Question

AsCl5 is less stable than SbCl5 why

Answer 1

The pentachlorides of the elements above and below arsenic in group 15, phosphorus pentachloride and antimony pentachloride are much more stable and the instability of AsCl5 appears anomalous. The cause is believed to be due to incomplete shielding of the nucleus in the elements following the first transition series (i.e. gallium, germanium, arsenic, selenium and bromine) which leads to stabilisation of their 4s electrons making them less available for bonding. This effect has been termed d-block contraction and is similar to the f-block contraction normally termed the lanthanide contraction.

Answer 2

Due to incomplete shielding of the nucleus in the elements following the first transition series which leads to stabilisation of their 4s electrons making them less available for bonding.