Question

The density of gaseous helium at \(25^{\circ} \mathrm{C}\) and normal atmospheric pressure is \(1.64 \times 10^{-4} \mathrm{~g} / \mathrm{mL}\). At the same temperature and pressure the density of argon gas is \(1.63 \times 10^{-3} \mathrm{~g} / \mathrm{mL}\). The mass of an atom of argon is almost exactly ten times the mass of an atom of helium. Provide a nanoscale explanation of why the densities differ as they do.

Short answer

Argon's higher atomic mass and similar volume occupation lead to greater density.

Step-by-step solution

Understanding Density

Density is a measure of mass per unit volume and is given by the formula \( \text{Density} = \frac{\text{Mass}}{\text{Volume}} \). In this context, we compare the densities of helium and argon gases.

Exploring Atomic Mass

The problem states that an atom of argon has almost ten times the mass of an atom of helium. This implies that on an individual atomic level, argon atoms are heavier than helium atoms.

Relationship between Atomic Mass and Density

Since density is mass per unit volume, if argon atoms are heavier (ten times the mass of helium) and assuming the volume occupied per atom at a given pressure and temperature is comparable, argon gas should be denser.

Analyzing Volume Occupied by Each Gas

Atoms in gases are separated by significant distances compared to their sizes. Argon, being a heavier atom, occupies a similar volume to helium under identical conditions (same temperature and pressure), due to its higher mass slightly more clustered spaces between atoms compared to helium.

Explaining Difference in Density

Because each argon atom is significantly heavier, in a given volume, the mass aggregate of argon is much higher than helium. This leads to argon having a higher density, which explains why argon's density \(1.63 \times 10^{-3} \mathrm{~g/mL}\) is greater than that of helium \(1.64 \times 10^{-4} \mathrm{~g/mL}\).

Nanoscale Perspective

On the nanoscale, fewer but heavier argon atoms compared to more but lighter helium atoms can occupy the same space, leading to a greater mass and density for argon.

Key concepts

Atomic Mass

Atomic mass is a fundamental concept in chemistry, referring to the mass of an atom. It is usually expressed in atomic mass units (amu), where one amu is defined as one-twelfth the mass of a carbon-12 atom. The atomic mass of an element depends on the number of protons and neutrons in its nucleus. For instance:

• Helium (He) has an atomic mass of approximately 4 amu, since it typically has 2 protons and 2 neutrons.
• Argon (Ar), on the other hand, has an atomic mass of about 40 amu, due to its 18 protons and 22 neutrons.

This difference in mass is crucial when it comes to understanding the properties of gases and their densities. Since each argon atom is roughly ten times heavier than a helium atom, argon naturally contributes more mass per atom when gases fill a container. This directly influences the density of a gas, as observed with helium and argon in similar conditions.

Nanoscale Explanation

On the nanoscale, atoms behave in intriguing ways that are essential for understanding the properties of gases. In a gaseous state, molecules are far apart from each other. However, the mass and volume relation of these atoms become significant on this tiny scale.

• Helium atoms, being lighter, spread out more freely to fill a space, resulting in less mass being concentrated in a given volume.
• Argon atoms, while being heavier, occupy similar volumes at a given temperature and pressure. Their increased mass results in more weight being packed into the same space.

This nanoscale understanding explains why gases with heavier atoms, like argon, exhibit higher densities than gases composed of lighter atoms, such as helium. Despite their atomic separation, the mass-to-volume ratio remains a pivotal factor, dictating the overall density regardless of how dispersed the atoms are.

Helium and Argon Comparison

When comparing helium and argon, several factors play a part in explaining why their densities differ under identical conditions. Both gases consist of atoms moving freely, especially at room temperature and atmospheric pressure.

• Argon is much heavier than helium, with each argon atom weighing about ten times as much as a helium atom.
• Despite this mass difference, both gases occupy similar volumes when in gaseous form. However, the result is that argon accumulates a higher mass per the same space, resulting in a greater density.

The dimension of atomic mass significantly impacts the density, given their vast differences in weight. Students exploring these concepts will understand that densities of gases become a balance of atomic mass and volume occupied, with argon naturally outpacing helium in heavier density due to its atomic structure and weight, which explains argon's higher density than helium.