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Chapter 5: Problem 25

Determine the molecular geometries of the following molecules: (a) GeH \(_{4} ;\) (b) \(\mathrm{PH}_{3} ;\) (c) \(\mathrm{H}_{2} \mathrm{S} ;\) (d) \(\mathrm{CHCl}_{3}.\)

Short Answer

Expert verified
Answer: (a) GeH4 has a tetrahedral geometry with bond angles of 109.5 degrees. (b) PH3 has a trigonal pyramidal geometry with bond angles less than 107.3 degrees. (c) H2S has a bent geometry with bond angles of approximately 104.5 degrees. (d) CHCl3 has a tetrahedral geometry with bond angles of 109.5 degrees.

Step by step solution

01

Determine the Central Atom and Surrounding Atoms

In GeH4, the central atom is Ge (germanium) and it is surrounded by four hydrogen atoms.
02

Electron Pair Calculation

The valence electron pairs in GeH4 can be calculated as follows: Germanium (Ge) has 4 valence electrons and each of the four Hydrogen atoms (H) has 1 valence electron. So, GeH4 has a total of 4 + 4(1) = 8 valence electrons.
03

VSEPR Theory Application

Now, using VSEPR theory, GeH4 has no lone pairs on the central atom and 4 bonding-pair electrons. This corresponds to a steric number 4, leading to a tetrahedral geometry for the molecule with bond angles of 109.5 degrees. (b) PH3
04

Determine the Central Atom and Surrounding Atoms

In PH3, the central atom is P (phosphorus) and it is surrounded by three hydrogen atoms.
05

Electron Pair Calculation

The valence electron pairs in PH3 can be calculated as follows: Phosphorus (P) has 5 valence electrons and each of the three Hydrogen atoms (H) has 1 valence electron. So, PH3 has a total of 5 + 3(1) = 8 valence electrons.
06

VSEPR Theory Application

Now, using VSEPR theory, PH3 has 1 lone pair on the central atom and 3 bonding-pair electrons. This corresponds to a steric number 4, leading to a trigonal pyramidal geometry for the molecule with bond angles less than 107.3 degrees due to the lone pair repulsion. (c) H2S
07

Determine the Central Atom and Surrounding Atoms

In H2S, the central atom is S (sulfur) and it is surrounded by two hydrogen atoms.
08

Electron Pair Calculation

The valence electron pairs in H2S can be calculated as follows: Sulfur (S) has 6 valence electrons and each of the two Hydrogen atoms (H) has 1 valence electron. So, H2S has a total of 6 + 2(1) = 8 valence electrons.
09

VSEPR Theory Application

Now, using VSEPR theory, H2S has 2 lone pairs on the central atom and 2 bonding-pair electrons. This corresponds to a steric number 4, leading to a bent geometry for the molecule with bond angles of approximately 104.5 degrees due to the lone pair repulsion. (d) CHCl3
10

Determine the Central Atom and Surrounding Atoms

In CHCl3, the central atom is C (carbon) and it is surrounded by one hydrogen atom and three chlorine atoms.
11

Electron Pair Calculation

The valence electron pairs in CHCl3 can be calculated as follows: Carbon (C) has 4 valence electrons, Hydrogen (H) has 1 valence electron and each of the three Chlorine atoms (Cl) has 7 valence electrons. So, CHCl3 has a total of 4 + 1 + 3(7) = 26 valence electrons.
12

VSEPR Theory Application

Now, using VSEPR theory, CHCl3 has no lone pairs on the central atom and 4 bonding-pair electrons. This corresponds to a steric number 4, leading to a tetrahedral geometry for the molecule with bond angles of 109.5 degrees.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

VSEPR Theory
Understanding molecular geometry begins with the Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory helps us predict the shape of molecules based on the repulsion between electron pairs in the valence shell of the central atom.

According to VSEPR, electron pairs arrange themselves as far apart as possible to minimize repulsion. This affects the shape of the molecule.
  • For molecules with no lone pairs, like GeH extsubscript{4}, the shape is determined by the positions of the bond pairs.
  • For those with lone pairs, like PH extsubscript{3} and H extsubscript{2}S, these take up more space and distort the shape.

These principles allow us to determine the three-dimensional structure of molecules, which is crucial for understanding their chemical behavior.
Valence Electrons
Valence electrons are the outermost electrons of an atom and play a key role in molecule formation and geometry. They are responsible for bonding as they interact with electrons of other atoms.

To find the number of valence electrons:
  • Look at the group number in the periodic table for main group elements.
  • For GeH extsubscript{4}, Ge contributes 4, and each H contributes 1, totaling 8.
  • In PH extsubscript{3}, P provides 5, and H contributes 3, summing up to 8.
  • For H extsubscript{2}S, S has 6, and H gives 2, reaching 8.

Understanding these basics helps predict and explain the structure and reactivity of molecules.
Tetrahedral Geometry
Tetrahedral geometry is common in molecules where a central atom is bonded to four others with no lone pairs.

Examples include GeH extsubscript{4} and CHCl extsubscript{3}, where respective central atoms have four bonds forming a shape like a pyramid with a triangular base.
  • This shape allows for equal bond angles of 109.5 degrees.
  • The spatial arrangement minimizes electron repulsion in such molecules.

This geometric configuration is important in chemistry as it affects the molecule's properties and how it interacts with other substances.
Trigonal Pyramidal Shape
The trigonal pyramidal shape occurs when three atoms bond to a central atom, which also has a lone pair.

PH extsubscript{3} is an example where the lone pair pushes the other three bonds downward, altering the ideal tetrahedral angle.
  • The bond angles are less than 109.5 degrees, typically around 107.3 degrees.
  • This asymmetry affects the physical properties and polarity of the molecule.

This geometry is significant for understanding the distinct chemical reactivity compared to molecules with symmetrical shapes.
Bent Molecular Shape
Bent molecular geometry is seen in molecules like H extsubscript{2}S, where there are two bonds and two lone pairs around the central atom.

This shape is similar to water and results from the lone pairs forcing the bonds closer together.
  • The bond angle is about 104.5 degrees due to the increased repulsion from lone pairs.
  • This geometry gives the molecule a polar characteristic, affecting how it interacts in chemical reactions.

Understanding this geometry is essential for predicting the behavior and properties of similar molecular structures.

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