Question

Determine the geometry about each interior atom in each molecule and sketch the molecule. (Skeletal structure is indicated in parentheses.) a. \(\mathrm{CH}_{3} \mathrm{OH}\left(\mathrm{H}_{3} \mathrm{COH}\right)\) b. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\left(\mathrm{H}_{3} \mathrm{COCH}_{3}\right)\) c. \(\mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{HOOH})\)

Short answer

CH3OH has C with tetrahedral geometry and O with bent geometry. CH3OCH3 has both C atoms with tetrahedral geometry. H2O2 has both O atoms with bent geometry.

Step-by-step solution

Identify Central Atoms

For each molecule, identify the central atom(s) around which the geometry will be determined. For CH3OH, the central atoms are C and O. For CH3OCH3, there are two central atoms, each C of the two CH3 groups. For H2O2, both O atoms are central atoms.

Draw Lewis Structures

Draw the Lewis structure for each molecule to determine the number of bonding pairs and lone pairs on each central atom. In CH3OH, C has 4 bonding pairs and no lone pairs, while O has 2 bonding pairs and 2 lone pairs. In CH3OCH3, each C has 4 bonding pairs and no lone pairs. In H2O2, each O has 2 bonding pairs and 2 lone pairs.

Apply VSEPR Theory

Use Valence Shell Electron Pair Repulsion (VSEPR) theory to predict the molecular geometry around each central atom. For CH3OH, C exhibits tetrahedral geometry, while O shows bent geometry. In CH3OCH3, both C atoms exhibit tetrahedral geometry. In H2O2, both O atoms show bent geometry.

Sketch the Molecule

Using the determined geometries, sketch the molecule showing the spatial arrangements of atoms. For CH3OH, C is at the center of a tetrahedron with H atoms at three corners and O at the fourth, O then bonds to H. In CH3OCH3, each C is at the center of a tetrahedron with H atoms at three of the corners and O bonding to both C atoms. In H2O2, each O is in a bent configuration with one H bonded to each O and a single bond between the O atoms.

Key concepts

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a fundamental concept used in chemistry to predict the shape of individual molecules based on the repulsion between electron pairs on the valence shell of atoms. When studying molecular geometry, it's vital to understand that electron pairs will arrange themselves to minimize the repulsive forces between them. This is because electrons carry a negative charge, and similar charges repel each other.

In applying VSEPR theory, we categorize electron pairs into two groups: bonding pairs, which are shared between atoms to form chemical bonds, and lone pairs, which are unshared and belong to a single atom. The arrangement of these electron pairs dictates the shape of the molecule. For instance, when an atom has four bonding pairs and no lone pairs, like in methane (CH₄), it adopts a tetrahedral geometry. However, if there are lone pairs, this can alter the geometry because lone pairs occupy more space and exert greater repulsive forces than bonding pairs. For example, the water molecule (H₂O), with two bonding pairs and two lone pairs on the oxygen atom, takes on a bent shape due to the repulsion of the lone pairs.

Lewis Structures

Lewis structures serve as a visual representation of the valence electrons in molecules and ions. These structures are named after Gilbert N. Lewis, who introduced them in 1916. Representing electrons as dots, Lewis structures depict the arrangement of electrons in the outermost shell, or valence shell, around atoms in a molecule.

To draw a proper Lewis structure, we count the valence electrons for each atom and pair them to form bonds while fulfilling the octet rule, where possible. The octet rule states that atoms tend to form bonds until they are surrounded by eight electrons, to resemble the electron configuration of noble gases. A correct Lewis structure is crucial for understanding the connectivity between atoms and predicting the distribution of bonding and non-bonding electron pairs. This, in turn, allows us to use the VSEPR theory to predict molecular geometry, as we know where the regions of electron density will be located around an atom.

Chemical Bonding

Chemical bonding refers to the force that holds atoms together in molecules and compounds. The three primary types of chemical bonds are ionic, covalent, and metallic. Ionic bonds occur when electrons are transferred from one atom to another, typically between metals and nonmetals, resulting in oppositely charged ions that attract each other. Covalent bonds, on the other hand, involve the sharing of electron pairs between atoms. These bonds form mostly between nonmetals, and the shared electrons help each atom achieve a full valence shell.

The type of bond and the method of electron sharing affect the molecule's properties, including its geometry, stability, and reactivity. In the context of molecular geometry, covalent compounds can have various shapes determined by the VSEPR theory. By understanding chemical bonding, we gain insights into how atoms connect and interact, informing everything from the physical properties of materials to the behavior of molecules in chemical reactions.