Group 0: properties and reactivity

In a nutshell

The elements of Group $$0$$​ are called noble gases. They are monatomic (single atoms), inert (unreactive) gases and this lesson will discuss why.

Atomic structure and reactivity 

In electron shells, there are:

1. A maximum of two electrons in the first shell
2. A maximum of eight electrons in the second shell 
3. A maximum of eight electrons in the third shell
   
   

Octet rule

The octet rule specifies that atoms are stable when they have a 'full' outer shell . A full outer shell is equal to eight electrons. However, hydrogen and helium are exceptions of this rule as they only have one shell (the first shell). The first shell is full when it has two electrons. Therefore, a full outer shell for helium and hydrogen is two electrons. Group numbers in the periodic table tell you how many electrons are in the outer shell of an atom. 

Atoms with incomplete outer shells are unstable so they react to form full outer shells by either ionic or covalent bonding. Noble gases have a full outer shell, which means they are already stable. This is why noble gases do not react with other elements and exist only as single atoms (monatomic). Noble gases do not form molecules or ionic compounds - they are inert (unreactive).

Electron configurations of helium ($$He$$​), neon ($$Ne$$​) and Argon ($$Ar$$​)

​

Melting points, boiling points and densities

Going down Group $$0$$​, the relative atomic mass ($$A_r$$) increases. This is associated with a greater number of electrons. This increases the strength of forces between atoms going down the group. As a result, atoms lower down the group are packed more tightly together within a given volume. Hence, density increases going down Group $$0$$​. 

Stronger forces between atoms is also associated with higher melting and boiling points. This is because a higher amount of (heat) energy is required to break the forces.  As a result, melting and boiling points increase going down Group $$0$$.​

Example

Why does krypton have a higher melting point than neon?

Krypton is below neon in Group $$0$$​,  so has a greater relative atomic mass ($$A_r=84$$​), compared to neon ($$A_r=20$$​). This means there are stronger forces between krypton atoms, compared to neon atoms. These stronger forces require more energy to break. Krypton therefore has a higher melting point than neon.