Question

Predict the molecular shape of each molecule. a. \(\mathrm{COS} \quad\) b. \(\mathrm{CF}_{2} \mathrm{Cl}_{2}\)

Short answer

The molecular shape of COS is linear, and the molecular shape of CF2Cl2 is tetrahedral.

Step-by-step solution

1. Identify the central atoms of each molecule

For molecule COS, the central atom will be the carbon (C) as it has the least electronegativity. Similarly, for CF2Cl2, the central atom will also be carbon.

2. Determine the number of valence electrons for each central atom

Carbon has 4 valence electrons. The other atoms in the molecules are as follows: Oxygen (O) has 6 valence electrons, Sulfur (S) has 6 valence electrons, Fluorine (F) has 7 valence electrons, and Chlorine (Cl) has 7 valence electrons.

3. Establish the electron geometry of each molecule

For molecule COS, the octet rule applies to the carbon atom. Carbon is bonded to one oxygen and one sulfur atom, making 2 bonding pairs, and there are no lone pairs on the carbon. Therefore, the electron geometry of COS is linear. For molecule CF2Cl2, the octet rule also applies to the carbon atom. Carbon is bonded to two fluorine atoms and two chlorine atoms, making 4 bonding pairs, and there are no lone pairs on the carbon. Therefore, the electron geometry of CF2Cl2 is tetrahedral.

4. Finalize molecular shapes for each molecule

The molecular shape for COS (with linear electron geometry) is linear, as there are only two bonding pairs around the central carbon. So, the molecular shape of COS will be a straight line with C in the middle and O and S on the sides. The molecular shape for CF2Cl2 (with tetrahedral electron geometry) is also tetrahedral, as there are four bonding pairs around the central carbon. So, the molecular shape of CF2Cl2 is tetrahedral with C at the center, and the F and Cl atoms arranged symmetrically around it. In conclusion, the molecular shape of COS is linear, and the molecular shape of CF2Cl2 is tetrahedral.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons in an atom and are critical in defining how an atom interacts and forms bonds with other atoms. In our exercises, both COS and CF subscript two is taken into account when calculating molecular shape. Let's break it down.

• Carbon has 4 valence electrons.
• Oxygen and Sulfur both have 6 valence electrons each.
• In CF₂Cl₂, both Fluorine and Chlorine have 7 valence electrons each.

Knowing how many valence electrons each atom has helps predict how atoms will bond. For instance, the valence electrons in carbon allow it to form four covalent bonds in CF₂Cl₂, accommodating a full outer shell in accordance with the octet rule.

Electron Geometry

Electron geometry is the spatial arrangement of all electron groups around a central atom, including lone pairs. It differs slightly from molecular shape, which only considers bonded atoms.

• For COS, carbon forms two bonds, one with oxygen and one with sulfur. The presence of two bonding pairs and no lone pairs makes its electron geometry linear.
• In CF₂Cl₂, carbon is attached to four atoms (two fluorine and two chlorine), resulting in four bonding pairs. Here, the electron geometry is tetrahedral.

Understanding electron geometry is crucial for predicting molecular shapes and angles, and it often follows general rules like the octet rule, which guides atoms in achieving stable configurations.

Bonding Pairs

Bonding pairs are pairs of electrons that are involved in chemical bonds between atoms. In essence, they "connect" atoms by sharing electrons, which contribute to the overall molecule's structure.

• In COS, carbon bonds once with oxygen and once with sulfur, resulting in two bonding pairs, supporting a linear shape.
• For CF₂Cl₂, carbon bonds with two fluorine atoms and two chlorine atoms, leading to four bonding pairs and a tetrahedral shape.

Recognizing the number and arrangement of bonding pairs helps predict molecular shape using models like VSEPR (Valence Shell Electron Pair Repulsion), which suggests that electron pairs arrange themselves to minimize repulsion.

Central Atom

The central atom is the nucleus around which the geometry of the molecule is constructed. It is usually the atom with the lowest electronegativity because it can best "share" its electrons with the surrounding atoms.

• For both COS and CF₂Cl₂, carbon is the central atom due to its ability to form multiple bonds.

The central atom determines the structure and shape of the molecule as it provides attachment points for other atoms, thus shaping the overall geometry. In the example of COS and CF₂Cl₂, it is clear how carbon serves as the nexus, creating linear and tetrahedral shapes respectively.