Explain why chromium has a higher melting point than scandium or zinc.

Provide a brief explanation for each of the following observed rankings:

i. Ranking based on increasing atomic radii Mo < Nb < Zr < Y.

ii. Ranking based on increasing density Y < Zr < Nb < Mo.

Rank the following transition metals in order of increasing atomic radius.

(a) Tc (b) Ru (c) Mo (d) Nb

Provide an explanation for the observed trend.

Considering measurements rounded to the nearest pm, the atomic radii of Nb (146 pm) and Ta (146 pm) are practically identical. Explain this observation.