Question

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."?

Answer 1

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

Explanation:

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

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

Here are some examples of σ bonds,

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

(From Google Sites)

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

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

(Adapted from Boundless)

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