Question

The physical properties of ${\mathrm{D}}_2\mathrm{O}$ differ from those of ${\mathrm{H}}_2\mathrm{O}$ because (a) D has a different electron configuration than O. (b) $\mathrm{D}$ is radioactive. (c) D forms stronger bonds with $\mathrm{O}$ than $\mathrm{H}$ does. (d) D is much more massive than H.

Short answer

The physical properties of D2O differ from those of H2O primarily because deuterium (D) is much more massive than hydrogen (H), as stated in option (d). The difference in mass between these isotopes affects properties like boiling point, density, and viscosity.

Step-by-step solution

Understanding the Molecules in Question

In this exercise, we are comparing two similar molecules, D2O (deuterium oxide) and H2O (water). These molecules are isomers and are chemically similar but have different physical properties due to the difference in the isotopes of hydrogen that are bonded with oxygen. In D2O, both hydrogens are replaced with deuterium, which is an isotope of hydrogen with an additional neutron.

Evaluating Option (a)

Option (a) states that D has a different electron configuration than O. However, electron configurations refer to the arrangement of electrons around the nucleus of an atom, not the isotopes or the atomic mass. The main difference between D2O and H2O is the presence of deuterium in D2O, which does not change the electron configuration. So, this option is incorrect.

Evaluating Option (b)

Option (b) states that D is radioactive. This is also incorrect. Deuterium is a stable isotope of hydrogen and is not radioactive.

Evaluating Option (c)

Option (c) states D forms stronger bonds with O than H does. However, although there might be some differences in bond strength between H and D due to the larger mass of deuterium, this is not the primary reason why the physical properties of D2O and H2O are different. The main difference arises from the mass difference between these isotopes, affecting physical properties like boiling point, density, and viscosity. So, this option is not the right answer.

Evaluating Option (d)

Option (d) states D is much more massive than H. This is the correct answer. Deuterium is about twice as massive as hydrogen due to the presence of an additional neutron. This difference in mass leads to different physical properties in D2O compared to H2O, such as a higher boiling and melting point, higher density, and different viscosity.

Key concepts

Deuterium

Deuterium is a fascinating isotope of hydrogen. Unlike regular hydrogen, which has only one proton in its nucleus, deuterium contains one proton and one neutron. This gives deuterium a greater mass compared to normal hydrogen.

• Deuterium is stable and non-radioactive, making it completely different from radioactive forms of hydrogen.
• Because it is an isotope, deuterium has the same number of electrons as regular hydrogen, meaning it shares the same electron configuration. This means its chemical properties are similar, but not identical to hydrogen.

Deuterium is used in various applications, such as in nuclear reactors and some scientific experiments, due to its stable nature.

Physical properties

The physical properties of a substance determine how it behaves under certain conditions, like temperature and pressure. When considering substances like water (${\text{H}}_2\text{O}$) and heavy water (${\text{D}}_2\text{O}$), these properties can differ noticeably.

In the case of ${\text{D}}_2\text{O}$ and ${\text{H}}_2\text{O}$, despite being chemically similar, their physical properties differ because of the isotopic composition. The presence of deuterium in ${\text{D}}_2\text{O}$ instead of regular hydrogen causes these differences. For example:

• Boiling and melting points: ${\text{D}}_2\text{O}$ has higher boiling and melting points compared to ${\text{H}}_2\text{O}$.
• Density: ${\text{D}}_2\text{O}$ is denser than ${\text{H}}_2\text{O}$, giving it the term "heavy water."
• Viscosity: ${\text{D}}_2\text{O}$ is slightly more viscous than ${\text{H}}_2\text{O}$.

These differences arise because the deuterium atom is more massive than regular hydrogen, impacting how ${\text{D}}_2\text{O}$ molecules interact with each other.

Mass difference

The concept of mass difference between isotopes is key to understanding their impact on physical properties. In this case, deuterium is almost twice as heavy as regular hydrogen because it includes an extra neutron in its nucleus.

This increase in mass directly affects the physical behavior of the molecules it forms, like in ${\text{D}}_2\text{O}$. Here’s how it matters:

• Larger mass typically results in increased intermolecular forces. This means that more energy, or heat, is required to change states, like from liquid to gas. Hence, ${\text{D}}_2\text{O}$ has a higher boiling point than ${\text{H}}_2\text{O}$.
• Density is another factor influenced by mass. Since each molecule of ${\text{D}}_2\text{O}$ is heavier, the substance overall becomes denser compared to ${\text{H}}_2\text{O}$.

Understanding mass difference helps explain why two seemingly similar substances, such as ${\text{H}}_2\text{O}$ and ${\text{D}}_2\text{O}$, exhibit various physical properties.