Zinc oxide

Zn 1
O 1

Formula unit Lattice

Choose Lattice to see four unit cells. Then drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

2 atoms in one formula unit of an ionic lattice.

The drawing shows one formula unit of an ionic lattice, not a molecule; zinc oxide has no molecular shape.

IUPAC name: zinc oxygen(2-)
Molar mass: 81.379(21) g/mol
Atoms: 2
SMILES: [O-2].[Zn+2]
InChIKey: RNWHGQJWIACOKP-UHFFFAOYSA-N
PubChem CID: 3007857

Made of

Zn O

Strongly heated, zinc oxide turns yellow, and the colour fades again as it cools. At room temperature it is a white or yellowish-white powder of hexagonal crystals, according to the Hazardous Substances Data Bank (HSDB). Its ions are Zn²⁺ from zinc and O²⁻ from oxygen. The solid does not dissolve in water, which breaks it down only slowly. CAMEO Chemicals, a US hazard database, records that the trace going into solution is neutral.

Acids and strong alkalis do attack it. HSDB calls zinc oxide amphoteric: with acids it gives zinc salts, and with strong alkalis it gives zincates, whose negative ions carry the zinc. Zinc’s oxide and hydroxide behave like those of aluminium and iron, among them aluminium oxide and iron(III) oxide, on Chem1’s account. Mildly acidic solutions dissolve them all, while only concentrated hydroxide makes them react as acids. Zinc oxide plays that acid part with no proton of its own to hand over.

Structure and bonding

A zinc ion of charge +2 and an oxide ion of charge −2 sit side by side in the drawing, two separate pieces with no bond between them. The pair is one formula unit of the crystal, one ion of each kind, and not a particle that exists by itself.

Two zinc ions and two oxide ions are all that one unit cell holds, the cell being the smallest block that repeats through the crystal. The Lattice view draws four such cells together. Every zinc ion has four oxide neighbours, and every oxide ion has four zinc neighbours; Coordination picks out one zinc ion with all four. The ions are spheres sized to their ionic radii. Turn sets the block revolving slowly, and Rotate left and Rotate right step it round.

HSDB gives the arrangement as hexagonal, the wurtzite crystal structure. Zinc sits in group 12, outside the Group 1 and 2 metals for which Chem1 finds the ionic model a really good fit.

Where it turns up

Marco Polo recorded zinc oxide being used as medicine in Persia in the 13th century, the US Geological Survey reports. Today it goes into ointments, pastes, bandages and dental cements, and calamine lotion contains a pink powder of zinc oxide with a little ferric oxide. A US rule dating from 1977 lets cosmetics carry it, those meant for the area around the eyes included.

Factories put it into white paints, ceramics, enamels and porcelain, and into car tyres, white glue, matches and white printing inks. It also serves as a fungicide and a herbicide. Animals get it too: the World Health Organization’s veterinary drug classification files it among remedies for diarrhoea. Crude zinc oxide is even recovered from the dust of electric arc furnaces, on the US Geological Survey’s account.

Why it matters

In sunscreen, zinc oxide guards against sunburn by absorbing or blocking ultraviolet radiation, as Medical Subject Headings (MeSH) describes sunscreen agents. A coarse powder shows as a white film on the skin. As particles under 100 nanometres across, HSDB notes, it vanishes from view yet screens as well as before. The US Food and Drug Administration followed a 2019 proposed rule on sunscreens with a 2021 proposed order. It would treat sunscreens containing zinc oxide, titanium dioxide or both as generally recognised as safe and effective.

The US Geological Survey ranks zinc oxide first among zinc chemicals by production volume. In the French process, boiled zinc’s vapour burns in air, and the flame’s turbulence and the surplus air set the powder’s crystal form. Rubber took half of all use in 1986. There zinc oxide activates and speeds vulcanisation, which ties rubber’s polyisoprene chains together through pairs of sulfur atoms so they cannot slip past one another. In white paint it serves as the pigment in place of lead carbonate.

Properties

Properties of zinc oxide Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 3007857.
Property	Value	Unit	Source
Formula (Hill order)	OZn
Molar mass	81.379(21) g/mol	g/mol	IUPAC/CIAAW 2024
Zinc atoms	1
Oxygen atoms	1
PubChem molecular weight	81.4 g/mol	g/mol	PubChem

Safety

Breathed in as a fume, zinc oxide can cause metal fume fever, which Los Alamos also calls oxide shakes or zinc chills. NIOSH, the US federal workplace-safety institute, lists chills, fever, aching muscles, nausea and vomiting among its symptoms. Under EU harmonised classification the compound is very toxic to aquatic life, and the harm persists. The PubChem Laboratory Chemical Safety Summary and the NIOSH Pocket Guide card set out precautions and exposure limits.

Questions

Is zinc oxide sunscreen bad for coral reefs?

The National Ocean Service of the US National Oceanic and Atmospheric Administration (NOAA) counts nano zinc oxide among the sunscreen chemicals that can harm marine life. NOAA also rates mineral sunscreen, made without chemical ultraviolet filters, as a better option with fewer effects on aquatic organisms. Under the EU's harmonised classification, zinc oxide is very toxic to aquatic life, its effects lasting a long time.

Why is zinc oxide in diaper rash cream?

The US Food and Drug Administration (FDA) monograph for over-the-counter skin protectants lists zinc oxide among its active ingredients. A product built on it may say that it helps treat and prevent diaper rash. A skin protectant, in the FDA's definition, shields damaged or exposed skin and mucous membranes from harmful or irritating stimuli for a while. Zinc oxide also has some antiseptic action, according to its PubChem record.

What causes metal fume fever from zinc oxide?

It follows breathing in freshly formed zinc oxide fume, and its cause is not known for certain. One proposal has the fume set off a timed series of inflammatory events, with the signal IL-6 then reaching the blood and helping to drive the fever. Another puts it down to fever-raising substances released as cells burst. Fume grows less reactive as it ages, since its particles clump together and settle out of the air.