Gold at a glance
- Atomic number
- 79
- Symbol
- Au
- Atomic mass
- 196.97 u
- Category
- Transition metal
- Group / period
- 11 / 6
- Electron configuration
- [Xe] 4f 14 5d 10 6s 1
- State at 25 °C
- Solid
- Melting point
- 1064 °C
- Boiling point
- 2856 °C
- Density
- 19.28 g/cm³
- Electronegativity
- 2.54
- 1st ionization energy
- 890 kJ/mol
- Oxidation states
- +3, +1
- Discovered
- Antiquity
Atomic structure
A neutral gold atom has 79 protons and 79 electrons, and its most common isotope carries about 118 neutrons. One atom weighs about 3.27 × 10⁻²⁵ kg, so a mole of gold (6.022 × 10²³ atoms) weighs 196.97 g. That nucleus holds 1.49 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. Its symbol Au comes from aurum. The electrons fill 6 shells (K 2, L 8, M 18, N 32, O 18, P 1), which gives the configuration [Xe] 4f 14 5d 10 6s 1. As a d-block element, both its outer s electrons and its inner d electrons can take part in bonding, which is why it shows more than one oxidation state. Its configuration breaks the simple filling order: one electron moves so that a subshell becomes half-full or full, which is more stable. It has only one stable isotope, Au-197. No other element has a stable isotope with mass number 197, so Au-197 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) gold is a solid. It melts at 1064 °C and boils at 2856 °C, so it stays liquid across a 1792-degree range. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 19.28 g/cm³, 19.3 times as dense as water. Its atoms have a radius of about 135 pm (Platinum next door is 135 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 2.54 on the Pauling scale. Pulling off its outermost electron takes 890 kJ/mol, more than Silver above it (731 kJ/mol). A bond between gold and chlorine would be polar covalent, with an electronegativity difference of 0.62. It releases 223 kJ/mol when it gains an extra electron. Its common oxidation states are +3, +1, so in compounds you will usually meet it as Au³⁺ and Au⁺. Group 11 also holds Copper, Silver and Roentgenium, which share its outer-shell pattern.
Where it comes from
It makes up 0.004 mg/kg of Earth's crust, the 75th most abundant element there. It has been known since ancient times, one of only 11 elements people used before modern chemistry.