How many of the following the second element has higher density than the first one ? 
1) Ag, Au 2) Hf, Zn 3) Zn, Hg 4) Na, Hf 5) Ca, W 6) Ta, Nb 

Third series of transition elements has a great density than second row of transition series, group IA and group IIA elements because the atomic size is not changing that much but atomic number is increasing, thereby having a high density.

• Au has a high density than Ag because Au is in third transition series and Ag in the second transition series.
• Hg is in the third transition element series whereas Zn is in the first transition element series.
• Hf and W are transition metals whereas Na and Ca are group IA and group IIA elements respectively.