Mercury at a glance
- Atomic number
- 80
- Symbol
- Hg
- Atomic mass
- 200.59 u
- Category
- Transition metal
- Group / period
- 12 / 6
- Electron configuration
- [Xe] 4f 14 5d 10 6s 2
- State at 25 °C
- Liquid
- Melting point
- -38.83 °C
- Boiling point
- 356.7 °C
- Density
- 13.53 g/cm³
- Electronegativity
- 2
- 1st ionization energy
- 1007 kJ/mol
- Oxidation states
- +2, +1
- Discovered
- Antiquity
Atomic structure
A neutral mercury atom has 80 protons and 80 electrons, and its most common isotope carries about 122 neutrons. One atom weighs about 3.33 × 10⁻²⁵ kg, so a mole of mercury (6.022 × 10²³ atoms) weighs 200.59 g. That nucleus holds 1.52 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. Its symbol Hg comes from hydrargyrum, meaning "liquid silver". The electrons fill 6 shells (K 2, L 8, M 18, N 32, O 18, P 2), which gives the configuration [Xe] 4f 14 5d 10 6s 2. 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. It has 7 stable isotopes: Hg-196, Hg-198, Hg-199, Hg-200, Hg-201, Hg-202 and Hg-204. No other element has a stable isotope with mass number 202, so Hg-202 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) mercury is a liquid. It melts at -38.83 °C and boils at 356.7 °C, so it stays liquid across a 395.6-degree range. To freeze it has to be cooled below -38.83 °C, colder than the freezing point of water. Only 11 elements have a lower melting point. Its density is 13.53 g/cm³, 13.5 times as dense as water. Its atoms have a radius of about 150 pm (Gold next door is 135 pm). In the solid, its atoms pack into a rhombohedral lattice.
Chemical behaviour
Its electronegativity is 2 on the Pauling scale. Pulling off its outermost electron takes 1007 kJ/mol, more than Cadmium above it (868 kJ/mol). A bond between mercury and chlorine would be polar covalent, with an electronegativity difference of 1.2. Its common oxidation states are +2, +1, so in compounds you will usually meet it as Hg²⁺ and Hg⁺. Group 12 also holds Zinc, Cadmium and Copernicium, which share its outer-shell pattern. Handle with care: as a pure element it is toxic.
Where it comes from
It makes up 0.085 mg/kg of Earth's crust, the 67th most abundant element there. 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 435.8 nm, 546.1 nm and 404.7 nm.