Which of the following elements has the greatest density at standard temperature and pressure (STP)?

• A. Iron (Fe)
• B. Lead (Pb)
• C. Gold (Au)
• D. Osmium (Os)

Answer: (D)

Verified step by step guidance

1. Understand that density is defined as mass per unit volume, given by the formula \(\text{density} = \frac{\text{mass}}{\text{volume}}\).
2. Recognize that the density of an element in its solid form depends on its atomic mass and how closely its atoms are packed in the crystal structure.
3. Look up or recall the densities of the given elements at standard temperature and pressure (STP): Iron (Fe), Lead (Pb), Gold (Au), and Osmium (Os).
4. Compare the densities by considering both atomic mass and atomic packing; Osmium is known to have one of the highest densities among naturally occurring elements due to its heavy atomic mass and tightly packed crystal lattice.
5. Conclude that Osmium (Os) has the greatest density at STP among the listed elements because it combines a high atomic mass with a very compact atomic arrangement.