Group 18 Elements – Noble Gases

  1. The “Noble Gases” are the last group in the Periodic Table, they also known as “inert gas”, due to their non-reactive behavior.
  2. This group consist of six elements, namely Helium, Neon, Argon, Kripton, Xenon and Radon.
  3. They are non-metallic, colourless gases at room temperature and pressure with very low melting points and boiling points.
  4. They form 1% of air, and most of this is argon.
  5. The size of atom increases down the group, due to the increase number of electron shell.
Name Proton number Electron arrangement Melting point Boiling point
Helium 2 2 -270°C -269°C
Neon 10 2.8 -249°C -246°C
Argon 18 2.8.8 -189°C -186°C
Krypton 36 -157°C -152°C
Xenon 54 -112°C -108°C
Radon 86 -71°C -62°C

Physical Properties

Solubility and Conductivity

  1. All noble gas are insoluble in water.
  2. They are not conductor of heat and electricity.

Melting Point and Boiling Point

  1. As show in the table above, the melting and boiling point of noble gases are very low.
  2. This is because all noble gases exist as monoatoms. The force in between all these atoms is the weak van de Waals’ Force.
  3. Therefore very little energy is needed to overcome this force during melting and boiling.


  1. The density of noble gases are very low.
  2. Nevertheless, the density increases steadily down the group.
  3. Density of a substance is given by the equation “Density=Mass/Volume”.
  4. Down the group, both the mass and the volume increase, but increase of mass is faster than the volume, hence the density increases down the group.

Chemical Properties

  1. All the noble gases are non-reactive elements.
  2. This is because their valence shell is full of electrons.
  3. In the chemical world, an atom is in the chemically most stable state if their valence shell is full with eight electrons (or 2 electrons for the first shell.).
  4. Therefore all noble gases do not react with other elements, due to their stable electronic structure.
  5. They exist as single atoms, that is they are monatomic.