Introduction to Ionic and Covalent Bonding

Because opposite charges attract, the resulting cations and anions are held together by strong electrostatic forces, creating an ionic bond.

Naming and Formulas for Ionic Compounds

When naming simple ionic compounds, you name the metal (cation) first, followed by the nonmetal (anion) with its ending changed to -ide. For example, a compound of lithium and sulfur is named lithium sulphide.

When writing chemical formulas, you must balance the positive and negative charges so that the overall charge of the compound is zero. You list the metal first, followed by the nonmetal, using subscripts to indicate the ratio of ions needed to achieve balance.

Note on Variable Charges: Some metals, particularly transition metals, can form more than one type of cation (e.g., Iron can be $\mathrm{Fe}^{2+}$ or $\mathrm{Fe}^{3+}$). In these cases, you must use a Roman Numeral in the name to specify the charge of the metal. For example, $\mathrm{FeCl}_{2}$ is iron(II) chloride.

Polyatomic Ions

Sometimes, groups of atoms stick together as a single unit and carry an overall charge; these are called polyatomic ions. Their names often end in -ate or -ite. When writing formulas involving these, use brackets to indicate the ratio of the group. For example, if you need three chlorate groups to balance a metal, you would write $\mathrm{Al(ClO_{3})_{3}}$.

Covalent Compounds: The Sharing of Electrons

While ionic bonds involve a total transfer, covalent bonds occur when two or more nonmetals share one or more pairs of electrons. Because the electrons are shared, these molecules are electrically neutral and do not form ions.

Naming Covalent Compounds

Naming covalent compounds requires the use of Greek prefixes to specify exactly how many atoms of each element are present. Common prefixes include mono-, di-, tri-, tetra-, penta-, and hexa-.

• Example: $\mathrm{N_{2}O_{3}}$ is named dinitrogen trioxide.
• Rule: Scientists generally omit the "mono-" prefix for the first atom unless it is carbon monoxide.

Polar vs. Nonpolar Covalent Bonds

The type of covalent bond depends on electronegativity—the measure of how strongly an atom attracts electrons.

1. Nonpolar Covalent: Occurs when atoms have similar electronegativity (difference $\le 0.4$). Electrons are shared equally.
2. Polar Covalent: Occurs when there is a moderate difference in electronegativity ($0.5$ to $1.7$). Electrons are shared unequally, creating partial charges: a partial negative ($\delta^-$) on the stronger atom and a partial positive ($\delta^+$) on the weaker atom.

Physical and Chemical Properties

We can distinguish between ionic and covalent compounds by observing their physical properties, which are characteristics that can be observed without changing the chemical identity of the substance.

Exam Tip: If you are given an unknown substance and told it has a very low melting point, you can predict it is likely a covalent compound. If it conducts electricity well in water, it is likely ionic.

Intermolecular Forces

While chemical bonds (intramolecular forces) hold atoms together within a molecule, intermolecular forces are the attractions between separate molecules. The strength of these forces determines physical properties like boiling points.

The forces are ranked from strongest to weakest as follows:

1. Ion-Ion Attractions: Found in ionic compounds.
2. Hydrogen Bonding: A strong version of dipole-dipole attraction occurring when Hydrogen is bonded to Fluorine, Oxygen, or Nitrogen.
3. Dipole-Dipole Attractions: Occur between polar covalent molecules.
4. Dispersion Forces: Weak, temporary attractions found in all molecules, but the primary force in nonpolar covalent compounds.