Tutoring chemistry, the periodic table is important. The tutor mentions a peculiarity of it.
The transition metals are the elements in the middle of the periodic table, starting the fourth row from the top. Their first element is scandium (Sc).
On some periodic tables, you’ll notice that scandium (Sc, 21) is under group IIIB, then follows titanium (Ti, 22) under IVB, and so on. However, if you continue across, you’ll notice that copper (Cu, 29) is under IB, then zinc (Zn, 30) is under IIB. Why do IB and IIB appear at the right side, while IIIB appears at the left?
Beginning with scandium, the 3d subshell is being filled, but 4s, in the shell above, already is. Scandium is 3d14s2. At nickel (Ni, 28), the 3d subshell has 8 electrons, the 4s, 2. However, at copper (Cu, 29), the 3d subshell gains two electrons to reach 3d10, while 4s drops to 4s1. Zinc has 3d104s2. Perhaps it’s the refilling of the outer s subshell that defines IB and IIB at the right side of the transition metals.
In the next period, silver (Ag, 47) has 5s1, while cadmium (Cd, 48) has 5s2. However, the filling of 4d happened back at palladium (Pd, 46).
One more period down, gold (Au, 79) has 6s1, while mercury (Hg, 80) has 6s2.
Mortimer, Charles E. Chemistry, 6th ed. Belmont: Wadsworth, 1986.
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.