Why so many ions are coloured
Transition metal ions with partly filled d orbitals absorb some visible light to promote an electron from one d level to another (a d–d transition). You see the colour that's left. Ions with d⁰ (Sc³⁺, Ti⁴⁺) or d¹⁰ (Zn²⁺, Cu⁺) have no such transition, so they're colourless.
Permanganate and dichromate are deeply coloured for a different reason, charge transfer from oxygen to the metal, since Mn(VII) and Cr(VI) are d⁰.
Check any formula's molar mass with the molar mass calculator, or see every element on the periodic table.
Questions students ask
What colour does barium give in the flame test?
Apple green. Calcium gives brick red, strontium crimson and sodium golden yellow.
Why is Zn²⁺ colourless?
Its 3d subshell is full (3d¹⁰), so no d–d transition can happen and no visible light is absorbed.
What is the formula of malachite?
CuCO₃·Cu(OH)₂, a basic copper carbonate and an ore of copper.
Which silver halide dissolves in ammonia?
AgCl dissolves easily, forming [Ag(NH₃)₂]⁺. AgBr dissolves partly and AgI hardly at all.