Question

Rank the following molecules in order of increasing polarity: \(\mathrm{H}_{2} \mathrm{~S}, \mathrm{H}_{2} \mathrm{Se}\), and \(\mathrm{H}_{2} \mathrm{O}\).

Short answer

\text{H}_2 \text{Se} < \text{H}_2 \text{S} < \text{H}_2 \text{O}

Step-by-step solution

Understand Polarity

Polarity of a molecule is determined by its dipole moment, which depends on the differences in electronegativity between the atoms and the shape of the molecule. Molecules with larger differences in electronegativity and asymmetric shapes tend to be more polar.

Determine Molecular Geometry

All three molecules \( \text{H}_2 \text{S}, \text{H}_2 \text{Se}, \text{H}_2 \text{O} \) have a bent shape due to the presence of two lone pairs on the central atom.

Compare Electronegativity Values

Electronegativity decreases down Group 16 in the periodic table. The electronegativity values are: Oxygen (3.44), Sulfur (2.58), Selenium (2.55). Therefore, \( \text{H}_2 \text{O} \) is the most electronegative, followed by \( \text{H}_2 \text{S} \) and then \( \text{H}_2 \text{Se} \).

Rank the Molecules by Polarity

Based on the electronegativity differences and the molecular geometry, \( \text{H}_2 \text{O} \) will be the most polar because of the greatest electronegativity difference and strong dipole moment. \( \text{H}_2 \text{S} \) will be next, followed by \( \text{H}_2 \text{Se} \).

Final Ranking

Therefore, the molecules ranked in order of increasing polarity are: \( \text{H}_2 \text{Se} < \text{H}_2 \text{S} < \text{H}_2 \text{O} \).

Key concepts

Electronegativity

Electronegativity is a chemical property that describes the ability of an atom to attract electrons towards itself. It is a crucial factor in determining the polarity of a molecule. The higher the electronegativity of an atom, the more it attracts electrons. In the context of Group 16 elements such as oxygen (O), sulfur (S), and selenium (Se), electronegativity decreases as you move down the group in the periodic table.

• Oxygen has an electronegativity of 3.44.
• Sulfur has an electronegativity of 2.58.
• Selenium has an electronegativity of 2.55.

Differences in electronegativity between atoms in a molecule lead to partial charges, creating dipole moments. Thus, understanding the electronegativity of each atom can help predict the overall polarity of the molecule.

Molecular Geometry

Molecular geometry is essential for determining the shape of a molecule, which in turn affects its polarity. The shape of a molecule is influenced by the number of bonds and lone pairs around the central atom. For example, the molecules \(\text{H}_2 \text{O}\), \(\text{H}_2 \text{S}\), and \(\text{H}_2 \text{Se}\), all have a bent shape due to the presence of two lone pairs on the central atom. This bent shape is critical because it allows the molecule to have regions of positive and negative charge, leading to an overall dipole moment.
Remember, shapes like linear, tetrahedral, or trigonal planar can also affect how charges are distributed within a molecule. However, for bent shapes of Group 16 elements mentioned here, the geometry helps to create asymmetric charge distribution, thus contributing to molecular polarity.

Dipole Moments

Dipole moments are vectors that measure the separation of positive and negative charges in a molecule. They are a product of the charge magnitude and the distance between charges. Dipole moments are crucial for understanding molecular polarity as they indicate how much the electrons are pulled toward one end of the molecule.
The larger the difference in electronegativity between the atoms, the larger the dipole moment. For water \(\text{H}_2 \text{O}\), the high electronegativity difference between hydrogen and oxygen results in a strong dipole moment. This makes \(\text{H}_2 \text{O}\) the most polar among the molecules listed. Conversely, \(\text{H}_2 \text{Se}\) has the smallest dipole moment due to a smaller electronegativity difference, making it the least polar.

Group 16 Elements

Group 16 elements are also known as the chalcogens and include oxygen, sulfur, and selenium. These elements exhibit interesting properties that influence the behavior of the molecules they form:

• As you move down Group 16, the size of the atoms increases, and the electronegativity decreases.
• Oxygen, being the most electronegative, form strong polar bonds, whereas selenium forms the least polar bonds within the group.
• The chemical properties, such as the formation of two bonds and the presence of lone pairs, remain consistent, giving rise to similar molecular shapes across the group.

Understanding the periodic trends of Group 16 elements helps predict and rationalize the observed polarity of their hydrides, such as \(\text{H}_2 \text{O}\) (water), \(\text{H}_2 \text{S}\) (hydrogen sulfide), and \(\text{H}_2 \text{Se}\) (hydrogen selenide).