Just as individual bonds in a molecule are often polar, molecules as a whole are also often polar because of the net sum of individual bond polarities. There are three possible structures for substances with the formula C2H2Cl2, two of which are polar overall and one of which is not. (b) Which of the three structures is nonpolar, and which two are polar? Explain.

Verified step by step guidance

1. Step 1: Understand the concept of polarity. A molecule is polar if it has a net dipole moment. This happens when there is an imbalance in the electron distribution in the molecule, causing a region of partial positive charge and a region of partial negative charge.
2. Step 2: Identify the three possible structures for C2H2Cl2. They are 1,1-dichloroethene (Cl2C=CH2), cis-1,2-dichloroethene (ClHC=CHCl), and trans-1,2-dichloroethene (ClHC=CHCl).
3. Step 3: Analyze the polarity of each structure. In 1,1-dichloroethene, the two chlorine atoms are on the same carbon atom, creating a polar molecule due to the difference in electronegativity between carbon and chlorine. In cis-1,2-dichloroethene, the two chlorine atoms are on the same side of the molecule, also creating a polar molecule.
4. Step 4: In trans-1,2-dichloroethene, the two chlorine atoms are on opposite sides of the molecule. This arrangement allows the dipole moments of the C-Cl bonds to cancel each other out, resulting in a nonpolar molecule.
5. Step 5: Therefore, the nonpolar structure is trans-1,2-dichloroethene, and the polar structures are 1,1-dichloroethene and cis-1,2-dichloroethene.

Key Concepts

Molecular Polarity

Molecular polarity refers to the distribution of electrical charge across a molecule, which determines whether it has a positive or negative end. This is influenced by the presence of polar bonds and the molecular geometry. A molecule is polar if it has a net dipole moment, meaning the vector sum of all bond dipoles does not cancel out.

Bond Polarity

Bond polarity arises from the difference in electronegativity between two atoms in a bond. When one atom is more electronegative, it attracts the shared electrons more strongly, creating a dipole moment. In C2H2Cl2, the C-Cl bonds are polar due to chlorine's higher electronegativity compared to carbon, affecting the overall polarity of the molecule.

Molecular Geometry

Molecular geometry describes the three-dimensional arrangement of atoms in a molecule. The shape of a molecule, determined by the VSEPR theory, influences how bond dipoles add up. For C2H2Cl2, different arrangements of chlorine and hydrogen atoms lead to distinct geometries, which can result in either a polar or nonpolar molecule depending on the symmetry of the structure.