Question

What are some examples of a closed shell?

Answer 1

https://en.wikipedia.org/wiki/Open_shell

Filled valence shell (General Chemistry, atoms)

or

No unpaired electrons , i.e. all orbitals either doubly-occupied or empty (Quantum Chemistry, molecules with a singlet state, such as `""^1 Sigma^+`)

An example of a closed-shell atom is `O^(2-)`, `F^(-)`, `Ne`, etc., seeing as how they all have a full octet.

`N_2` is one example of a closed-shell molecule, since you can see in the diagram below that none of its electrons are unpaired.