Physical and Chemical Properties of Group 18 Elements
Elements in Group 18 of the Periodic Table are:
Helium, Neon, Argon, Krypton, Xenon, Radon
These elements are known as noble gases.
Physical properties of noble gases
1. Table shows some physical properties of Group 18 elements.
|Atomic radius (nm)||0.050||0.070||0.094||0.109||0.130||–|
|Melting point (°C)||-272||-249||-189||-157||-112||-71|
|Boiling point (°C)||-269||-246||-185||-152||-107||-62|
|Density (g dm-3)||0,18||0.84||1.66||3.73||5.89||9.73|
2. Atomic radius (atomic size)
- The atomic radii (atomic sizes) of noble gases increase when going down Group 18 from helium to radon.
- This is because the number of shells occupied with electrons increases when going down the group.
3. Melting and boiling points
- All noble gases have very low melting and boiling points. They exist as gases at room temperature and pressure.
- This is because noble gases consist of single atoms (monoatomic molecules) held together by weak van der Waals forces of attraction.
- Only a small amount of heat energy is required to overcome the weak interatomic forces of attraction during melting and boiling.
Trend of change down the group
- The melting and boiling points of noble gases increase when going down Group 18.
- This is because the atomic sizes of the noble gases increase when going down the group from helium to radon, thus the van der Waals forces of attraction between the atoms become stronger.
- Hence, more heat energy is required to overcome the stronger interatomic forces of attraction during melting and boiling.
- All noble gases have very low densities.
- The densites of noble gases increase when going down Group 18.
All noble gases are colourless gases which are insoluble in water.
6. Electrical and heat conductivity
- All noble gases cannot conduct electricity.
- All noble gases are poor conductors of heat.
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Unreactive (Inert) properties of noble gases
Table shows the electron arrangements of the atoms of noble gases.
- Each helium atom has only 2 electrons in one filled electron shell.
This electron arrangement is known as the duplet electron arrangement.
- The outermost shells of the atoms of the other noble gases have 8 electrons.
This electron arrangement is known as the octet electron arrangement.
- The duplet electron arrangement of helium and the octet electron arrangement of the other noble gases are very stable.
- Hence, atoms of noble gases do not release electrons, accept electrons or share electrons among each other or with atoms of other elements.
- Thus, noble gases exist as monatomic molecules and do not react with the other elements or compounds. This means that noble gases are inert chemically or chemically unreactive.
Uses of Group 18 elements
- The inert property of noble gases enable them to be used in various fields in our daily life.
- Helium, neon and argon are the most commonly used noble gases.
- To fill weather balloons and airships.
This is because helium gas is very light and non-flammable (unreactive).
Hydrogen, the lightest gas, is not used for the above purposes because it is flammable.
- A mixture of 80% helium and 20% oxygen is used as an artificial atmosphere for divers.
Helium is used in preference to nitrogen because helium is less soluble than nitrogen in the blood. Nitrogen can cause a decompression sickness called bends’.
- Used in thermometers to measure very low temperatures
- Used as a protective atmosphere for growing crystals of silicon and germanium.
Silicon and germanium are used to make microchips.
- Used to fill advertising light bulbs. These bulbs can light up advertisement boards with a reddish-orange colour.
- Small neon lamps are widely used as indicator lights to show that a circuit is ‘on’.
- Used to fill electric bulbs.
This is because argon is chemically inert. Thus, the hot tungsten filament in the bulb does not react with it. As a result, argon gas prevents the tungsten filament from being oxidised.
- Used to provide an inert atmosphere for welding.
This is because argon gas prevents the hot metals from reacting with oxygen in the air.
- Argon is used as a carrier gas in liquid-gas chromatography because it is unreactive.
- Used to fill high speed photographic flash lamps.
- Used in lasers to repair the retina of the eye.
- Used in electron tubes and stroboscopic lamps.
- Used in bubble chambers.
- Used to treat cancer because it is radioactive.