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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 central atom bonded to three other atoms and one lone pair, resulting in a slightly distorted, asymmetrical structure with bond angles less than 109.5 degrees. A tetrahedron, on the other hand, has a central atom bonded to four other atoms with no lone pairs, creating a perfectly symmetrical structure with bond angles of exactly 109.5 degrees. Trigonal pyramid geometries can exhibit molecular polarity, while tetrahedral geometries may be nonpolar if the surrounding atoms are the same.

Step by step solution

01

Trigonal Pyramid Geometry

A molecule with a trigonal pyramid geometry consists of a central atom bonded to three other atoms, with one lone pair of electrons on the central atom. This geometry can be found in a molecule where the central atom has four electron pairs, three of which are bonding pairs, and one is a lone pair. The trigonal pyramid shape has one atom at the apex and three atoms forming the base. Due to the presence of the lone pair, the bond angles between the three bonding pairs are less than the perfect tetrahedral angle (109.5 degrees), typically around 107 degrees. An example of a molecule with trigonal pyramid geometry is ammonia (NH₃).
02

Tetrahedron Geometry

A molecule with a tetrahedral geometry consists of a central atom bonded to four other atoms, with no lone pairs on the central atom. This geometry can be found in a molecule where the central atom has four bonding electron pairs. The tetrahedral shape has a perfectly symmetrical structure with each of the four atoms being of equal distance from the central atom, and all bond angles between the atoms are equal to 109.5 degrees. An example of a molecule with tetrahedral geometry is methane (CH₄).
03

Differences between Trigonal Pyramid and Tetrahedron Geometries

There are several key differences between trigonal pyramid and tetrahedral molecular geometries: 1. In a trigonal pyramid, the central atom is bonded to three other atoms and has one lone pair, while in a tetrahedron, the central atom is bonded to four other atoms with no lone pairs. 2. The bond angles in a trigonal pyramid are slightly less than 109.5 degrees due to the presence of the lone pair on the central atom. In a tetrahedron, all bond angles are exactly 109.5 degrees. 3. Trigonal pyramid geometry results in a slightly distorted, asymmetrical structure, while tetrahedron geometry has a perfectly symmetrical structure. 4. Molecules with a trigonal pyramid geometry can exhibit molecular polarity, while those with a tetrahedron geometry can be nonpolar if the atoms surrounding the central atom are the same.

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Most popular questions from this chapter

Consider the \(\mathrm{SCl}_{2}\) molecule. (a) What is the electron configuration of an isolated \(\mathrm{S}\) atom? (b) What is the electron configuration of an isolated \(\mathrm{Cl}\) atom? (c) What hybrid orbitals should be constructed on the \(S\) atom to make the \(\mathrm{S}-\mathrm{Cl}\) bonds in \(\mathrm{SCl}_{2}\) ? (d) What valence orbitals, if any, remain unhybridized on the \(\mathrm{S}\) atom in \(\mathrm{SCl}_{2}\) ?

(a) Is the molecule \(\mathrm{BF}_{3}\) polar or nonpolar? (b) If you react \(\mathrm{BF}_{3}\) to make the ion \(\mathrm{BF}_{3}^{2-}\), is this ion planar? (c) Does the molecule \(\mathrm{BF}_{2} \mathrm{Cl}\) have a dipole moment?

The energy-level diagram in Figure 9.36 shows that the sideways overlap of a pair of \(p\) orbitals produces two molecular orbitals, one bonding and one antibonding. In ethylene there is a pair of electrons in the bonding \(\pi\) orbital between the two carbons. Absorption of a photon of the appropriate wavelength can result in promotion of one of the bonding electrons from the \(\pi_{2 p}\) to the \(\pi_{2 p}^{*}\) molecular orbital. (a) Assuming this electronic transition corresponds to the HOMO-LUMO transition, what is the HOMO in ethylene? (b) Assuming this electronic transition corresponds to the HOMO-LUMO transition, what is the LUMO in ethylene? (c) Is the \(\mathrm{C}-\mathrm{C}\) bond in ethylene stronger or weaker in the excited state than in the ground state? Why? (d) Is the \(\mathrm{C}-\mathrm{C}\) bond in ethylene easier to twist in the ground state or in the excited state?

The highest occupied molecular orbital of a molecule is abbreviated as the HOMO. The lowest unoccupied molecular orbital in a molecule is called the LUMO. Experimentally, one can measure the difference in energy between the HOMO and LUMO by taking the electronic absorption (UV-visible) spectrum of the molecule. Peaks in the electronic absorption spectrum can be labeled as \(\pi_{2 \mathrm{p}}-\pi_{2 \mathrm{p}}{ }^{*}\), $\sigma_{2 s}-\sigma_{2 s}{ }^{*},$ and so on, corresponding to electrons being promoted from one orbital to another. The HOMO-LUMO transition corresponds to molecules going from their ground state to their first excited state. (a) Write out the molecular orbital valence electron configurations for the ground state and first excited state for \(\mathrm{N}_{2}\). (b) Is \(\mathrm{N}_{2}\) paramagnetic or diamagnetic in its first excited state? (c) The electronic absorption spectrum of the \(\mathrm{N}_{2}\) molecule has the lowest energy peak at \(170 \mathrm{nm}\). To what orbital transition does this correspond? (d) Calculate the energy of the HOMO-LUMO transition in part (a) in terms of \(\mathrm{kJ} / \mathrm{mol}\). (e) Is the \(\mathrm{N}-\mathrm{N}\) bond in the first excited state stronger or weaker compared to that in the ground state?

What hybridization do you expect for the atom that is underlined in each of the following species? (a) $\underline{\mathrm{O}}_{2}^{-} ;(\mathbf{b}) \underline{\mathrm{N}} \mathrm{H}_{4}^{+} ;$ (c) \(\mathrm{SCN}^{-}\) (d) \(\underline{\mathrm{Br}} \mathrm{Cl}_{3}\)
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