Question

For each pair of molecules decide which molecule is polar and explain why it is polar while the other is not. (a) \(\mathrm{SO}_{2}, \mathrm{CO}_{2}\) (c) \(\mathrm{SeCl}_{2}, \mathrm{BeCl}_{2}\) (b) \(\mathrm{SO}_{2}, \mathrm{SO}_{3}\) (d) \(\mathrm{CH}_{4}, \mathrm{CH}_{3} \mathrm{I}\)

Short answer

SO2, SeCl2, and CH3I are polar because of a difference in electronegativity between the atoms and the asymmetrical arrangement of these atoms. Whereas, CO2, BeCl2, SO3 and CH4 are non-polar due to the symmetrical arrangement of the atoms causing the polarities of the bonds to cancel out.

Step-by-step solution

Determine Polarity of SO2 and CO2

**SO2(Sulfur Dioxide):** Oxygen is more electronegative than sulfur, so the electron pair is pulled closer to the oxygen(s). Add to that the bent shape of SO2, causes the molecule to be polar. \n\n**CO2(Carbon Dioxide):** Although oxygen is more electronegative than carbon which would cause polarity, CO2 is a linear molecule, meaning that the polarities of the two C=O bonds are equal and opposite, effectively cancelling out. So, CO2 is non-polar.

Determine Polarity of SeCl2 and BeCl2

**SeCl2(Selenium Dichloride):** Chlorine is more electronegative than selenium and the molecule is also bent. Hence, the molecule is polar. \n\n**BeCl2(Beryllium Dichloride):** Chlorine is more electronegative than beryllium. However, BeCl2 is a linear molecule which means the polarities from the two Be-Cl bonds cancel each other out. So, BeCl2 is non-polar.

Determine Polarity of SO2 and SO3

**SO2(Sulfur Dioxide):** As mentioned earlier, the bent shape and difference in electronegativities between sulfur and oxygen makes SO2 a polar compound. \n\n**SO3(Sulfur Trioxide):** Even though oxygen is more electronegative than sulfur, SO3 is a trigonal planar molecule. Therefore, the dipoles in the S-O bonds cancel out, leading SO3 to be a non-polar molecule.

Determine Polarity of CH4 and CH3I

**CH4(Methane):** Carbon and hydrogen have almost the same electronegativities so there's no polar bonds in the molecule. Even though there's four C-H bonds, the tetrahedral arrangement of the methane molecule causes the bond dipoles to cancel, making it non-polar. \n\n**CH3I(Methyl iodide):** Carbon is less electronegative than iodine and the structure of CH3I isn't symmetrical. Therefore, the molecule has a net dipole and is polar.

Key concepts

Electronegativity

Electronegativity refers to the tendency of an atom to attract a shared pair of electrons in a chemical bond. Imagine it like a game of tug-of-war between atoms over electrons.
Some atoms have a stronger attraction or pull for the electrons due to their higher electronegativity. You can think of these atoms as the stronger players in the game.
For example, in the molecule sulfur dioxide ( SO_2 ), oxygen is more electronegative than sulfur. This uneven distribution of electron density leads to a separation of charge, contributing to SO_2 being polar.

• Oxygen, chlorine, and fluorine generally have high electronegativities.
• Elements like carbon and sulfur have moderate electronegativities.
• Elements like beryllium and iodine have lower electronegativities.

Electronegativity differences create polar bonds especially when you pair high electronegativity atoms with lower ones. In contrast, methane ( CH_4 ) has carbon and hydrogen, two atoms with nearly equal electronegativities, which results in non-polar bonds.

Molecular Geometry

The shape of a molecule significantly influences its polarity. Molecular geometry determines how bond polarities within a molecule add up or cancel out.
There are many types of molecular shapes that affect whether a molecule is polar or not:

• Bent Shape: Molecules like SO_2 and SeCl_2 are polar due to their bent geometry. The angles within the molecule cause the bond dipoles not to cancel out, leading to a net dipole moment.
• Linear Shape: Examples include CO_2 and BeCl_2, where a linear shape ensures the polarities cancel each other out, making them non-polar.
• Tetrahedral Shape: As seen in methane, the symmetrical tetrahedral shape helps in balancing the bond polarities, leading to non-polarity.
• Trigonal Planar Shape: Like in SO_3, this shape allows the dipoles to cancel out thus making it non-polar.

Understanding molecular geometry helps in predicting the overall molecule polarity, as symmetry often plays a crucial role in determining if the polarities cancel or stack.

Bond Polarity

Bond polarity arises when there's a difference in electronegativity between the atoms in a bond, leading to unequal sharing of electrons. You can imagine it as one atom having more pull on the electrons than the other.
This uneven sharing causes a dipole moment, where one end of the bond becomes slightly negative, and the other becomes slightly positive.
The result of these dipole moments not canceling each other out will lead to a polar molecule. Let's look at some examples:

• In CH_3I, carbon has a lower electronegativity than iodine, creating a positive dipole on the iodine side. The molecule's overall asymmetry maintains the dipole moment, making it polar.
• Conversely, CH_4 has all bonds ( C-H ) nearly equal in electronegativity with a symmetric geometry, leading to a cancellation of any dipole and thus a non-polar result.

So while bond polarity is about the uneven electronegativity, the whole molecule’s polarity depends on whether these bond polarities work together or against each other in the molecular geometry.