The term 'valency' is best defined as an element's:

How many valence electrons are present in a neutral atom of Oxygen (atomic number 8)?

According to Hund's rule, electrons occupy degenerate orbitals singly before pairing to minimize electron–electron repulsion. Which of the following neutral atoms in its ground state possesses the highest number of unpaired electrons?

True or False: In neutral atoms, the $3d$ subshell is filled before the $4s$ subshell because it belongs to a lower principal energy level ($n=3$).

According to the Pauli exclusion principle, when two electrons occupy the same orbital, they must have __________ spins.