Both Ti and Hf behave as though they possess the same number of valence electrons. Which of the subshells in the electron configuration of Hf behave as valence orbitals?

1s

2s

2p

3s

3p

4s

3d

4p

5s

4d

5p

6s

4f

5d

Do the electron configuration of both atoms.

Ti:

Atomic number 22.

Abbreviated electron configuration: [Ar] 3d^2 4s^2

Valence electrons 4s^2

Hf:

Atomic number: 72

Abbreviated electron configuration: [Xe] 4f^14 5d^2 6s ^2

The subshells that behave as valence orbitals are, generally, those of the highest energy level.

In this case those are 6s.

Answer: 6s