Noble gases header

Higher and foundation tier

The Noble gases

noble gases symbolThe noble gases are found in the last column of the periodic table, group 0 (or sometimes it is called group 8). The noble gases are helium, neon, argon, krypton, xeon and radon. Chemically they are very unexciting as they tend not to react with other elements, with the exception mainly of fluorine, but even then severe conditions are needed and the compounds formed are not very stable- some even decompose explosively! So why don't the noble gases react? Well if you remember elements only react if they can achieve full last electron shells, since the noble gases all have full last shells they have no reason to react. Helium the first noble gas has 2 electrons in its last shell, which is full. All the other noble gases have 8 electrons in their last electron shell.

Most of the gases we have discussed so far go around in pairs as diatomic molecules e.g. oxygen gas (O2), nitrogen gas (N2), hydrogen gas, (H2) and all the halogens : fluorine gas (F2), Chlorine gas (Cl2), Bromine (Br2), iodine (I2). However noble gases are different, since they are reluctant to react they go around as single atoms, NOT molecules. They are often called monatomic gases (mono = one) since they consist of individual atoms.

noble gases in cylinder

Physical properties

Since there is no real chemistry to study for the noble gases as they rarely react we can use the periodic table to predict their physical properties e.g. trends in the melting points (m.p.), boiling points (b.p.) and their density.

Now the noble gases consist of individual atoms, so their melting and boiling points will be very low. So what trend might we expect in the m.p. and b.p. as we go down group 0 from helium to radon? Well as we go down the group the atomic mass increases so we might expect the heavier atoms to have the highest melting and boiling points and this is exactly the trend we find. However this is not the whole story.

Atoms and molecules in all substances will attract each other, these weak forces of attraction are called intermolecular bonds (think of an international as a competition between different countries), well intermoleular bonds are forces of attraction between adjacent atoms and molecules. They are caused by uneven electron distributions as atoms and molecules approach each other. These intermolecular bonds are very weak when compared to normal covalent and ionic bonds but they can have a large effect on the physical properties of atoms and molecules.

The helium atoms in the diagram below as we know will not bond or join with each other as they already have full last shells. However as the atoms in the helium gas get close to each other the atoms attract each other weakly - these are the intermolecular bond. These intermolecular bonds are a type of bonding called Van der Waals bonding, and it is very weak.

However the strength of this intermolecualr bonding increases as the atoms get larger in diameter. Radon being the largest noble gas will have more and also stronger intermolecular bonds than helium. This is simply because it is a larger atom and has more surface area to form bonds with neighbouring atoms. The larger atomic mass and also increase in the amount and strength of the intermolecualar bonding as we go down group 8 means that the melting and boiling points increase from helium to radon at the bottom of the group.

intermolecular bonding in noble gases

The table below clearly shows the trend in the melting points (m.p.) and boiling points (b.p.) of the group 0 noble gases. You will notice that the melting and boiling points are very very low, this is completely expected as noble gases consist of individual atoms with weak intermolecular forces between these small individual atoms.

Noble gas Helium (He) Neon (Ne) Argon (Ar) Krypton (Kr) Xenon (Xe) Radon (Rn)
melting point/oC -272 -248 -189 -157 -111 -71
boiling point/oC -268 -246 -185 -153 -108 -61

Densities of gases

Density is how mass is inside a given volume. In the example below we have two gases, helium and radon inside two identical boxes. As you probably already know gases are mostly empty space with only a few atoms present. According to Avogadro's law equal volumes of gases will contain the same number of particles, in this case atoms. So the two identical boxes obviously have the same volume, and equal volumes of gases contain the same number of particles, so the boxes will contain the same number of atoms- obviously! Since radon atom are heavier than helium atoms the box with the radon atoms weighs more than the box with the helium atoms and since density is how mass is inside a given volume the radon gas is more dense than the helium gas.

noble gases density

Trends in the density of noble gases

balloons filled with noble gases The density of the noble gases increases as you descend group 0. If you consider that the air contains approximately 80% nitrogen (N2) with a Mr of 14x2=28, and 20% oxygen (O2) with a Mr of 16x2=32, this gives an average mass of the air as 24. So any gas with a mass less than this will "float" or rise and any gas with a mass more than this will sink (for more detailed calculation vist pages on mole calculations). Since the densities of the noble gases increase as you descend group 0 then :

Uses of noble gases

argon inside lightbulb Having little or no chemistry the uses of the noble gases are limited. Most uses rely on the fact that the noble gases have almost no chemical reactivity e.g. Light bulb filaments are made of the metal tungsten which glows white hot when the bulb is in use. If the light bulb was simply filled with air then the filament would burn out almost as soon as it was switched on. So filling the bulb with an inert noble gas like argon will prevent the filament from burning out.
protecting a weld using a noble gas During welding a metal is heat well above its melting point and it can easily be oxidised by reacting with oxygen in the air. This will weaken the metal and spoil the weld. So to protect the hot metal during welding it is given a blanket of inert argon gas. Since argon is denser than air and chemically unreactive it will keep out the oxygen and prevent the metal from oxidising. Other uses for argon gas is in argon lasers which are used in eye surgery and for creating halographs.
If neon gas is placed in a glass tube and a large enough voltage applied across it then the gas can be made to emit light. These neon light and signs are common on most high streets where they are used on night clubs, resturants, arcades and advertising signs. Las Vegas lights Helium is mainly used in weather balloons and since its boiling point is so low, -2690C, liquid helium is used to cool super-conducting magnets that hospital body scanners need to work properly e.g. MRI scanners. The main use for krypton is to fill energy saving light bulbs and photography flashes.

Key Points

Practice questions

Check your understanding - Questions on noble gases.