Which element has the lowest density among all known elements?

• A. Helium
• B. Oxygen
• C. Lithium
• D. Hydrogen

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. Recall that among elements, density depends on atomic mass and how closely atoms pack in a given state (solid, liquid, or gas).
3. Consider the physical states of the elements at standard conditions: Helium, Oxygen, and Hydrogen are gases, while Lithium is a solid.
4. Recognize that hydrogen, being the lightest element with the smallest atomic mass, has the lowest density among all elements, especially in its gaseous form.
5. Therefore, by comparing atomic masses and states, conclude that hydrogen has the lowest density among the given options and all known elements.