• Each carbon atom has four electrons in its outer shell (electronic configuration: 1s22s22p2)
• Carbon atoms share these four electrons in four covalent bonds with other atoms to achieve a?full outer shell configuration
• These electrons are found in orbitals within the respective atoms
• When forming a covalent bond, the orbitals overlap in such a way to form two types of bonds
  • Sigma bonds (σ)
  • Pi bonds (π)

• When carbon atoms use only three of their electron pairs to form a σ bond, each carbon atom will have a p orbital which contains?one?spare electron
• When the p orbitals of two carbon atoms overlap with each other, a π bond is formed (the π bond contains two electrons)
• The two orbitals that form the π bond lie above and below the plane of the two carbon atoms to maximise bond overlap

σ bonds

• Sigma?(σ) bonds are formed from the?end to end?overlap?of atomic orbitals
• s orbitals overlap this way as well as p orbitals

Sigma orbitals can be formed from the end to end overlap of s orbitals

• The electron density in a σ bond is symmetrical about a line joining the nuclei of the atoms forming the bond
• The pair of electrons is found between the nuclei of the two atoms
• The electrostatic attraction between the electrons and nuclei bonds the atoms to each other

Hydrogen

• The hydrogen atom has only one s orbital
• The s orbitals of the two hydrogen atoms will overlap to form a σ bond

π bonds

• Pi?(π) bonds are formed from the?sideways?overlap?of?adjacent?p orbitals
• The two lobes that make up the π bond lie?above?and?below?the?plane?of the σ bond
• This maximises overlap of the p orbitals
• A single π bond is drawn as?two electron clouds,?one arising from each lobe of the p orbitals
• The two clouds of electrons in a π bond represent?one?bond containing?two electrons

π orbitals can be formed from the sideways overlap of p orbitals

Ethene

• Each carbon atom uses?three?of its?four?electrons to form σ bonds
• Two σ bonds are formed with the hydrogen atoms
• One σ bond is formed with the other carbon atom
• The fourth electron from each carbon atom occupies a p orbital which overlaps?sideways?with another p orbital on the other carbon atom to form a π bond
• This means that the C-C is a?double bond: one σ and one π bond

Each carbon atom in ethene forms two sigma bonds with hydrogen atoms and one σ?bond with another carbon atom. The fourth electron is used to form a π?bond between the two carbon atoms