Question

Describe a polar molecule.

Short answer

A polar molecule consists of two or more atoms bonded together with polar covalent bonds due to differences in electronegativity, resulting in an unequal sharing of electrons. This creates a dipole moment, where one side of the molecule has a partial positive charge, and the other side has a partial negative charge. An example of a polar molecule is water (H2O), where the oxygen atom is more electronegative than the hydrogen atoms, causing the oxygen to have a partial negative charge and the hydrogen atoms to have partial positive charges. The bent geometry of the water molecule ensures an uneven distribution of these charges, making water a polar molecule.

Step-by-step solution

Define a molecule

A molecule is the smallest particle of a substance that has the chemical properties of that substance. Molecules consist of two or more atoms bonded together through chemical bonds, such as covalent, ionic, or metallic bonds.

Introduce the concept of polarity

Polarity is a property of molecules that arises due to differences in electronegativity between the bonded atoms. Electronegativity is the ability of an atom to attract electrons towards itself in a chemical bond. When atoms with different electronegativities form a bond, the electrons are unequally shared, causing one side of the molecule to be slightly more negative and the other side more positive. This results in a dipole moment, which is a measure of the polarity of the molecule.

Describe the characteristics of a polar molecule

A polar molecule has the following characteristics: 1. The molecule contains polar covalent bonds, meaning that the electrons are not shared equally between the bonded atoms due to differences in their electronegativities. 2. The molecule has an overall dipole moment, where one side of the molecule has a partial positive charge and the other has a partial negative charge.

Provide an example of a polar molecule

An example of a polar molecule is water (H2O). In a water molecule, two hydrogen atoms are covalently bonded to an oxygen atom. Oxygen is more electronegative than hydrogen, which means it attracts the shared electrons in the bond more strongly. As a result, the oxygen atom has a partial negative charge, while the two hydrogen atoms have partial positive charges. The bent geometry of the water molecule, with an angle of 104.5° between the hydrogen-oxygen-hydrogen atoms, ensures that these charges are not evenly distributed, resulting in an overall dipole moment and making water a polar molecule.

Key concepts

Chemical Bonds

Chemical bonds are the fundamental forces that hold atoms together within a molecule. There are three primary types of bonds: covalent, ionic, and metallic. Covalent bonds form when two atoms share one or more pairs of electrons, enabling them to complete their outer shells and become more stable. Ionic bonds, on the other hand, are formed when electrons are transferred from one atom to another, resulting in positively and negatively charged ions that attract each other. Metallic bonds involve the sharing of free electrons among a lattice of metal atoms. In covalent bonds, the sharing can be either equal or unequal, depending on the atoms involved. If the sharing is unequal due to differences in electronegativity, this can lead to the creation of polar molecules, with slightly charged positive and negative ends.

Electronegativity

Electronegativity refers to the ability of an atom to attract electrons towards itself when forming a chemical bond. This is a crucial concept in understanding the polarity of molecules, as it is the unequal sharing of electrons that creates dipoles within a molecule. Electronegativity is a periodic trend; generally, atoms become more electronegative across a period from left to right, and less electronegative down a group. This is because atoms with more protons have a greater pull on shared electrons, and closer valence shells are more efficient at attracting electrons. The differences in electronegativity between atoms in a bond can lead to the partial charges that characterize polar molecules. For instance, in a water molecule, the difference in electronegativity between hydrogen and oxygen leads to an unequal sharing of electrons, making water polar.

Dipole Moment

A dipole moment is a vector quantity that measures the extent of the polarity in a molecule. It arises when there is a separation of positive and negative charges due to the uneven distribution of electrons brought about by the differences in electronegativity of atoms. The dipole moment is determined not just by the magnitude of the charge but also the distance between the charges; it is the product of the charge difference and the distance between the charges. A molecule with a dipole moment has distinct ‘poles’ – hence, the term polar molecule. Not all molecules with polar bonds have a dipole moment because the molecular geometry can result in the cancellation of individual bond dipoles, as seen in carbon dioxide. However, in water, the unique V-shaped geometry prevents such cancellation, resulting in a strong dipole moment.

Properties of Molecules

The properties of molecules, such as boiling and melting points, solubility, and reactivity, are greatly influenced by whether they are polar or nonpolar. Polar molecules generally have higher boiling and melting points compared to similar-sized nonpolar molecules because of the strong dipole-dipole interactions that require more energy to overcome. They tend to dissolve well in polar solvents (like water) because 'like dissolves like', meaning substances with similar properties (like polarity) mix well. Conversely, nonpolar molecules have weaker intermolecular forces and often dissolve in nonpolar solvents. Reactivity is also influenced by polarity; polar molecules with a high dipole moment have uneven electron distribution, which often makes them more reactive in chemical reactions. Understanding the polar or nonpolar character of a molecule can help predict its behavior in different chemical contexts.