Question

Use Lewis structures to predict the molecular polarities for sulfur difluoride, sulfur tetrafluoride, and sulfur hexafluoride.

Short answer

Sulfur difluoride (SF2) has a bent molecular geometry which leads to a net dipole moment, making it a polar molecule. Sulfur tetrafluoride (SF4) has a seesaw molecular geometry with a net dipole moment, making it polar as well. Sulfur hexafluoride (SF6) has an octahedral molecular geometry with symmetrical distribution of fluorine atoms, causing individual dipole moments to cancel out and resulting in a nonpolar molecule.

Step-by-step solution

Draw Lewis structures for each molecule

We will first draw the Lewis structures for sulfur difluoride (SF2), sulfur tetrafluoride (SF4), and sulfur hexafluoride (SF6). For SF2: Sulfur has 6 valence electrons, and each fluorine atom has 7 valence electrons. So, the total number of valence electrons in SF2 is (6 + 7×2) = 20. For SF4: Sulfur has 6 valence electrons, and each fluorine atom has 7 valence electrons. So, the total number of valence electrons in SF4 is (6 + 7×4) = 34. For SF6: Sulfur has 6 valence electrons, and each fluorine atom has 7 valence electrons. So, the total number of valence electrons in SF6 is (6 + 7×6) = 48.

Determine the electron pair geometry and molecular geometry for each molecule

Next, we will determine the electron pair geometry and molecular geometry for each molecule using the VSEPR theory (valence shell electron pair repulsion). For SF2: There are 4 electron groups (2 single bonds and 1 lone pair) around sulfur, resulting in a trigonal planar electron pair geometry. However, since there are only two bonding pairs, the molecular geometry is bent. For SF4: There are 5 electron groups (4 single bonds and 1 lone pair) around sulfur, leading to a trigonal bipyramidal electron pair geometry. The molecular geometry is seesaw. For SF6: There are 6 electron groups (6 single bonds and no lone pairs) around sulfur, resulting in an octahedral electron pair geometry. The molecular geometry is also octahedral.

Predict the molecular polarity for each molecule

Lastly, we will predict the molecular polarity for each molecule by considering the molecular geometry and the electronegativity of fluorine compared to sulfur. For SF2: The molecular geometry is bent, and fluorine atoms are highly electronegative compared to sulfur. Thus, there is a net dipole moment and the molecule is polar. For SF4: The molecular geometry is seesaw, and since sulfur is less electronegative than fluorine, the molecule has a net dipole moment and is polar. For SF6: The molecular geometry is octahedral, and it has a symmetrical distribution of fluorine atoms around sulfur. This causes the individual dipole moments to cancel out, resulting in a nonpolar molecule. In summary, sulfur difluoride (SF2) and sulfur tetrafluoride (SF4) are polar molecules, while sulfur hexafluoride (SF6) is a nonpolar molecule.

Key concepts

Molecular Polarity

Molecular polarity is an essential concept in chemistry, dictating how a molecule behaves in different environments. It's all about the distribution of electrical charge across a molecule. This depends on both the molecular geometry and the presence of polar bonds.
In a polar molecule like sulfur difluoride (SF2), there is an unequal sharing of electrons leading to a net dipole moment. This happens because fluorine is much more electronegative than sulfur, pulling the electrons toward itself. The bent shape of SF2 further ensures that these dipole moments don't cancel out, resulting in a polar molecule.
In contrast, sulfur hexafluoride (SF6) is a perfect example of a nonpolar molecule. Although SF6 has polar S-F bonds, the symmetrical octahedral shape leads to the cancellation of dipole moments, balancing the charge distribution. This symmetry results in no net dipole moment, making the molecule nonpolar.

VSEPR Theory

Understanding the shape of molecules begins with the Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory is essential for predicting the arrangement of electron pairs around a central atom, thereby determining the molecular shape.
The basic idea is that electron pairs, including both bonding and lone pairs, will arrange themselves as far apart as possible to minimize repulsion. For example, in SF2, the two bonding pairs and the lone pairs around the sulfur atom adopt a bent shape due to electron pair repulsion.
Similarly, SF4 adopts a seesaw geometry due to the five electron groups forming around the sulfur atom. The added complexity of having a lone pair causes an asymmetrical shape. Lastly, with SF6, the six bonding pairs arrange themselves evenly around the sulfur atom, resulting in an octahedral geometry.

Valence Electrons

Valence electrons play a significant role in determining how atoms bond and form molecules. These are the electrons in the outermost shell of an atom, and they are crucial for forming chemical bonds.
For sulfur difluoride (SF2), each sulfur atom contributes 6 valence electrons, while each fluorine atom offers 7. The total number of valence electrons determines how these atoms bond and influences the structural shape.
In sulfur tetrafluoride (SF4), the sulfur atom's 6 valence electrons help accommodate its bonding with four fluorine atoms and one lone pair. Moving to sulfur hexafluoride (SF6), we observe the sulfur atom using its valence electrons to form strong bonds with six fluorine atoms, achieving a full outer shell arrangement.

Molecular Geometry

Molecular geometry refers to the three-dimensional shape of a molecule, determined by the arrangement of atoms around a central atom. This shape is crucial for understanding a molecule's chemical behavior and properties.
In sulfur difluoride (SF2), the molecular geometry is bent due to the presence of lone pairs on the sulfur atom, pushing the bonded fluorine atoms closer. This shape contributes to the molecule's polarity.
For sulfur tetrafluoride (SF4), the molecular geometry is described as a seesaw shape. This is due to the presence of a lone pair that distorts the symmetrical arrangement of the bonding pairs.
Lastly, the molecular geometry of sulfur hexafluoride (SF6) is octahedral, providing symmetry because all six fluorine atoms are evenly spaced around the central sulfur atom, leading to nonpolarity.