Question

Use the VSEPR theory to predict a probable shape of the molecule \(\mathrm{F}_{4} \mathrm{SCH}_{2}\), and explain the source of any ambiguities in your prediction.

Short answer

The probable shape of the molecule can be seen as a combination of seesaw around the Sulfur atom and linear around the Carbon atom. The ambiguity in prediction arises due to the presence of two central atoms leading to distinct geometric arrangements at each central atom.

Step-by-step solution

Identify the Central atom

First, it is important to recognize the central atom which is usually the one with the lowest electronegativity. In \( \mathrm{F}_{4} \mathrm{SCH}_{2} \), S (Sulfur) is the central atom, being less electronegative compared to F (Fluorine) and C (Carbon).

Calculate the Total Number of Valence Electron Pairs

Next, calculate the total number of valence electrons for the molecule which is the sum of valence electrons of all atoms. For \( \mathrm{F}_{4} \mathrm{SCH}_{2} \) this would be: \( 4*7 (F) + 6 (S) + 4 (C) + 2*1 (H) = 40 \) electrons. Since each bond represents a pair of electrons, there are 20 electron pairs in total.

Predict Molecular Shape

Going by the VSEPR theory, electron pairs immerse themselves around the central atom in a manner to minimize electron-pair repulsion which determines the shape. The Sulfur atom is surrounded by 6 atoms (4 Fluorine and 2 Hydrogen bonded to Carbon) and therefore, adopts an octahedral geometry. The Carbon atom appears linear since it has two atoms attached (Sulfur and Hydrogen). The molecule has a seesaw shape when focusing on the Sulfur atom, but has a linear shape when focusing on Carbon. The ambiguity hence arises from the different focuses and geometric arrangements of either the sulfur or carbon atom.

Explain Ambiguities

The molecule is unusual as it involves more than one central atom, Sulfur and Carbon, leading to ambiguity. When focusing on Sulfur, the shape can be predicted as seesaw resulting from the octahedral electron geometry with one pair of non-bonding electrons. However, for Carbon, its geometry appears as linear due to its connection to two different atoms.

Key concepts

Valence Electrons Calculation

Understanding how to calculate valence electrons is vital in predicting molecular shapes using the VSEPR theory. Valence electrons are the electrons in the outermost shell of an atom that can participate in forming chemical bonds.

For example, in the compound \( \mathrm{F}_{4} \mathrm{SCH}_{2} \), each fluorine atom contributes 7 valence electrons, sulfur contributes 6, carbon contributes 4, and each hydrogen atom contributes 1. Adding these up gives a total of 40 valence electrons. It's crucial to get the number of valence electrons correct as this will determine how the atoms bond and ultimately the shape of the molecule.

Here's a simple bullet-point summary on how to calculate valence electrons:

• Determine the group number of each element in the periodic table, which corresponds to the number of valence electrons for main-group elements.
• For transition metals and other exceptions, use known charge states or electron configurations to determine valence electrons.
• Add them together to find the total for the molecule.
• Remember to account for charges, adding extra electrons for negative charges or subtracting them for positive charges.

Molecular Shape Prediction

Predicting the shape of a molecule involves understanding how atoms bond and arrange themselves in three-dimensional space, a process governed by the VSEPR theory, which stands for Valence Shell Electron Pair Repulsion. According to this theory, electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion.

In the example of \( \mathrm{F}_{4} \mathrm{SCH}_{2} \), the sulfur atom is surrounded by six areas of electron density, suggesting an octahedral geometry. However, because two hydrogens are bonded to carbon and not to sulfur directly, the shape becomes skewed, adopting what is known as a 'seesaw' geometry for the sulfur-centered part of the molecule.

To predict molecular shapes, follow these steps:

• Identify the central atom(s).
• Calculate the number of valence electrons.
• Use the VSEPR theory to determine the electron-pair geometry.
• Consider the presence of lone pairs which can alter the shape from the basic geometry.
• Visualize the three-dimensional structure bearing in mind the repulsion between electron pairs.

The complexity arises when, as in our molecule, there are several atoms that could be considered 'central', leading to different geometrical interpretations depending on the atom in focus.

Electron-Pair Repulsion

The concept of electron-pair repulsion is at the heart of the VSEPR theory. Electrons are negatively charged and, as a result, repel each other. In a molecule, where electrons are shared between atoms, this repulsion influences the final shape as the molecule adopts a geometry that minimizes this repulsion.

In molecules with multiple bonding partners, such as \( \mathrm{F}_{4} \mathrm{SCH}_{2} \), it's essential to consider that double or triple bonds count as one region of electron density in the same way as a single bond when predicting shape. Lone pairs of electrons hold a more significant repulsion effect compared to bonded pairs, and this must be accounted for when predicting the molecular shape. Also, the shape around each atom may differ, leading to a complex overall structure.

Key takeaways include:

• Electron pairs around a central atom repel each other.
• Geometry of molecules strive to minimize this repulsion.
• Bonded electron pairs and lone pairs are treated differently due to their repulsion strengths.
• Double and triple bonds are considered as one region of electron density in terms of shape prediction.