Gold (Au)

Gold is yellow because of relativity. In a gold atom the inner electrons move at a substantial fraction of the speed of light, which contracts the 6s orbital and shrinks the gap it has to jump, so gold absorbs blue light instead of ultraviolet and reflects the rest — silver, with the same outer configuration but lighter, stays white. It is extraordinarily malleable: one gram beats out to a sheet about half a square metre in area and 100 nanometres thick, thin enough to see green light through. All the gold ever mined would form a cube roughly 22 metres on a side.

Properties

Atomic number

79

Symbol

Au

Standard atomic weight

196.97 u

Category

Transition metal

Group

11

Period

6

Block

d-block

Electron configuration

[Xe] 6s¹ 4f¹⁴ 5d¹⁰

Electrons per shell

2, 8, 18, 32, 18, 1

Melting point

1,337.33 K — 1064.2 °C — 1947.5 °F

Boiling point

3,129 K — 2855.8 °C — 5172.5 °F

Density

19.282 g/cm³

Electronegativity

2.54 (Pauling)

Oxidation states

+3, +1

Discovered

Known since antiquity

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s¹ 4f¹⁴ 5d¹⁰

Noble gas shorthand

[Xe] 6s¹ 4f¹⁴ 5d¹⁰

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 18 · P: 1

What it is used for

• Jewellery and bullion, which is most of all gold ever mined
• Bonding wire and connector plating in electronics
• Infrared-reflecting coatings on spacecraft and astronaut visors
• Dental crowns and inlays
• Gold nanoparticles in lateral-flow tests — the red line on a pregnancy test

Anomalous electron configuration: [Xe] 6s¹ 4f¹⁴ 5d¹⁰ rather than the predicted 6s² 5d⁹.

Neighbours in the table

Group 11

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Gold sits in group 11 with copper, silver, roentgenium.