Cadmium at a glance
- Atomic number
- 48
- Symbol
- Cd
- Atomic mass
- 112.41 u
- Category
- Transition metal
- Group / period
- 12 / 5
- Electron configuration
- [Kr] 4d 10 5s 2
- State at 25 °C
- Solid
- Melting point
- 321.1 °C
- Boiling point
- 766.9 °C
- Density
- 8.69 g/cm³
- Electronegativity
- 1.69
- 1st ionization energy
- 868 kJ/mol
- Oxidation states
- +2
- Discovered
- 1817
Atomic structure
A neutral cadmium atom has 48 protons and 48 electrons, and its most common isotope carries about 66 neutrons. One atom weighs about 1.87 × 10⁻²⁵ kg, so a mole of cadmium (6.022 × 10²³ atoms) weighs 112.41 g. That nucleus holds 1.38 neutrons for every proton. The electrons fill 5 shells (K 2, L 8, M 18, N 18, O 2), which gives the configuration [Kr] 4d 10 5s 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: Cd-106, Cd-108, Cd-110, Cd-111, Cd-112, Cd-114 and Cd-116. Its most common isotope, Cd-114, has the same mass number as Sn-114: isobars, nuclei of different elements with 114 nucleons each.
Physical properties
At room temperature (25 °C) cadmium is a solid. It melts at 321.1 °C and boils at 766.9 °C, so it stays liquid across a 445.8-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 8.69 g/cm³, 8.69 times as dense as water. Its atoms have a radius of about 155 pm (Silver next door is 160 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 1.69 on the Pauling scale. Pulling off its outermost electron takes 868 kJ/mol, less than Zinc above it (906 kJ/mol). A bond between cadmium and chlorine would be polar covalent, with an electronegativity difference of 1.5. Its common oxidation states are +2, so in compounds you will usually meet it as Cd²⁺. Group 12 also holds Zinc, Mercury 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 0.15 mg/kg of Earth's crust, the 66th most abundant element there. In the human body it accounts for about 0.00007 % of body mass. It was first identified in 1817, making it the 50th element to be discovered (counting the 11 known since antiquity together). When excited it glows at its own set of colours; its brightest visible lines are at 508.6 nm, 643.8 nm and 480.0 nm.