Question

An oxide of copper which is red in colour has the formula:
A. CuO
B. $C{{u}_{2}}O$
C. $Cu{{O}_{2}}$
D. $C{{u}_{2}}{{O}_{2}}$

Answer 1

Hint: Copper is a d-block element and so it is also called transition element and is capable of d-d transitions. This characteristic of copper and other d-block elements is responsible for the colour of the compounds of those metals.

Complete step by step solution:
-The atomic number of copper is 29 and the electronic configuration of Cu can be represented as $\left[ Ar \right]3{{d}^{10}}4{{s}^{1}}$ . This is why copper belongs to d-block elements as it has such configuration. It loses electrons to become cation and so it is a metal.
-When 1 electron of copper is lost, it attains the covalency of +1 by removing the electrons of the outer 4s orbital and reaches the stability of a full-filled d-orbital.
-In +1 oxidation state, we can see that all the 10 electrons of the d-orbital are present in their shell and so there is no vacancy in the orbital when copper is in +1 oxidation state.
-Under these conditions, copper cannot show any transition and so its compounds will not be coloured due to lack of any unpaired electrons which are responsible for the transitions.
-Thus the compounds of Cu (+1) are generally colourless. When Cu loses one more electron from the d-subshells to become Cu (+2), then there are 9 electrons left in the orbital with 1 unpaired electron. Then it can show transition and thus forms coloured compounds.
-The colour of the compounds formed depend on the energy released during the transitions. Copper in its oxidation state of +2 generally forms compounds of colours within white and blue.
-There are exceptions in the copper (+1) compounds of oxide and sulphide. This is because they are solids and the arrangement of electrons depends on the crystal lattice of the compounds.
-In copper(I) oxide, the energy bandgap is 2.1 electron volts and the photons below this energy bandgap will not be absorbed and those above this will get absorbed.
-All the colours except red are absorbed by the oxide due to his energy band gap. Only red colour is not absorbed and so the colour of the compound becomes red as this is the only colour emitted by the oxide.
-The chemical formula for this copper oxide is $C{{u}_{2}}O$ and it is red in colour.

Therefore the correct option is B.

Note: The colour of the compound depends on the d-d transition but only in the complex formation. When we consider the compound in solid state, then the orientation of the compound molecules is different as they are present in the crystal lattice where every atom has a fixed position for itself. So colour formation in such compounds do not need d-d transitions.