Unit 13 Properties and uses of group 18 elements
Key unit competence: Analyse the origin of inertness of group 18 elements (noble gases) to explain their chemical behaviour and uses.
Key ideas
- Group 18 (noble gases): He, Ne, Ar, Kr, Xe, Rn. They occur in air (argon about 0.93%) and helium in natural gas; obtained by fractional distillation of liquid air (helium from natural gas). Radon is radioactive.
- They are monatomic, colourless, odourless gases with very low melting and boiling points that increase down the group (London forces increase with atomic size).
- Inertness: full outer shells (He 1s2, others ns2 np6) and very high first ionisation energies, so they have little tendency to gain, lose or share electrons.
- Ionisation energy decreases down the group, so heavier noble gases can react with very electronegative elements: Bartlett made XePtF6 in 1962; xenon fluorides XeF2, XeF4, XeF6 and oxides (XeO3) are known.
- Shapes from VSEPR: XeF2 linear (3 lone pairs), XeF4 square planar (2 lone pairs), XeF6 distorted octahedral.
- Uses: helium in balloons and airships (light, non-flammable), diving gas mixtures and cooling MRI magnets; neon in advertising signs (red glow); argon as an inert atmosphere in welding and light bulbs; krypton in lasers and flash lamps; xenon in car headlamps and anaesthesia; radon in radiotherapy (but a health hazard in homes).
Common misconceptions
- "Noble gases cannot form any compounds." No: Xenon and krypton form compounds with fluorine and oxygen.
- "Helium is in group 18 because it has 8 outer electrons." No: Helium has only 2 electrons, but its shell is full (1s2).
- "Noble gases have high boiling points because they are unreactive." No: They have very low boiling points because only weak London forces act between single atoms.
Worked examples
Example 1. Explain why the noble gases are chemically inert.
Solution
Their atoms have full outer shells: helium 1s2, the others ns2 np6.
They have very high first ionisation energies and no tendency to gain electrons (zero or positive electron affinity).
So they rarely lose, gain or share electrons and exist as single atoms.
Example 2. Give four uses of noble gases and link each to a property.
Solution
Helium: balloons (low density, non-flammable).
Neon: advertising signs (glows when electricity passes).
Argon: welding and light bulbs (inert atmosphere).
Xenon: car headlamps and anaesthesia (bright light, dissolves in fats).
Practice questions
Write the configuration of neon (Z = 10).
Show answer
1s2 2s2 2p6.
Common mistake: Writing 2p8.
Why are noble gases monatomic?
Show answer
Their atoms are stable with full shells and do not bond to each other.
Common mistake: Saying they are too light.
Give two uses of argon.
Show answer
Light bulbs, welding shield gas, double-glazing (any two).
Common mistake: Saying balloons.
Name the first noble gas compound.
Show answer
XePtF6 (xenon hexafluoroplatinate).
Common mistake: Saying XeF2 was first.
State the shape of XeF4.
Show answer
Square planar.
Common mistake: Saying tetrahedral.
Why is helium preferred to hydrogen in airships?
Show answer
Helium is non-flammable; hydrogen is explosive.
Common mistake: Saying helium is lighter.
Explain the trend in boiling points of noble gases.
Show answer
They increase down the group because London forces increase with atomic size.
Common mistake: Saying ionic bonding increases.
Why does xenon form compounds with fluorine and oxygen only?
Show answer
They are the most electronegative elements, able to remove Xe's electrons.
Common mistake: Saying xenon is a metal.
Name one hazard of radon.
Show answer
It is radioactive and can cause lung cancer.
Common mistake: Saying it is explosive.
How is helium separated from natural gas?
Show answer
By cooling and liquefying the other gases (fractional distillation), leaving helium.
Common mistake: Saying by electrolysis.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The electron configuration of argon is: a. single atoms 2. Neon is used in: b. 1s2 2s2 2p6 3s2 3p6 3. Noble gases exist as: c. have full outer electron shells 4. Noble gases are unreactive mainly because they: d. advertising signs Show answer
1 → b (1s2 2s2 2p6 3s2 3p6) · 2 → d (advertising signs) · 3 → a (single atoms) · 4 → c (have full outer electron shells)
- 18 electrons.
- Neon glows red-orange with electricity.
- They are monatomic.
- Stable configurations.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
square planar · plants · natural gas · fractional distillation of liquid air- Helium is obtained mainly from .
- Noble gases are obtained industrially by .
- The shape of XeF4 is .
Show answer
1: natural gas · 2: fractional distillation of liquid air · 3: square planar
- It accumulates underground from radioactive decay.
- They separate by boiling point.
- Four bonding pairs and two lone pairs.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Helium is used in balloons because it is: a. increases 2. Down group 18, boiling point: b. Radon 3. Which noble gas is radioactive? c. lasers and flash lamps 4. Krypton is used in: d. light and non-flammable Show answer
1 → d (light and non-flammable) · 2 → a (increases) · 3 → b (Radon) · 4 → c (lasers and flash lamps)
- Safer than hydrogen.
- Stronger London forces.
- Radon forms from uranium decay.
- Bright light when excited.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
Neil Bartlett · decreases · argon · increases- The most abundant noble gas in air is .
- First ionisation energy down group 18 .
- The first noble gas compound was made by .
Show answer
1: argon · 2: decreases · 3: Neil Bartlett
- About 0.93% of air.
- Outer electrons are further from the nucleus.
- He made a xenon compound in 1962.