Final answer:
Scandium (Sc) is the transition metal with the smallest atomic mass; its noble-gas notation is [Ar]4s²3d²1.
Step-by-step explanation:
The transition metal with the smallest atomic mass is Scandium (Sc) which has an atomic number of 21. To write its noble-gas notation, we first identify the noble gas in the periodic table that comes before scandium, which is Argon (Ar), with an atomic number of 18. The electron configuration for Argon is [Ar]. Scandium has three more electrons than Argon, which fill into the 4s orbital and the 3d orbital. Therefore, the noble-gas notation for Scandium is [Ar]4s23d1.