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Chapter 9: Problem 15

How does a trigonal pyramid differ from a tetrahedron so far as molecular geometry is concerned?

Short Answer

Expert verified
A trigonal pyramid has a lone pair causing less than 109.5掳 angles, while a tetrahedron has all atoms without lone pairs and 109.5掳 angles.

Step by step solution

01

Define Tetrahedron Geometry

A tetrahedron is a molecular geometry where a central atom is surrounded by four atoms positioned at the vertices of a tetrahedron. The bond angles in a perfect tetrahedron are approximately 109.5 degrees, and all four sides are identical.
02

Define Trigonal Pyramidal Geometry

In trigonal pyramidal geometry, a central atom is bonded to three atoms, with one lone pair of electrons on the central atom. This lone pair creates a repulsion that pushes the three bonded atoms slightly closer together, resulting in bond angles of less than 109.5 degrees, often around 107 degrees.
03

Identify Key Geometric Differences

The key difference between these two geometries is the presence of a lone pair in trigonal pyramidal geometry. This lone pair distorts the shape, leading to smaller bond angles than in a perfect tetrahedral structure, while a tetrahedron lacks lone pairs and maintains its symmetric shape.

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Key Concepts

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

Tetrahedron
A tetrahedron in molecular geometry involves a central atom surrounded by four atoms situated at the corners of a tetrahedron. This shape is highly symmetrical and is known for its balanced structure. Each of the bond angles in a tetrahedron is approximately 109.5 degrees, which is significant for maintaining the symmetrical shape.
  • The central atom forms four equivalent bonds with the surrounding atoms.
  • The geometry is pivotal in molecules like methane (\( CH_4 \)) where the tetrahedral shape maximizes the distance between bonded pairs, minimizing repulsion.
      • Due to the symmetric arrangement, all four faced angles and edges of a tetrahedron are equal. This uniformity is what distinguishes it from other geometrical forms where unsymmetrical forces come into play.
Trigonal Pyramidal Geometry
Trigonal pyramidal geometry is characterized by a central atom bonded to three atoms with one lone pair on the central atom. This creates a shape similar to a pyramid, but with a significant distinction due to the lone pair. The presence of this lone pair slightly distorts the geometry compared to a perfect tetrahedron, which affects the bond angles.
  • The lone pair repels the bonded electrons more strongly than the bonds repel each other.
  • This causes a reduction in bond angles to less than 109.5 degrees, usually around 107 degrees.
One example of a molecule with trigonal pyramidal geometry is ammonia (\( NH_3 \)). Here, the nitrogen atom has three hydrogen atoms bonded and one lone pair of electrons, leading to the slightly compressed angles. This geometry exhibits a skewed symmetry due to the lone pair's influence.
Lone Pairs Effect
Lone pairs have a profound effect on molecule shape and geometry. Unlike bonded pairs of electrons, lone pairs are localized around the central atom and strongly repel bonded electron pairs. This repulsion alters the spatial arrangement of the molecule.
  • Lone pairs occupy more space than bonding pairs, leading to a decrease in bond angles.
  • This is the reason why trigonal pyramidal shapes typically have smaller angles than a regular tetrahedral shape.
The effect of lone pairs is a key aspect in molecular geometry, as they can drastically change the shape and consequently the properties of a molecule. Understanding the lone pair's influence helps predict molecular behavior and reactivity, which is crucial in chemistry.

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Most popular questions from this chapter

Consider the molecule \(\mathrm{PF}_{4}\) Cl. (a) Draw a Lewis structure for the molecule, and predict its electron-domain geometry. (b) Which would you expect to take up more space, a \(\mathrm{P}-\mathrm{F}\) bond or a \(\mathrm{P}-\mathrm{Cl}\) bond? Explain. (c) Predict the molecular geometry of \(\mathrm{PF}_{4}\) Cl. How did your answer for part (b) influence your answer here in part (c)? (d) Would you expect the molecule to distort from its ideal electron-domain geometry? If so, how would it distort?

An \(\mathrm{AB}_{2}\) molecule is described as having a tetrahedral geometry. (a) How many nonbonding domains are on atom A? (b) Based on the information given, which of the following is the molecular geometry of the molecule: (i) linear, (ii) bent, (iii) trigonal planar, or (iv) tetrahedral?

A compound composed of \(6.7 \% \mathrm{H}, 40.0 \% \mathrm{C},\) and \(53.3 \% \mathrm{O}\) has a molar mass of approximately \(60 \mathrm{~g} / \mathrm{mol}\). (a) What is the molecular formula of the compound? (b) What is its Lewis structure if the two \(\mathrm{O}\) are bonded to \(\mathrm{C} ?(\mathbf{c})\) What is the geometry and hybridization of the \(\mathrm{C}\) atom that is bonded to \(2 \mathrm{O}\) atoms? (d) How many \(\sigma\) and how many \(\pi\) bonds are there in the molecule?

Consider the Lewis structure for acetic acid, which is known as vinegar: (a) What are the approximate bond angles about each of the two carbon atoms, and what are the hybridizations of the orbitals on each of them? (b) What are the hybridizations of the orbitals on the two oxygen atoms, and what are the approximate bond angles at the oxygen that is connected to carbon and hydrogen? (c) What is the total number of \(\sigma\) bonds in the entire molecule, and what is the total number of \(\pi\) bonds?

(a) An \(\mathrm{AB}_{6}\) molecule has no lone pairs of electrons on the \(\mathrm{A}\) atom. What is its molecular geometry? (b) An \(\mathrm{AB}_{4}\) molecule has two lone pairs of electrons on the A atom (in addition to the four B atoms). What is the electron-domain geometry around the A atom? (c) For the \(\mathrm{AB}_{4}\) molecule in part (b), predict the molecular geometry.
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