Copper at a glance
- Atomic number
- 29
- Symbol
- Cu
- Atomic mass
- 63.546 u
- Category
- Transition metal
- Group / period
- 11 / 4
- Electron configuration
- [Ar] 3d 10 4s 1
- State at 25 °C
- Solid
- Melting point
- 1085 °C
- Boiling point
- 2562 °C
- Density
- 8.96 g/cm³
- Electronegativity
- 1.9
- 1st ionization energy
- 746 kJ/mol
- Oxidation states
- +2, +1
- Discovered
- Antiquity
Atomic structure
A neutral copper atom has 29 protons and 29 electrons, and its most common isotope carries about 34 neutrons. One atom weighs about 1.06 × 10⁻²⁵ kg, so a mole of copper (6.022 × 10²³ atoms) weighs 63.546 g. That nucleus holds 1.17 neutrons for every proton. Its symbol Cu comes from cuprum. The electrons fill 4 shells (K 2, L 8, M 18, N 1), which gives the configuration [Ar] 3d 10 4s 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 2 stable isotopes: Cu-63 and Cu-65. No other element has a stable isotope with mass number 63, so Cu-63 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) copper is a solid. It melts at 1085 °C and boils at 2562 °C, so it stays liquid across a 1477-degree range. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 8.96 g/cm³, 8.96 times as dense as water. Its atoms have a radius of about 135 pm (Nickel next door is 135 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 1.9 on the Pauling scale. Pulling off its outermost electron takes 746 kJ/mol. A bond between copper and chlorine would be polar covalent, with an electronegativity difference of 1.3. It releases 119 kJ/mol when it gains an extra electron. Its common oxidation states are +2, +1, so in compounds you will usually meet it as Cu²⁺ and Cu⁺. Group 11 also holds Silver, Gold and Roentgenium, which share its outer-shell pattern.
Where it comes from
It makes up 60 mg/kg of Earth's crust, the 26th most abundant element there. In the human body it accounts for about 0.0001 % of body mass. It has been known since ancient times, one of only 11 elements people used before modern chemistry. When excited it glows at its own set of colours; its brightest visible lines are at 521.8 nm, 515.3 nm and 510.6 nm. In a flame test its compounds colour the flame blue-green.