Orbital overlap was an idea first introduced by Linus Pauling

Linus Pauling

Linus Carl Pauling was an American chemist, biochemist, peace activist, author, and educator. He was one of the most influential chemists in history and ranks among the most important scientists of the 20th century...

 to explain the molecular bond angles observed through experimentation and is the basis for the concept of orbital hybridisation

Orbital hybridisation

In chemistry, hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules. It is an integral part...

. s orbitals are spherical and have no directionality while p orbitals are oriented 90° to one another. A theory was needed therefore to explain why molecules such as methane (CH₄) had observed bond angles of 109.5°. Pauling's theory was that bonds are created by the overlap of adjacent atomic orbitals of two atoms. The greater the overlap of the two orbitals, the stronger the bond. The resulting bond is known as an sp hybrid and it exhibits strength greater than that of an s or p orbital.