who is the stronger (bully) -- CARTMAN
or STAN ?
- WhAt happened to STAN and Why ?
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- The following day Kyle wakes up early and runs over to piggy and starts
playing with it and having a grand ol’ time!
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- Until Stan, having woken up a little late, gets around to piggy’s sty to
find Kyle spending some quality time with it.
- Stan, remembering how Cartman treated him the other day…
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- Who is the stronger (bully) -- Kyle
or STAN ?
- WhAt happened to Kyle and Why ?
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- The following day, while Kenny was trying to get some one-one time with
piggy…
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- Who is the stronger (bully) -- Kenny
or KYLE ?
- WhAt happened to Kenny and Why ?
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- The following day, Kenny, remember how he got displaced by Stan, woke up
early so that he could cozy up to piggy.
- He groggily walks over to the sty
to find…
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- Cook had to come along and console Kenny!!
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- Who is the stronger (bully) -- Cartman
or Kenny ?
- WhAt happened to Kenny and Why ?
- Who is the biggest bully of them all?
- What is the hierarchy of the bullies?
- Who would end up with piggy if Kyle were to come along and find Cartman
with it? Why?
- Who would end up with piggy if Stan were to come along and find Kenny
with piggy? Why?
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- Similarly, more reactive element will displace a less reactive one from
a compound (and take its place).
- Work out the Halogen reactivity series (most reactive at the top) based
on the experiment you performed in the lab.
- Pure Bromine, Br2, is a RED liquid (at room temperature & pressure).
- Pure Chlorine, Cl2, is a GREEN gas (at room temperature & pressure).
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- We will restrict our studies to the three middle halogens (Cl2-I2).
- A halogen HIGHER UP in the periodic table will DISPLACE one LOWER DOWN
in table from a solution.
- In general: X2 + 2Y- ® 2X- + Y2
- Chlorine will displace any halide ion below it in the table from a
solution.
- Cl2 (aq) + 2Y-(aq) ® 2Cl-(aq) + Y2 (aq)
Where Y- = Br- or I-
- Bromine will displace iodide ions from a solution.
- Br2 (aq) + 2I-(aq) ® 2Br-(aq) + I2 (aq)
- Evidence for displacement comes from changes in the appearance of the
reaction mixture which you observed in the lab.
- So, of the three, chlorine is the most reactive, followed by bromine
and then iodine.
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- The question though is how come reactivity of halogens decreases going
down the group whereas reactivity of group 1 metals increases going down
the group?
- You are reminded here once again that elements react with other elements
because they are energetically unstable.
- Atoms of noble gases are energetically stable while the rest of the
elements in the periodic table are not.
- So, all atoms of other elements try to attain a similar electronic
structure as those of noble gases and they do so by reacting with other
elements.
- Now, the halogens also have two options for achieving the same
electronic structure as a noble gas: gain 1 electron or lose 7
electrons.
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- Which one is easier and therefore more likely?
- Gaining 1 electron.
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- Which one is easier and therefore more likely?
- Gaining 1 electron.
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- As with group 1 metals, halogens will also undergo the process that is easier.
- While for group 1 metals it was easier for them to lose 1 electrons
instead of gaining 7, for halogens, it is easier for them to gain 1
electrons instead of losing 7!!
- Not only that remember that the electrons in the different shells in an
atom are attracted by the protons in the nucleus and the strength with
which they are attracted is different for electrons in different shells.
- The closer the shell is to the nucleus, the more strongly the electrons
in it are attracted by the nucleus and therefore the more reactive the
halogen.
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- Going Down Group 7
reactivity goes down.
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- Going Down Group 7
- HARDER to GAIN LAST OUTER
ELECTRON
- So LESS
- CHEMICALLY
- REACTIVE
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- Going down Group 7
- Bigger atoms
- one extra full shell of electrons for each period you go down
- Less Reactive
- Harder to gain last electron needed as nucleus is farther away from
the valence shell
- In other words when the following reaction take place
- Cl2 (aq) + 2Y-(aq) ® 2Cl-(aq) + Y2 (aq)
Where Y- = Br- or I-
- Chlorine is able to gain the electrons from bromide
and iodide ion more readily because they don’t hold the electrons as
strongly as if chlorine were holding it in its valence shell.
- In other words
- Br2 (aq) + Cl-(aq) ® No reaction
- Because chlorine is able to hold the electron more strongly than
bromine would hold it.
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- Swimming in pools treated with chlorine gives our eyes a sting.
- A British firm has found an alternative which involves electrolyzing the
water using two electrodes made of silver-copper alloy.
- The process of electrolysis releases copper and silver ions.
- Silver ions kill bacteria and copper ions kills algae.
- As the dead bacteria and algae are charged, they clump together forming
solids which can be removed by filtration.
- Other alternatives to chlorine are hydrogen peroxide and ozone.
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- 1. J05/2/5. The halogens are a group of diatomic non-metals showing a
trend in color, state and reactivity.
- (a) In this description, what is the meaning of
- (i) diatomic, [1]
- (ii) state? [1]
- (b) The table gives some information about some of the halogens.
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- (i) Complete the last column in the table to show the state of each of
the halogens at room temperature. [2]
- (ii) State the colour of bromine. [1]
- (iii) Suggest a value for the boiling point of iodine. [1]
- (c) Complete the word equation for the reaction of chlorine with
potassium iodide.
- chlorine + potassium iodide ® [2]
- (d) (i) Draw a diagram to show the electronic structure of a chlorine
molecule. Show only the outer electrons. [2]
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- (ii) State a use of chlorine. [1]
- (f) Astatine, At, is below iodine in Group VII of the Periodic Table.
- (i) In which Period of the Periodic Table is astatine? [1]
- (ii) How many protons does astatine have in its nucleus? [1]
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- 2. N03/2/4c. Bromine is extracted from seawater by treatment with
chlorine. When chlorine is bubbled through a solution of potassium
bromide, the solution turns orange-red.
- (i) What does this tell you about the reactivity of chlorine compared
with bromine? [1]
- (ii) Write a word equation for this reaction. [1]
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- (d) In order to get the maximum yield of bromine from seawater, acid is
added during the extraction procedure. The graph shows how the yield of
bromine changes with pH.
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- 3. J05/3/1. Three of the halogens in Group VII are:
- (a) (i) How does their colour change down the Group? [1]
- (ii) How does their physical state (solid, liquid or gas) change down
the Group? [1]
- (iii) Predict the colour and physical state of fluorine.
- colour
- physical state [2]
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- (b) Describe how you could distinguish between aqueous potassium bromide
and aqueous potassium iodide.
- test
- result with bromide
- result with iodide [3]
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- 4. J02/3/4. Bromine is one of the halogens in Group VII.
- (a) (i) Predict which halogen has the lightest colour. [1]
- (ii) Predict which halogens are solids at room temperature. [1]
- (b) Bromine is obtained from the bromide ions in sea water. Sea water
is concentrated by evaporation. Chlorine gas is bubbled through the
solution. Chlorine displaces bromide ion converting it into bromine.
- (i) Complete the following equation.
- Cl2 + ......Br– ®................... + .................. [2]
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Notes