[kennykwong]

i learnt in class that when going down the group 2, the atom will tend to form ionic bond rather than covalent bond because of the polarising power is decrease down the group 2 (or other cation).
however, i wonder if anyone can tell me WHEN it will start to form ionic bond. the when i refer to is in what condition for example when the ratio of the polarising power and polarisability is equal to what.
thanks for replying.

[Schrödinger]

First of all, no bond is completely ionic, although bonds can be 100% covalent (Cl₂, O₂, N₂, etc). Hence, there is no demarcation line as such. The ionic character or percentage increases as you move down the group for your cation. That's all you will be able to say. There are formulae to calculate %ionic character. You could define your own demarcation line for ionic by saying "I consider 80% ionic to be ionic". That's completely up to you.