Is an element is defined by the number of
neutrons its nucleus contains?

An observation that requires measurement is called quantitative observable or qualitative

Particles vibrate in a rigid structure and do not move relative to their neighbors.

What are the first two quantum numbers for the electrons located in subshell 4d? what are the first three quantum numbers for the electrons located in subshell 2s? how many electrons can be held in a sublevel l = 3? how many electrons can be held in the energy level n = 4? how many electrons in an atom can share the quantum numbers n = 4 and l = 3?