In general, the Halogens are a very similar set of non-metals. They all exist as diatomic
molecules, X , and oxidise metals to form halides. The halogen oxides are acidic, and
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the hydrides are covalent. Fluorine is the most electronegative element in the periodic
table. Generally, electronegativity and oxidising ability decrease on descending the
Group. The result of this decreasing electronegativity is increased covalent character
in the compounds, so that AlF is ionic whereas AlCl is covalent.
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N.B. Fluorine shows some anomalies because of the small size of its atom and ion.
This allows several F atoms to pack around a different central atom, as in AlF 3-
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compared with AlCl -. The F-F bond is also unexpectedly weak because the small
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size of the F atom brings the lone pairs closer together than in other halogens, and
repulsion weakens the bond.
The most characteristic chemical feature of the halogens is their ability to oxidise.
Fluorine has the strongest oxidising ability, so other elements which combine with
fluorine have their highest possible oxidation number. Fluorine is such a strong oxi-
dising agent that it must be prepared by electrolysis. Chlorine is the next strongest
oxidising agent, but it can be prepared by chemical oxidation. Most elements react
directly with chlorine, bromine and iodine, with decreasing reactivity going down
the Group, but often the reaction must be activated by heat or UV light. Chlorine,
bromine and iodine disproportionate in the presence of water and alkalis.
Oxides and Oxoacids
There are no fluorine oxides as F is more electronegative than oxygen (O). Chlorine,
bromine and iodine each form several oxides which are thermally unstable, such as
chlorine dioxide ClO . The only fluorine oxoacid, HOF, is unstable at room tempe-
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rature, but there are many oxoacids of the other halogens. The best known salts of
these are; hypochlorite, chlorate(I) CIO-, chlorite, chlorate(III) ClO -, hypochlorate,
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chlorate(V) CIO -, perchlorate, chlorate(VII) ClO - . These are all powerful oxidising
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agents.
Halides
The halogens can combine with each other to form interhalogens and polyhalide
ions. Polyhalide ions have the general formula [Y-X-Y]-. It is not possible for F to
represent X in a polyhalide ion as it cannot expand its octet.
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Hydrides
Hydrogen halides have the general formula HX. HF is a colourless liquid which
boils at 19·5°C, and all the other hydrogen halides are colourless gases. HF is a
liquid due to the extensive hydrogen bonding which occurs between molecules. All
the hydrogen halides dissolve easily to give acidic solutions, the most widely used
being hydrochloric acid, HCl. All except HF are typical acids; they liberate carbon
dioxide from carbonates and form salts with basic oxides. HF is a weak acid because
the H-F bond is very strong, and because hydrogen-bonding occurs between F- and
HF in solution.
Organic Compounds
The halogens form organic compounds which are best known for their industrial and
environmental impact, such as PVC, DDT and TCP.
Oxidation States and Electron Affinities
Fluorine in all its compounds has an assigned oxidation number of -1, as it is the
most electronegative of all the elements. The other halogens show a wide range of
oxidation numbers, and the redox chemistry of these halogens is important. The
oxidation numbers most commonly shown are odd; there are few compounds with
even oxidation numbers and they are often thermally unstable. Chlorine is the third
most electronegative element after F and O. The halide ions are readily formed by
accepting one electron, as this completes an octet of valence electrons. The electron
affinity decreases on descending the Group
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NOBLE/INERT GASES (Group 0)
The noble gases (He, Ne, Ar, Kr, Xe, and Rn) also known as the inert gases, are
located in Group VIII of the periodic table.
Properties
The noble gases are relatively non reactive. This is because they have a complete
valence shell. They have little tendency to gain or lose electrons. The noble gases
have high ionization energies and negligible electronegativities. The noble gases have
low boiling points and are all gases at room temperature.
Summary of Common Properties
Physical Properties:
1. Helium has a boiling point of 4.2K under 1 atm pressure, the lowest boiling
point of any substance.
2. When an electric current is passed through a tube which contains neon gas
at a low pressure gas, it has a red-orange glow.
3. Argon is colourless, odourless and tasteless. It makes up 0.934% of air.
4. Krypton is a dense, colorless, odorless atmospheric gas.
5. Xenon is a heavy, unreactive stable gas. When Xenon atoms are sufficiently
energized, they produce a brilliant white light.
Chemical properties:
1. noble gases exist as single, monatomic gases atoms
2. all noble gases have eight valence electrons, a completely filled outer energy
level of electrons (‘filled’ outer shell means two electrons in the outer shell for
He and eight electrons in the outer shell for the others). This stable electron
arrangement accounts for the nonreactive nature of the noble gases.
Problem set Unit 3 (accounts for 20 %).
1. 1s22s22p63s23p4, is the electronic configuration that belongs to which period
and group of the periodic table of elements?
A) 3, 4,
b) 4, 16,
c) 3, 16,
d) 4, 3.
2. Which element in the period 3 has both metallic and nonmetallic properties
a) Na,
b)Al,
c) Si, d) Ar.
3. Which element is more likely to form a compound with Krypton,
a) F,
b) Cl,
c) Xe, I, Br.
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4. Which group contains an element that is a liquid at room temperature,
a) 1,
b) 7,
c) 8,
d) 18.
5. In which group do elements form usually form oxides with the formula M O ?,
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a) 1,
b) 3,
c) 13,
d) 2.