ChemSphere
48Cd112.414

Cadmium

Transition metal · Period 5 · Group 12 · d-block

Brilliant pigments, rechargeable batteries and a toxic legacy

Atomic mass
112.414 u
State at 25 °C
Solid
Electronegativity
1.69
Melting point
321.1 °C
Boiling point
766.9 °C
Configuration
[Kr] 4d10 5s2
Building atom…
48 protons 64 neutrons 48 electrons

Nucleus drawn at reduced count for clarity. Drag to rotate.

Cadmium, explained

Video lesson in production

Bhavya’s explainer for this element will play here.

Overview

Cadmium is a soft, bluish-white metal in group 12, below zinc and above mercury. Its configuration, [Kr] 4d10 5s2, has a full d subshell that takes no part in bonding, so cadmium behaves like a main-group metal and forms compounds almost exclusively in the +2 state. Its chemistry closely resembles zinc's, which explains why the two occur together in ores and why cadmium is obtained as a by-product of zinc refining. The resemblance is also the root of cadmium's toxicity: the body takes it up through pathways meant for zinc and calcium, then holds it for decades.

Cadmium compounds have useful optical and electronic properties. Cadmium sulfide and selenide range from bright yellow through orange to deep red, and they were prized pigments for artists from the mid-19th century. Cadmium telluride absorbs sunlight efficiently and is used in thin-film solar panels, while nanocrystals of cadmium selenide, known as quantum dots, glow in colours set by their size. Cadmium-113 absorbs slow neutrons very strongly, so cadmium is used in reactor control rods. Its main traditional use, the nickel-cadmium battery, is in decline because of concerns about disposal.

Discovery

In 1817 Friedrich Stromeyer, an inspector of pharmacies in Göttingen, examined zinc carbonate that turned yellow when heated even though it contained no iron. He traced the colour to the oxide of a new metal. Karl Hermann found the same element independently around the same time. Stromeyer named it after cadmia, the Latin name for zinc carbonate ore (calamine), from the Greek kadmeia, linked to the legendary Cadmus, founder of Thebes.

Where it’s found
Cadmium occurs at about 0.1 parts per million in the crust, mainly as an impurity in zinc ores such as sphalerite, and nearly all of it is recovered during zinc refining.
What it’s used for
  • Nickel-cadmium batteries
  • Thin-film solar panels
  • Yellow and red pigments
  • Reactor control rods
  • Quantum-dot displays
  • Corrosion-resistant plating
In living things
Cadmium has no role in humans and is highly toxic, damaging the kidneys and bones. One marine diatom is known to use cadmium in place of zinc in an enzyme when zinc is scarce.
Safety
Cadmium and its compounds are toxic and carcinogenic; inhaling fumes from heated cadmium can be fatal, so work with it requires ventilation and strict controls.

How it compares

Where Cadmium sits among all 118 elements on the trends that shape the table.

  • Electronegativity1.69Pauling

    #45 of 100 elements with data

  • Ionization energy868kJ/mol

    #27 of 104 elements with data

  • Atomic radius155pm

    #40 of 95 elements with data

  • Electron affinity-68kJ/mol

    #101 of 109 elements with data

  • Density8.65g/cm³

    #51 of 114 elements with data

  • Melting point594K

    #78 of 110 elements with data

Density
8.65 g/cm³
Atomic radius
155 pm
Covalent radius
136 pm
Molar heat
26.02 J/(mol·K)
Appearance
silvery bluish-gray metallic
Discovered by
Karl Samuel Leberecht Hermann

Inside the atom

48 electrons arranged in 5 shells: 2 · 8 · 18 · 18 · 2.

Orbital diagram

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 · shorthand [Kr] 4d10 5s2

  1. 1s2
  2. 2s2
  3. 2p6
  4. 3s2
  5. 3p6
  6. 4s2
  7. 3d10
  8. 4p6
  9. 5s2
  10. 4d10

Each box is an orbital holding up to two electrons of opposite spin. Highlighted subshells hold the valence electrons that decide how Cadmium bonds.

Ionization energies

Energy to remove each electron in turn, on a log scale. Each electron is harder to remove than the last, because the remaining ones feel more of the nucleus.

Signature reactions

The chemistry Cadmium is known for.

  1. Precipitation

    Precipitating cadmium sulfide

    Bubbling hydrogen sulfide through a cadmium salt solution gives a bright yellow precipitate of cadmium sulfide. This colour is a classic test for cadmium and the basis of cadmium yellow pigment.

    Cd2+(aq)+H2S(g)CdS(s)+2H+(aq)

    Conditions: acidic aqueous solution

  2. Redox · Electrochemistry

    Nickel-cadmium cell discharge

    Cadmium is oxidized at the negative electrode while nickel oxyhydroxide is reduced at the positive. Because the solid products stay on the electrodes, the reaction can be reversed by charging hundreds of times.

    Cd(s)+2NiO(OH)(s)+2H2O(l)Cd(OH)2(s)+2Ni(OH)2(s)

    Conditions: alkaline KOH electrolyte, about 1.2 V

  3. Redox · Combustion

    Burning in air

    Heated cadmium burns to form brown cadmium oxide fumes. These fumes are the main hazard when cadmium-plated parts are welded or brazed.

    2Cd(s)+O2(g)2CdO(s)

    Conditions: heated above melting point (321 °C)

Where you’ll meet it

  • Nickel-cadmium battery

    Nickel-cadmium battery

    NiCd cells use a cadmium negative electrode and powered cordless tools for decades.

    Photo: Wikimedia Commons
  • Cadmium pigments

    Cadmium pigments

    Cadmium sulfide and selenide give artists' paints their brilliant yellows, oranges and reds.

    Photo: Wikimedia Commons
  • Greenockite

    Greenockite

    Greenockite, a yellow cadmium sulfide mineral, is one of the few true cadmium ores.

    Photo: Wikimedia Commons
  • Quantum dot

    Quantum dot

    Cadmium selenide quantum dots glow in colours that depend on the size of each nanocrystal.

    Photo: Wikimedia Commons

Research

No research on Cadmium yet

When Bhavya publishes a project that studies Cadmium, it will be linked here.

See all research