The element with the lowest density and smallest atomic weight is which one of the following: (a) aluminum, (b) argon, (c) helium, (d) hydrogen, or (e) magnesium.
The Correct Answer and Explanation is:
The correct answer is: (d) hydrogen.
Explanation
Hydrogen is the element with both the lowest density and the smallest atomic weight among the listed options. Let’s explore why this is the case.
Hydrogen (H) is the first element on the periodic table, with an atomic number of 1 and an atomic weight of approximately 1.008 atomic mass units (amu). It consists of only one proton and one electron (in its most common isotope, protium, it has no neutrons). This makes it the lightest element in existence.
In terms of density, hydrogen gas at standard temperature and pressure (STP) has a density of about 0.08988 grams per liter (g/L), which is the lowest of all elements. Its extremely low mass and high velocity at room temperature cause hydrogen molecules to occupy a larger volume compared to their mass, resulting in very low density.
Let’s compare this to the other elements listed:
- Helium is the next lightest element, with an atomic weight of about 4.0026 amu and a density of 0.1786 g/L at STP—still very low, but higher than hydrogen.
- Argon, a noble gas like helium, has a significantly higher atomic weight (39.95 amu) and density (~1.784 g/L).
- Aluminum (atomic weight ~26.98 amu) and magnesium (~24.31 amu) are metals and much denser in solid form (Al: ~2.70 g/cm³, Mg: ~1.74 g/cm³).
Thus, hydrogen’s unique position as the lightest and least dense element is due to its simple atomic structure and the physical properties of gases. These characteristics also make hydrogen a prime candidate for use in applications like fuel cells and rocket propulsion, where lightweight and high energy density are critical.
