Question

In which of the following molecular shapes would you expect to find one or more unshared pairs of electrons on the central atom? (a) bent (c) trigonal planar (b) tetrahedral (d) trigonal pyramidal

Short answer

The molecular shapes bent and trigonal pyramidal would have one or more unshared pairs of electrons on the central atom.

Step-by-step solution

Analyze the Bent Structure

In a bent molecular structure, like Water (H2O), the central atom typically has two bonded pairs and two lone pairs of electrons. Thus, it indeed possesses one or more unshared pairs of electrons.

Investigate the Trigonal Planar Molecule

In the trigonal planar molecular structure, like Boron Trifluoride (BF3), the central atom typically has three bonded pairs of electrons and no lone pair. Thus, it does not possess any unshared pairs of electrons.

Examine the Tetrahedral Molecule

In the tetrahedral molecular structure, like Methane (CH4), the central atom typically has four bonded pairs of electrons and no lone pair. Thus, it does not possess any unshared pairs of electrons.

Check the Trigonal Pyramidal Structure

In the trigonal pyramidal molecule shape like Ammonia (NH3), the central atom typically has three bonded pairs and one lone pair of electrons. Thus, it indeed possesses one or more unshared pairs of electrons.

Key concepts

Lone Pairs

Lone pairs refer to the pairs of valence electrons that are not shared between atoms. These electrons are located on the central atom and do not participate in bonding. Lone pairs can heavily influence the shape and properties of molecules. For example, in water (H₂O), the lone pairs on the oxygen atom cause the molecule to adopt a bent shape. The presence of lone pairs can also affect the angles between bonded atoms, generally creating less than the typical angle if they were not present.

• Lone pairs repel bonded pairs of electrons, resulting in changes to molecular geometry.
• The strength of this repulsion is greater than that between bonded pairs.
• Molecules with lone pairs often have additional polarity due to the uneven electron distribution.

Understanding lone pairs is crucial when predicting and explaining the 3D structure of molecules.

Central Atom

The central atom in a molecule is typically the atom to which other atoms are bonded. It serves as the foundation or core of the molecule's structure. In the context of molecular geometry, the central atom is essential in determining how the molecule's overall shape is established. For example, in ammonia (NH₃), nitrogen is the central atom bonded to three hydrogen atoms, forming a trigonal pyramidal shape.

• The central atom is often the atom with the highest valence or the atom that can form the most bonds.
• It plays a key role in the bond angles and overall molecular structure.
• The number of lone pairs and bonding pairs at the central atom determines the molecular geometry.

Selecting the correct central atom is step one in understanding a molecule's 3D shape and properties.

Electron Pairs

Electron pairs in a molecule include both the pairs involved in bonding between atoms (bonded pairs) and those not involved in bonding (lone pairs). These pairs are crucial to determining the geometry of the molecule as they occupy space around the central atom. For instance, in the case of a tetrahedral molecule like methane (CH₄), there are four bonded pairs and no lone pairs.

• Bonded pairs help form molecular bonds by sharing electrons between atoms.
• Lone pairs are non-bonding and located on the central atom, affecting shape and angles.
• The spatial arrangement of all electron pairs affects the molecule's structural shape.

Analyzing electron pairs helps predict molecular geometry, with frameworks like the Valence Shell Electron Pair Repulsion (VSEPR) theory being used to understand the interactions and shapes based on these pairs.