Question

In each pair, choose the one with the more polar bonds. (Use the periodic table to answer the question.) (a) \(\mathrm{PCl}_{3}\) or \(\mathrm{AsCl}_{3}\) (b) \(\mathrm{SF}_{2}\) or \(\mathrm{GeF}_{4}\) (c) \(\mathrm{SiCl}_{4}\) or \(\mathrm{SCl}_{2}\) (d) \(\mathrm{SrO}\) or \(\mathrm{Sn} \mathrm{O}\)

Short answer

The compounds with more polar bonds are (a) \(\mathrm{PCl}_{3}\), (b) \(\mathrm{SF}_{2}\), (c) \(\mathrm{SCl}_{2}\), and (d) \(\mathrm{SrO}\).

Step-by-step solution

Understanding Polarity

Bond polarity is determined by the difference in electronegativity between the two atoms involved in the bond. The greater the difference, the more polar the bond. The periodic table trends show that electronegativity decreases from top to bottom within a group and increases from left to right across a period.

Compare Polar Bonds in PCl3 and AsCl3

Phosphorus (P) is above arsenic (As) in the periodic table, meaning P is more electronegative than As. Since both molecules have chlorine (Cl), which has the same electronegativity, the P-Cl bonds are more polar than the As-Cl bonds.

Compare Polar Bonds in SF2 and GeF4

Sulfur (S) is above germanium (Ge) in the periodic table, which means S is more electronegative than Ge. Therefore, the S-F bonds in SF2 are more polar than the Ge-F bonds in GeF4.

Compare Polar Bonds in SiCl4 and SCl2

Sulfur (S) is more electronegative than silicon (Si). Therefore, the S-Cl bonds in SCl2 are more polar than the Si-Cl bonds in SiCl4.

Compare Polar Bonds in SrO and SnO

Tin (Sn) is more electronegative than strontium (Sr). However, since oxygen (O) is much more electronegative than both Sr and Sn, the Sr-O bond will be more polar due to the larger difference in electronegativity between Sr and O compared to the difference between Sn and O.

Key concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry that describes how strongly an atom attracts electrons in a chemical bond. It's an abstract quantity that can't be measured directly, but scientists have developed scales to compare the relative electronegativities of different elements. One of the most commonly used is the Pauling scale.

Elements with high electronegativity, such as oxygen, fluorine, and chlorine, tend to pull shared electrons toward themselves more strongly than elements with lower electronegativity. This tug-of-war over electrons determines the polarity of the bond - when two atoms with differing electronegativities form a bond, the shared electrons are likely to be found closer to the atom with the higher electronegativity.

Periodic Table Trends

The periodic table is not just a layout of elements, it's a map of chemical properties, including electronegativity. There are trends in electronegativity that you can follow as you move across and down the table: electronegativity increases from left to right across a period, and decreases from top to bottom within a group.

Why does this trend exist? Across a period, atoms have more protons in their nuclei, which pulls electrons closer, enhancing electronegativity. Down a group, as you move to larger atoms with more electron shells, the distance between the nucleus and the valence electrons increases. This greater distance reduces the nucleus' hold on the shared electrons, decreasing electronegativity. Understanding these trends can help predict the behavior of elements in chemical reactions and is key to understanding polarity in compounds.

Polar Bonds

Polar bonds are the result of the interaction between atoms with different electronegativities. When such atoms link together, the electrons in the bond are not shared equally. This unequal sharing creates a dipole moment—a separation of electric charge leading to a molecule with a negatively charged end and a positively charged end.

For instance, in a hydrochloride acid (HCl) molecule, chlorine is more electronegative than hydrogen, which creates a polar bond with a partial negative charge on the chlorine atom and a partial positive charge on the hydrogen atom. Polar bonds are essential for properties like solubility, melting point, boiling point, and reactivity. They influence how molecules interact with one another and are central to the structure and function of a vast array of substances, from water to proteins in our body.