A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in a carbonyl group with a carbon atom and an oxygen atom. Other common double bonds are found in azo compounds (N=N), imines (C=N) and sulfoxides (S=O). In skeletal formula the double bond is drawn as two parallel lines (=) between the two connected atoms; typographically, the equals sign is used for this.

Double bonds are stronger than single bonds and double bonds are also shorter. The bond order is two. Double bonds are also electron-rich, which makes them reactive.

The type of bonding can be explained in terms of orbital hybridization. In ethylene each carbon atom has three sp² orbitals and one p-orbital. The three sp² orbitals lie in a plane with 120° angles. The p-orbital is perpendicular to this plane. When the carbon atoms approach each other, two of the sp² orbitals overlap to form a sigma bond. At the same time, the two p-orbitals approach (again in the same plane) and together they form a pi-bond. For maximum overlap, the p-orbitals have to remain parallel, and, therefore, rotation around the central bond is not possible. This property gives rise to cis-trans isomerism. Double bonds are shorter than single bonds because p-orbital overlap is maximized.

Sir Harold (Harry) Walter Kroto, FRS (born 7 October 1939 as Harold Walter Krotoschiner), is a British chemist and one of the three recipients to share the 1996 Nobel Prize in Chemistry with Robert Curl and Richard Smalley.

He is the Francis Eppes Professor of Chemistry at the Florida State University, which he joined in 2004; prior to that he spent a large part of his career at the University of Sussex, where he holds an emeritus professorship.

Kroto was born in Wisbech, Cambridgeshire, England, to Edith and Heinz Krotoschiner, with his name being of Silesian origin. His father's family came from Bojanowo, Poland, and his mother's from Berlin, Germany. Both his parents were born in Berlin but came to Great Britain in the 1930s as refugees from the Nazis because his father was Jewish.

He was raised in Bolton, Lancashire, England, and attended Bolton School, where he was a contemporary of the highly acclaimed actor Ian McKellen. In 1955, the family name was shortened to Kroto.

As a child, he became fascinated by a Meccano set. Kroto credits Meccano — amongst other things — with developing skills useful in scientific research. He developed an interest in chemistry, physics, and mathematics in secondary school, and because his sixth form chemistry teacher (Harry Heaney – who subsequently became a University Professor) felt that the University of Sheffield had the best chemistry department in the United Kingdom, he went to Sheffield.