Meaning- "Sigma bonds can occur between any kind of atomic orbitals; the only requirement is that the atomic orbital overlap happens directly between the nuclei of atoms."?

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Answer 1 · 2017-06-12T15:59:45

It is saying that the bonding electrons must be directly between the nuclei of the bonding atoms.

Sigma (σ) bonds are formed by head-on overlapping of atomic orbitals.

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This puts most of the electron density along the internuclear axis between the two nuclei.

Here are some examples of σ bonds,

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Hydrogen has an $s-s$ $"s-s"$ σ bond, hydrogen fluoride has an $s-p$ $"s-p"$ σ bond, and fluorine has a $p-p$ $"p-p"$ σ bond.

Ethane Ethane
(From Google Sites)

Ethane has an ${sp}^{3} {-sp}^{3} l C-C$ $"sp"^3"-sp"^3color(white)(l) "C-C"$ σ bond and ${sp}^{3} -s C-H$ $"sp"^3"-s C-H"$ σ bonds.

You can also have $p-d$ $"p-d"$ and $d-d$ $"d-d"$ σ bonds.

d sigma d sigma
(Adapted from Boundless)

In all of these σ bonds, the bonding electrons are on the internuclear axis between the two nuclei.