Question

Give an example of a molecule or polyatomic ion that has the following features. (a) three bonded atoms, no unshared electrons on the central atom (b) three bonded atoms, one unshared pair of electrons on the central atom (c) two bonded atoms, two unshared pairs of electrons on the central atom

Short answer

(a) Boron trifluoride (BF3), (b) Ammonia (NH3), and (c) Water (H2O)

Step-by-step solution

Identifying Molecule for (a)

For feature (a) - three bonded atoms and no unshared electrons on the central atom, an example can be Boron trifluoride (BF3). Boron (B) is the central atom connected with three Fluorine (F) atoms and Boron doesn't have any unshared electron pairs.

Identifying Molecule for (b)

For feature (b) - three bonded atoms and one unshared pair of electrons on the central atom. An example can be Ammonia (NH3). Nitrogen (N) is the central atom connected with three Hydrogen (H) atoms and Nitrogen has one unshared electron pair.

Identifying Molecule for (c)

For feature (c) - two bonded atoms and two unshared pairs of electrons on the central atom. An example can be Water (H2O). Oxygen (O) is the central atom connected with two Hydrogen (H) atoms and Oxygen has two unshared electron pairs.

Key concepts

Chemical Bonding

Chemical bonding is a fundamental concept in chemistry that involves the joining of atoms to form molecules or polyatomic ions. At its most basic level, chemical bonding is about the forces that hold atoms together.

There are three primary types of chemical bonds: ionic, covalent, and metallic. Ionic bonds occur between metals and non-metals where electrons are transferred from one atom to another. Covalent bonds, on the other hand, involve the sharing of electron pairs between atoms, typically non-metals. Metallic bonds are found in metals, where electrons are free to move around many positively charged atomic nuclei.

Considering the electron configurations and the desire for atoms to achieve a stable electron arrangement, often akin to the nearest noble gas, atoms will either share or transfer electrons to obtain a full valence shell. This drive towards stability is a key motivator for the formation of chemical bonds. For instance, in the example of Boron trifluoride (BF3) provided in the exercise, boron atoms bond covalently with three fluorine atoms sharing electrons to achieve stable electron arrangements.

Valence Shell Electron Pair Repulsion Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is an important tool for predicting and understanding the three-dimensional arrangement of atoms within a molecule. According to VSEPR theory, electron pairs around a central atom will arrange themselves as far apart as possible to minimize the repulsion between their negative charges.

This theory assists us in understanding the molecular structure by considering the number of shared and unshared electron pairs in the valence shell of the central atom. Each pair of electrons, whether involved in a bond or unshared, occupies space around the central atom, and these pairs repel each other. This repulsion causes the molecule to adopt a shape that minimizes the repulsion, thereby determining the molecule's geometry.

For example, in ammonia (NH3), the nitrogen atom has one unshared pair of electrons and three bonds with hydrogen atoms. According to VSEPR theory, these four regions of electron density repel each other and arrange themselves tetrahedrally, with the unshared pair exerting slightly more repulsion and thus slightly distorting the shape, resulting in a trigonal pyramidal shape for the molecule.

Molecular Structure

Molecular structure refers to the spatial arrangement of atoms within a molecule and the chemical bonds that hold those atoms together. It not only involves the position of the atoms but also the orientation of chemical bonds and the distribution of electrons within the molecule.

The molecular structure is critical for determining the properties and behavior of the molecule, including its reactivity, polarity, phase of matter, color, magnetism, and biological activity. Thanks to modern theories like VSEPR and advanced technologies like X-ray crystallography and spectroscopy, scientists can predict and determine these structures with excellent accuracy.

In water (H2O), for instance, the oxygen atom has two bonded pairs and two unshared pairs of electrons, leading to a bent molecular shape as per VSEPR theory. This specific structure leads to the polar nature of water, which is responsible for its unique properties, such as its high boiling point and its exceptional ability to dissolve many substances. Understanding molecular structure is therefore essential for grasping the complexities of chemical reactions and the functionality of different molecules in various environments.