All practice pages/Interatomic theory/Elements
Element 24, a transition metal
The metal that keeps stainless steel stainless, puts the shine on chrome, and makes rubies red and emeralds green.
Period 4, group 6. The other transition metals are tinted, and every cell opens its own article.
Chromium has 24 electrons, one for every proton, so the atom carries no charge. Below are the same 24 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹
Short form: [Ar] 3d⁵ 4s¹, where [Ar] stands for the 18 electrons argon already has.
That list runs by shell. In the order the subshells fill it reads [Ar] 4s¹ 3d⁵, because a subshell fills by its energy, not by its shell number.
The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14. Shell 3 here is 3s² 3p⁶ 3d⁵, and 2 + 6 + 5 is 13.
Chromium sits in period 4 and has 4 shells. The period number counts the shells.
The outer shell holds 1 electron. Those are the ones chromium can lose, gain or share, so they are the ones that decide what it bonds with.
Like other transition metals, chromium loses its outer s electrons, and often some d electrons as well, to form positive ions, and it can form more than one kind.
Mined almost entirely as chromite, FeCr₂O₄, mostly in South Africa, Kazakhstan, India and Turkey. A trace of chromium in aluminum oxide makes ruby red, and in beryl it makes emerald green.
Louis-Nicolas Vauquelin found it in 1797 in crocoite, a bright orange lead mineral from Siberia, and isolated the metal the next year. He named it from the Greek chroma, color, because its compounds come in so many colors.
Most goes into stainless steel, which needs at least about 10.5% chromium to form the thin, self-healing oxide film that stops rust. Chrome plating protects and decorates car parts, faucets and tools. Chromium compounds also tan leather, color paints and glass, and preserve wood.
Chromium in the +6 state, as in chromates and dichromates, is toxic and causes cancer, so its use is tightly controlled. Chromium metal and chromium(III) compounds are far less harmful.