Question

(a) What conditions must be met if a molecule with polar bonds is nonpolar? (b) What geometries will signify nonpolar molecules for ${\mathrm{AB}}_2,{\mathrm{AB}}_3,$ and ${\mathrm{AB}}_4$ geometries?

Short answer

A molecule with polar bonds can be nonpolar if: (1) the central atom has no lone pairs of electrons, (2) the surrounding atoms are identical or have the same electronegativity, and (3) the molecule has a symmetrical geometry, leading to the cancellation of the net electric dipole moment arising from the polar bonds. For the general formulas AB₂, AB₃, and AB₄, nonpolar geometries include linear for AB₂, trigonal planar for AB₃, and tetrahedral for AB₄, provided that the polar bonds' dipoles cancel out due to symmetry.

Step-by-step solution

Understanding Polarity

Polarity refers to the separation of electric charges within a molecule, leading to a net electric dipole moment. A molecule can have polar bonds, meaning that the atoms in those bonds have a notable difference in electronegativity – the ability of an atom to attract electrons in a bond towards itself. However, a molecule itself can still be nonpolar if the net electric dipole moment is zero because the polar bonds are arranged symmetrically, canceling out each other's effects.

Conditions for Nonpolarity in Molecules with Polar Bonds

A molecule with polar bonds can be nonpolar if the following conditions are met: 1. The central atom has no lone pairs of electrons. 2. The surrounding atoms are identical or have the same electronegativity. 3. The molecule has a symmetrical geometry, leading to the cancellation of the net electric dipole moment arising from the polar bonds. It's important to note that if these conditions are not satisfied, then the molecule will be polar.

Identifying Nonpolar Geometries for AB₂, AB₃, and AB₄

Now we will discuss different geometries of molecules with the general formulas AB₂, AB₃, and AB₄, and determine if they can form nonpolar molecules given the presence of polar bonds. 1. AB₂: The simplest geometry for AB₂ is linear, where the two B atoms are opposite each other (180° angle) with respect to the central A atom. With this configuration, the polar bonds' dipoles point in opposite directions and cancel each other, resulting in a nonpolar molecule. 2. AB₃: One possible geometry for AB₃ is trigonal planar, with bond angles of 120°. In this case, if all three B atoms are the same and no lone pairs are present on the central atom A, the dipoles of each polar bond would cancel out due to symmetry, and the molecule would be nonpolar. Another possible geometry is trigonal pyramidal, which will always result in a polar molecule. 3. AB₄: For AB₄, the geometry can either be tetrahedral or square planar. In the case of a tetrahedral geometry, if all four B atoms are the same and no lone pairs are present on the central atom A, the polar bonds' dipoles would cancel out each other due to symmetry, making the molecule nonpolar. In contrast, a molecule with square planar geometry will always result in a polar molecule. In conclusion, molecules with the general formulas AB₂, AB₃, and AB₄ can be nonpolar with the following geometries, provided that the polar bonds' dipoles cancel out due to symmetry: linear for AB₂, trigonal planar for AB₃, and tetrahedral for AB₄. However, other possible geometries, such as trigonal pyramidal and square planar, will result in polar molecules.

Key concepts

Polar Bonds

Polar bonds occur when two atoms in a molecule have differing electronegativity values. This means one atom pulls electrons more strongly than the other, creating a partial charge difference between them. Electronegativity is like the atom's ability to "grab" electrons tightly in a chemical bond. For instance, consider a bond between hydrogen and fluorine in a hydrogen fluoride (HF) molecule. Here, fluorine is more electronegative than hydrogen, making the bond polar.

• Polar bonds are characterized by differences in electronegativity.
• A polar bond can lead to an electric dipole moment.
• Common in molecules like water and HF.

Polar bonds are key in forming polar molecules, yet a molecule with polar bonds isn't necessarily polar. It's the overall structure and arrangement of these bonds within the molecule that determines its polarity.

Nonpolar Molecules

Nonpolar molecules are those where the net dipole moment is zero despite having polar bonds. This intriguing phenomenon happens when the polar bonds are arranged such that their dipoles cancel each other. The conditions contributing to molecule nonpolarity include:

• The central atom having no lone electron pairs to disrupt symmetrical balance.
• All surrounding atoms being identical, or having similar electronegativity.
• A symmetrical molecular geometry allowing dipole moments to negate each other.

An example would be carbon dioxide ( CO₂ ), which has polar bonds but is linear, causing dipoles to cancel out. Thus, understanding the arrangement of bonds is crucial in determining if a molecule is nonpolar.

Molecular Geometry

Molecular geometry describes how atoms are arranged in a molecule, impacting molecular polarity. It is the three-dimensional shape that is essential to understanding how polar bonds interact within the molecule. The shape of a molecule is determined by the number of atoms bonded to the central atom and the presence of lone pairs of electrons.

• Linear: Often seen in molecules like BeCl₂, where the B atoms are opposite, creating a nonpolar molecule.
• Trigonal Planar: Found in molecules such as BF₃ where all atoms are in one plane, possibly nonpolar if symmetrically arranged.
• Tetrahedral: Seen in CH₄ where the symmetry helps cancel out the dipoles.

Molecular geometry is pivotal in understanding a molecule's polarity because it determines the direction and arrangement of dipole moments.

Symmetry in Molecules

Symmetry plays a vital role in determining the polarity of molecules. When a molecule is symmetric, it means the atoms and the bonds are arranged in a way that allows any dipole moments to counterbalance each other, leading to a nonpolar molecule. Symmetry can help understand why molecules like CO₂ remain nonpolar despite having polar bonds.

The Importance of Symmetry:

Symmetry helps in figuring out molecular polarity as:

• If bonds are symmetrically arranged, the net dipole moment becomes zero.
• It aids in canceling out dipoles arising from opposite directions.
• Examples of symmetric and therefore nonpolar molecules include methane (CH₄) and boron trifluoride (BF₃).

For students, recognizing symmetry can simplify understanding whether the molecules will behave as polar or nonpolar without manual calculations of dipole moments.