Question

Give all the ideal bond angles \(\left(109.5^{\circ}, 120^{\circ},\right.\) or \(\left.180^{\circ}\right)\) in the following molecules and ions. (The skeleton does not imply geometry.) (a) \(\mathrm{Cl}-\mathrm{S}-\mathrm{Cl}\) (b) \(\mathrm{F}-\mathrm{Xe}-\mathrm{F}\) (c) CC(O)CN (d) C=CCC#N

Short answer

Answer: 109.5°

Step-by-step solution

a) \(\mathrm{Cl}-\mathrm{S}-\mathrm{Cl}\)#

First, determine the number of electron pairs around the central sulfur atom. Sulfur has 6 valence electrons, and each chlorine contributes one electron. Hence, we have a total of 4 electron pairs in the valence shell of Sulfur. According to VSEPR theory, these electron pairs will repel each other to minimize their interactions, and they will adopt a tetrahedral geometry. Therefore, the bond angle in \(\mathrm{Cl}-\mathrm{S}-\mathrm{Cl}\) is 109.5°.

Key concepts

VSEPR Theory

Understanding the shape of molecules is crucial in chemistry, and the Valence Shell Electron Pair Repulsion (VSEPR) theory provides a simple yet powerful way to predict molecular geometries. VSEPR theory states that electron pairs around a central atom will arrange themselves to be as far apart as possible. This happens because electron pairs, being negatively charged, repel each other.

The shapes determined by VSEPR theory are not random but are consistent with specific numbers of bonding and non-bonding electron pairs. For instance, two electron pairs will adopt a linear arrangement with a 180° angle, while three will form a trigonal planar shape with 120° angles. As the numbers increase, the shapes and angles become more complex, covering tetrahedral, trigonal bipyramidal, and octahedral arrangements, among others.

In the exercise, students identify the molecular geometry by using the VSEPR theory which helps in predicting the ideal bond angles in molecules and polyatomic ions.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of the atoms within a molecule. It differs from molecular structure because it does not take into account the lengths of bonds or the electronic transitions, focusing purely on atom positions and bond angles.

Factors affecting molecular geometry include the number of bonds, the presence of lone pairs of electrons, and the electronic repulsion between them. For example, a molecule with four single bonds and no lone pairs will exhibit a tetrahedral geometry, while the addition of lone pairs can distort this geometry leading to different shapes like trigonal pyramidal or bent.

The molecular geometry is essential for determining the molecule's properties such as polarity, reactivity, color, and biological activity. Thus, recognizing the molecular geometry is a vital skill for students dealing with chemical formulas and reactions.

Electron Pair Repulsion

At the heart of VSEPR theory lies the concept of electron pair repulsion. Electrons, with their negative charge, interact electromagnetically. Pairs of electrons in the valence shell of an atom, whether they are involved in bonding or not, will repel each other due to their like charges.

The repulsion isn't limited to bonded electron pairs, as lone pairs (non-bonded pairs) also exert force, often affecting the molecule's geometry more significantly than bonding pairs. It's interesting to note that lone pairs occupy more space and therefore repel more strongly than bonding pairs, as they are closer to the nucleus and not shared between atoms. This repulsion dictates the angles between the bonds, leading to the ideal bond angles for different molecular geometries.

Explaining this to students can help them understand the spatial arrangement in molecules and how it affects the substance's properties and reactivity.

Tetrahedral Geometry

Tetrahedral geometry is one of the fundamental shapes in molecular geometry emanating from the principles of VSEPR theory. When there are four electron pairs around the central atom, whether bonding or non-bonding, they will adopt a tetrahedral arrangement.

In a perfect tetrahedron, all the bond angles are equal, and the value is approximately 109.5°. This angle minimizes electron pair repulsion and is seen in molecules like methane (CH4). However, if one or more of the electron pairs are non-bonding, the angles can be slightly less due to the greater repulsion of lone pairs as compared to bonding pairs.

Students can visualize a tetrahedron as a three-dimensional triangular pyramid to grasp this spatial concept better, and understand that every corner represents the position where an atom or lone pair would be located.