Unit 3: Bonding
Bonds are connections between atoms. A solid grasp of valence shell electron pair repulsion (VSEPR) theory will help you understand how elements that differ by one or two atomic numbers behave.
According to VSEPR theory, the number of electrons an element has corresponds with its chemical properties. For example, sodium differs from neon and potassium by one atomic number, but it resembles potassium, not neon. Sodium and potassium both have one valence electron, which explains their similar properties, while neon is a stable element with eight valence electrons. We use VSEPR to predict the three-dimensional structure, or geometry, of molecules.
Completing this unit should take you approximately 5 hours.
3.1: Chemical Bonds
Now that we understand atomic structure and electron configurations, we are ready to learn how valence electrons combine to form chemical bonds between atoms.
To begin our exploration of bonding, we need to define the two main types of bonds: covalent, and ionic. Covalent bonds occur mostly between nonmetal atoms. In a covalent bond, the electrons are shared between atoms. Ionic bonds occur between metal ions and nonmetal ions, or polyatomic ions. In an ionic bond, the positively charged metal ion or ions are attracted to the negatively charged nonmetal ion or ions. It is an electrostatic interaction.
3.2: Molecular Structure
Now that we can draw Lewis structures, we can determine the shape, or molecular geometry, of molecules. This is important because the shape of a molecule often determines its reactivity, its intermolecular forces, and other properties.
For example, carbon dioxide, CO2, and water, H2O, are both small molecules with three atoms total. However, CO2 has a linear shape. This makes it nonpolar, and it is a gas at room temperature. Water has a bent, or v shape. This makes water a polar molecule, and, as we know, it is a liquid at room temperature.
The main way we understand molecular geometry is through Valence Shell Electron Pair Repulsion Theory, or VSEPR. The idea of VSEPR is that pairs of electrons in the valence, or outer shell, in a molecule will be repelled and get as far apart as possible in three-dimensional space. VSEPR is based on Lewis dot structures.