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.

• A. The observed ranking in atomic radii is caused by the increasing number of electrons as you go from left to right of the periodic table while the observed ranking in density is caused by the increasing mass but decreasing radius as you go from left to right of the periodic table. 
• B. The observed ranking in atomic radii is caused by the increasing nuclear charge as you go from left to right of the periodic table while the observed ranking in density is caused by the increasing mass and increasing radius as you go from left to right of the periodic table. 
• C. The observed ranking in atomic radii is caused by the increasing nuclear charge as you go from left to right of the periodic table while the observed ranking in density is caused by the increasing mass but decreasing radius as you go from left to right of the periodic table. 
• D. The observed ranking in atomic radii is caused by the increasing number of electrons as you go from left to right of the periodic table while the observed ranking in density is caused by the increasing mass and increasing radius as you go from left to right of the periodic table.

Answer: (B)