Question

What does it mean to say that a bond is polar? Give two examples of molecules with polar bonds. Indicate in your examples the direction of the polarity.

Short answer

A bond is considered polar when there is an unequal distribution of electrons between two atoms due to a difference in electronegativity. In a polar bond, the more electronegative atom has a partial negative charge, and the less electronegative atom has a partial positive charge. Two examples of molecules with polar bonds are HCl and H2O. In HCl, the polarity goes from the hydrogen (H) atom to the chlorine (Cl) atom. In H2O, the polarity is directed from each hydrogen (H) atom to the central oxygen (O) atom.

Step-by-step solution

Define Bond Polarity

Bond polarity refers to the unequal distribution of electrons between the two atoms in a bond, resulting from a difference in electronegativity between the bonded atoms. The more electronegative atom attracts the shared electrons, causing a partial negative charge on that atom, while the less electronegative atom has a partial positive charge.

Factors of Bond Polarity

One of the factors contributing to bond polarity is the difference in electronegativity between the bonded atoms. Electronegativity is a measure of the ability of an atom to attract electrons in a bond. A bond becomes more polar as the electronegativity difference between the two atoms increases.

Example 1: HCl (Hydrogen Chloride) Molecule

In the HCl molecule, the hydrogen (H) atom has an electronegativity value of 2.1, while the chlorine (Cl) atom has a value of 3.0. The electronegativity difference is 0.9, which indicates that the bond is polar. The more electronegative chlorine atom attracts the shared electrons, resulting in a partial negative charge (δ-) on the Cl atom and a partial positive charge (δ+) on the H atom. The direction of polarity in the HCl molecule is from the hydrogen atom to the chlorine atom.

Example 2: H2O (Water) Molecule

In the H2O molecule, the oxygen (O) atom has an electronegativity value of 3.4. The two hydrogen (H) atoms have an electronegativity value of 2.1. The electronegativity differences are 1.3, indicating that the O-H bonds are polar. The more electronegative oxygen atom attracts the shared electrons, resulting in a partial negative charge (δ-) on the O atom and a partial positive charge (δ+) on the H atoms. The direction of polarity in the H2O molecule is from each hydrogen atom to the central oxygen atom.

Key concepts

Electronegativity

Understanding electronegativity is fundamental when it comes to predicting and explaining bond polarity in molecules. Electronegativity can be thought of as the magnetic pull an atom exerts on electrons. It is a measure of an atom's ability to attract and hold onto electrons when it forms a chemical bond.

Atoms on the periodic table show a trend in electronegativity; it generally increases as you move from left to right across a period and decreases as you move down a group. This implies that elements like fluorine (F) have very high electronegativity values and tend to strongly attract electrons, while elements like cesium (Cs) have low electronegativity and weakly attract electrons.

The Pauling scale is commonly used to quantify electronegativity, with the most electronegative element, fluorine, set at 4.0. When two different atoms bond, the difference in their electronegativity values determines the bond’s polarity. If the difference is significant (greater than 0.5 on the Pauling scale), the bond is considered polar.

Polar Molecules

Molecules that have polar bonds do not always result in a polar molecule. The molecule's shape or geometry can influence its overall polarity. Polar molecules are those in which the distribution of electrons around the molecule is uneven, leading to a molecule that has electrically charged areas.

How Geometry Affects Polarity

For instance, if a molecule is symmetrical, like carbon dioxide (CO2), the polarities of the bonds may cancel out, resulting in a nonpolar molecule. Conversely, water (H2O) has a bent shape that prevents the bond polarities from canceling out, making it a polar molecule.

Polar molecules tend to have a slight electrical field due to the differences in charge distribution. This can lead to various intermolecular interactions like hydrogen bonding, which is a significant factor in the properties of substances like water, with implications for solubility, melting and boiling points.

Partial Charges

The concept of partial charges emerges when we talk about polar molecules. These partial charges are symbolized by δ+ (delta plus) and δ- (delta minus), indicating areas of relative positive and negative charge within the molecule. The unequal sharing of electrons in polar bonds creates these partial charges.

The greater the difference in electronegativity between the bonded atoms, the more pronounced the partial charges. For instance, in hydrochloric acid (HCl), the chlorine atom becomes δ- because it is more electronegative and attracts the shared electrons more strongly than hydrogen, leaving hydrogen with a δ+ charge.

Significance of Partial Charges

Partial charges are crucial when it comes to the molecule's reactivity and its interactions with other molecules. They are the reason why polar molecules can dissolve ionic compounds and why substances with similar polarities tend to be miscible (mix well) with each other.