Introduction to Chemical Bonding

Primary Types of Chemical Bonds

Chemical bonds are categorized into three main types based on how the valence electrons behave:

1. Ionic Bonding

Ionic bonds occur through the transfer of valence electrons from one atom to another. This typically happens between a metal and a nonmetal. The metal loses electrons to become a positive cation, while the nonmetal gains those electrons to become a negative anion. The bond itself is the electrostatic force that holds these oppositely charged particles together.

Ionic compounds often organize into a crystal lattice, a 3D geometric arrangement where every positive ion is surrounded by negative ions and vice versa. These substances generally have high melting and boiling points and are brittle.

2. Covalent Bonding

Unlike ionic bonds, covalent bonds involve the sharing of electrons between atoms, usually two nonmetals. This sharing allows both atoms to achieve a full outer shell.

Covalent bonds can be classified by their strength and polarity:

• Bond Order: Atoms can share one pair (single bond), two pairs (double bond), or three pairs (triple bond) of electrons. Triple bonds are shorter and stronger than single bonds.
• Polarity: If electrons are shared equally, the bond is nonpolar. If one atom is more electronegative and "hogs" the electrons, the bond is polar covalent, creating partial charges.

3. Metallic Bonding

Metallic bonds occur between metal atoms. Instead of transferring or sharing electrons in pairs, metals form a "sea of electrons". In this model, valence electrons are delocalized, meaning they are not held to any one particular atom but can flow freely around the positive metal ions. This "sea" is what allows metals to be excellent conductors of electricity and heat.

Representing Chemistry: Formulas and Structures

To communicate the composition of substances, chemists use specific notations. A compound is a pure substance made of two or more different elements. This is represented by a chemical formula.

• Subscripts: Small numbers following a symbol indicate how many atoms of that element are present (e.g., $H_{2}O$ has 2 hydrogen atoms).
• Coefficients: Large numbers placed before a formula indicate the total number of molecules (e.g., $3O_{2}$ means 3 molecules of oxygen gas).

To visualize covalent compounds, we use Lewis Structures (or dot diagrams). These diagrams show how valence electrons are distributed, including bonding pairs (shared electrons) and lone pairs (nonbonding electrons).

Pro-Tip: The HONC 1234 Rule

• Hydrogen: forms 1 bond
• Oxygen: forms 2 bonds
• Nitrogen: forms 3 bonds
• Carbon: forms 4 bonds

Molecular Geometry and VSEPR Theory

The physical shape of a molecule is not random; it is determined by the Valence Shell Electron-Pair Repulsion (VSEPR) theory. Because electrons are negatively charged, they repel each other. Consequently, electron pairs (both bonding pairs and lone pairs) will arrange themselves as far apart as possible to minimize this repulsion.

When determining shape, we distinguish between two concepts:

1. Electron Geometry: The arrangement of all electron domains (bonds and lone pairs) around the central atom.
2. Molecular Geometry: The more specific arrangement of only the atoms around the central atom.

For example, a central atom with three electron domains might have a "trigonal planar" electron geometry, but if one of those domains is a lone pair, the molecular shape is described as "bent".