Question

Using the molecular formula \(\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{Cl}_{2}\), draw the structural formula of (a) a molecule that is polar and \((\mathrm{b})\) a molecule that is nonpolar.

Short answer

The molecular formula is C2H4Cl2. The polar molecule's structural formula is: H | H--C--Cl | C--Cl | H The nonpolar molecule's structural formula is: H--C--Cl | H--C--H | Cl

Step-by-step solution

Draw the skeletal structures of the molecules

First, we should draw the skeletal structures for the two molecules, keeping in mind that carbon atoms can form 4 bonds, and hydrogen and chlorine atoms can form 1 bond, each.

Draw the structural formula of the polar molecule

To create a polar molecule with a molecular formula of C2H4Cl2, we can place the two chlorine atoms on one of the carbon atoms, as such: H | H--C--Cl | C--Cl | H This molecule has an asymmetrical arrangement of atoms, with both chlorine atoms on one side, providing a difference in electronegativity, resulting in a polar molecule.

Draw the structural formula of the nonpolar molecule

To create a nonpolar molecule with a molecular formula of C2H4Cl2, we can place the two chlorine atoms on opposite ends of the molecule, as such: H--C--Cl | H--C--H | Cl In this symmetrical arrangement, the electronegativity differences between the atoms are balanced or canceled out, thus creating a nonpolar molecule.

Key concepts

Polar Molecules

When we talk about polar molecules, we are referring to molecules that have an uneven distribution of electron density. This arises due to differences in electronegativity between bonded atoms. In simple terms, electronegativity is the ability of an atom to attract electrons. When an atom with higher electronegativity forms a bond with an atom with lower electronegativity, the electrons are not shared equally. This creates a dipole moment, with a partial positive charge on one end and a partial negative charge on the other end.

A real-world analogy could be a game of tug-of-war, where one team (the more electronegative atom) is stronger than the other, causing the rope (electrons) to be pulled more towards the stronger team, creating an imbalance. For example, in a water molecule (H_2O), oxygen is more electronegative than hydrogen, which makes water a polar molecule with oxygen having a slight negative charge and hydrogens a slight positive charge.

Nonpolar Molecules

Conversely, nonpolar molecules possess a uniform distribution of electron charge. This means they have either atoms with similar electronegativities or symmetrical structures that allow the different electronegativities to cancel each other out. Consequently, these molecules do not have a net dipole moment. Imagine that tug-of-war again, but this time both teams are equally strong, so the rope doesn't move to one side; this represents the equal sharing of electrons.

Common examples of nonpolar molecules include diatomic gases like nitrogen (N_2) and methane (CH_4). In the exercise given, by placing the chlorine atoms at opposite ends of the ethylene molecule (C_2H_4Cl_2), a nonpolar molecule is formed due to the balance of electronegativity across the molecule.

Structural Formula

The structural formula is a representation that shows not only the number of atoms but also their arrangement in space. It allows us to picture how the atoms are bonded to one another in a molecule. Like a blueprint in construction, a structural formula can give us valuable insights into the nature of the molecule, such as whether it's polar or nonpolar, and can help predict the properties and behaviors of the molecule.

In the context of the exercise, the structural formula for C_2H_4Cl_2 helps us understand why one arrangement could be polar while the other could be nonpolar. Creating diagrams of molecules enables students to visualize the three-dimensional arrangement of atoms and predict molecular polarity effectively.

Electronegativity

Electronegativity is a crucial concept in chemistry, indicating how strongly an atom can attract electrons in a chemical bond. Think of it as a measure of how 'hungry' an atom is for electrons. The Pauling scale is often used to quantify electronegativity, with fluorine being the most electronegative atom with a value of about 4.0. The greater the difference in electronegativity values between two atoms, the more polar the bond between them will be.

This concept is essential when trying to determine molecular polarity. For instance, when looking at the molecule C_2H_4Cl_2 in the exercise, the difference in electronegativity between carbon and chlorine leads to the formation of polar covalent bonds. However, as demonstrated in the exercise solutions, whether the entire molecule is polar depends on the geometric arrangement of these polar bonds.