Explain the following
(i) $C u^{+}$ is colourless and $c u^{2 +}$ is coloured,
(ii) $Z n$ shows only + 2 Oxidation state.

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Solution

(1) $C u^{+}$ is colourless and $c u^{2 +}$ is coloured : The colour of transition metal ions or their compound due to the partially filled $(n - 1) d$ orbitals, i.e. the unpaired electrons in $(n - 1) d$ orbitals. When white light ta on such a Metal compound some light energy corresponding to certain colouris absorbed and elections are excited to the set of higher energy. This type of promotion is called $d - d$ transition. The other colour of white light are transmitted which provide complementary coloursand compound appears coloured.
The transition metal ions which have completely filled orbitals or do not possess unpaired electron are colourless because there is no probability of $d - d$ transition.
(ii) $Z n$ shows only $+ 2$ oxidation state : $Z n$ represent only $+ 2$ Oxidationstate because due to participation of $n s$ and $(n - 1) d$ electrons inbonding, all transition elements show different oxidation states and form various compounds. Minimum oxidation state of these elements is $+ 2$ which is due to involvement of electrons of outer most orbit. Higher oxidation states of these elements are due to participation of d electrons of penultmateshall along with $n s$ electrons because the $s$ orbital of outer msot shell and $d$ orbital of penultimateshell arevery nearer to each other on energy scale. Therefore when some energy is supplied, electrons from $(n - 1) d$ subshell are also removed and different oxidation are found.

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