Question

Refrigerant The refrigerant known as freon-14 is an ozone-damaging compound with the formula \(\mathrm{CF}_{4}\) . Why is the CF_ molecule nonpolar even though it contains polar bonds?

Short answer

The CF_4 molecule is nonpolar despite containing polar C-F bonds due to its symmetric tetrahedral geometry. This symmetric arrangement causes the bond dipoles from each C-F bond to cancel out, resulting in the overall molecule being nonpolar.

Step-by-step solution

Understanding Bond Polarity

Polar bonds occur when atoms with different electronegativity values share electrons. The difference in electronegativity leads to an unequal distribution of electron density, creating a bond dipole moment. In a polar bond, the more electronegative atom has a partial negative charge, while the less electronegative atom has a partial positive charge.

Understanding Molecular Polarity

Molecular polarity depends on both bond polarity and the molecule's shape. If a molecule contains polar bonds and the molecular geometry doesn't cancel the individual bond dipoles, the molecule is polar. However, if the molecule's geometry results in cancellation of bond dipoles, the molecule becomes nonpolar.

Examining the CF_4 Molecular Structure

In CF_4, a carbon (C) atom is bonded to four fluorine (F) atoms. The geometry of the CF_4 molecule is tetrahedral, with a bond angle of 109.5 degrees. The molecule has a symmetrical arrangement, where the C atom is in the center, and each F atom is on one corner of the tetrahedron.

Analyzing the Role of Bond Dipoles

The bond dipoles in the molecule denote the difference in electronegativity between bonded atoms. In the case of CF_4, F is more electronegative than C, resulting in polar C-F bonds. However, due to the molecule's three-dimensional symmetric tetrahedral shape, the bond dipoles from each C-F bond cancel out, making the overall molecule nonpolar. Thus, even though CF_4 contains polar bonds, the symmetry of the molecule's geometric arrangement ensures that their individual bond dipoles are canceled out, resulting in a nonpolar molecule.

Key concepts

Polar Bonds

Polar bonds occur when two atoms in a molecule have different electronegativities, causing an unequal sharing of electrons. The atom with the higher electronegativity will pull the electron density closer, leading to a partial negative charge, while the other atom has a partial positive charge. This creates a bond dipole moment.

In a bond dipole, it's essential to understand that the direction of the dipole is from the less electronegative atom towards the more electronegative one. For example, in carbon-fluorine (C-F) bonds, fluorine is more electronegative than carbon, creating polar bonds with a dipole directed towards the fluorine atom.

• Electronegativity differences between atoms lead to polarity.
• The larger the difference, the more polar the bond.
• C-F bonds are polar due to the significant electronegativity difference between carbon and fluorine.

Understanding these basics is crucial as the behavior and polarity of molecules are often determined by the nature of these polar bonds.

Tetrahedral Geometry

A molecule with a tetrahedral geometry has a central atom bonded to four other atoms placed at the corners of a tetrahedron. This arrangement is characterized by 109.5-degree bond angles, which is a typical feature of four single bonds around a central atom like in methane (CH₄) or freon-14 (CF₄).

The symmetry inherent in tetrahedral molecules plays a vital role in their overall polarity. Each bond dipole in a tetrahedral molecule can counterbalance the others, depending on the central atom's position relative to the surrounding atoms. This balanced arrangement in CF₄ means the net dipole moment is zero, rendering the molecule nonpolar despite having individual polar C-F bonds.

• Four single bonds form a tetrahedron around the central atom.
• Any polar bonds arranged symmetrically cancel each other's dipole moments.
• CF₄ exemplifies how molecular geometry influences overall molecular polarity.

Electronegativity

Electronegativity is a measure of an atom's ability to attract shared electrons in a chemical bond. It's a crucial concept in determining the nature of bonds between atoms.

Elements with high electronegativity, like fluorine, have a strong tendency to attract electrons. In contrast, elements like carbon have lower electronegativity. The larger the difference in electronegativity between two bonded atoms, the more polar the bond is due to the unequal sharing of electrons.

• Fluorine has one of the highest known electronegativity values.
• A large difference in electronegativity results in polar bonds.
• Understanding electronegativity helps explain the formation of molecules like CF₄ with polar C-F bonds that can still result in nonpolar molecules due to symmetry.

Grasping this concept helps in predicting bond types and molecular interactions, central to mastering molecular polarity.