The compound with molecular formula $$C_{6}H_{6}$$, which gives only one monobromo derivative and takes up four moles of hydrogen per mole for complete hydrogenation has ________ electrons.

For any hydrocarbon $$C_nH_m$$, the number of degrees of unsaturation (also called index of hydrogen deficiency, IHD) is calculated by the relation
$$\text{IHD}=\,\frac{(2n+2)-m}{2}\,.$$

Here $$n=6$$ and $$m=6$$, so
$$\text{IHD}=\,\frac{(2\times6+2)-6}{2}=\,\frac{14-6}{2}=4.$$ Each unit of IHD represents either one $$\pi$$-bond or one ring.

The statement says that the compound “takes up four moles of hydrogen per mole for complete hydrogenation”.
• One mole of $$H_2$$ adds across a double bond (or one of the two $$\pi$$ bonds in a triple bond).
• A ring does not consume $$H_2$$ during hydrogenation.

Since four moles of $$H_2$$ are absorbed, all four units of IHD must be $$\pi$$ bonds; there are no rings contributing to IHD. Thus the molecule contains four distinct $$\pi$$ bonds.

Each $$\pi$$ bond is formed by a pair of electrons, therefore the total number of $$\pi$$ (unsaturated) electrons is
$$4 \times 2 = 8.$$

Hence, the compound possesses 8 electrons in its $$\pi$$ system (unsaturated electrons).