Chemical periodicity of other elements — practice questions

Let the oxidation state of chromium be $x$. Oxygen almost always has an oxidation state of -2 in compounds. The overall charge of the ion is -2. Therefore, the sum of the oxidation states must equal -2. Equation: $2(x) + 7(-2) = -2$ $2x - 14 = -2$ $2x = 12$ $x = +6$

The oxidation state of chromium is +6.

1. Sodium chloride is a compound containing the $Na^+$ ion. The sodium ion has an electronic configuration of $1s^2 2s^2 2p^6$. It has no d-electrons, and its outer electron shell is full. Therefore, no electrons can be promoted by absorbing energy from the visible spectrum, and the solution is colourless.
2. Copper(II) chloride contains the $Cu^{2+}$ ion. The copper(II) ion is a transition metal ion with the electronic configuration $[Ar] 3d^9$. It has a partially filled d sub-shell.
3. In aqueous solution, the $Cu^{2+}$ ion is surrounded by ligands (water molecules), which causes its d-orbitals to split into different energy levels.
4. Electrons can be promoted from the lower energy d-orbitals to the higher energy d-orbitals by absorbing light from the red-orange part of the visible spectrum. The remaining light, which is blue-green, is transmitted, causing the solution to appear blue-green.