Why Calcium is PARAMAGNETIC instead of fully filled orbitals?

Calcium is paramagnetic because it is a metal.

The of calcium is [Ar]4s².

You would expect calcium to be diamagnetic. An isolated calcium atom is as.

But solid calcium has a metallic structure. It consists of an array of cations immersed in a sea of electrons.

The electrons are no longer trapped in a single 4s orbital. Instead, they are in an extended electron cloud.

Half of the electrons have an "up" spin, and half have a "down" spin.

When we apply a magnetic field, some of the electrons are free to line up with the field, so calcium metal is paramagnetic.