Chemical Bonding and Nomenclature: Covalent and Ionic Compounds

Chemical Bonding and Molecular Representation

Sharing Electrons (Covalent Bonds)

When two or more atoms share electrons to bond together, a covalent bond is formed. The resulting collection of atoms is called a molecule. Molecules can be represented in several ways, each providing different information about the compound.

• Chemical formula: Uses element symbols and subscripts to indicate the types and numbers of atoms present. Chemical formulas can be empirical or molecular.

• Empirical formula: The simplest whole-number ratio of elements in a compound. Examples:

  • Ethane (C2H6): empirical formula is CH3

  • Benzene (C6H6): empirical formula is CH

  • Caffeine (C8H10N4O2): empirical formula is C4H5N2O

• Molecular formula: Shows the actual number and type of atoms in a single molecule of a compound. It is a whole-number multiple of the empirical formula. Examples: CO2, SiO2, CH4, C2H6, O2, H2O, NH3, P2O5

• Structural formula: Illustrates how atoms are bonded (using lines for bonds). Each line represents a pair of shared electrons. The structural formula does not necessarily indicate the molecule's shape, but it shows which atoms are connected.

Three-Dimensional Molecular Models

• Ball-and-stick model: Shows atoms as spheres and bonds as sticks, representing the relative orientation of atoms in a molecule.

• Space-filling model: Depicts the relative sizes of atoms and their spatial arrangement, giving a more realistic view of molecular shape.

Ionic Bonding

Forces of Attraction and Ion Formation

Ionic bonding involves the electrostatic attraction between oppositely charged ions (cations and anions). These forces are usually much stronger than covalent bonds. Ionic compounds typically form between metals (which lose electrons to become cations) and nonmetals (which gain electrons to become anions).

• Cation: A positively charged ion, usually a metal.

• Anion: A negatively charged ion, usually a nonmetal.

On the periodic table, metals are found on the left and center, while nonmetals are on the right.

Naming Simple Compounds (Nomenclature)

Common Names vs. Systematic Names

Historically, many compounds were given common names (e.g., saltpeter for KNO3, Epsom salts for MgSO4), but as the number of known compounds grew, a systematic naming scheme became necessary. In general chemistry, we focus on inorganic compounds and use systematic nomenclature rules.

Binary Compounds

Binary compounds are composed of two elements. The rules for naming these compounds are as follows:

1. The cation is always named first, followed by the anion.

2. A monatomic (single atom) cation takes its name from the element.

3. A monatomic anion is named by taking the root of the element name and adding -ide.

Examples:

Compound	Ions Present	Name
NaCl	Na+ and Cl-	Sodium Chloride
KI	K+ and I-	Potassium Iodide
KBr	K+ and Br-	Potassium Bromide
CaS	Ca2+ and S2-	Calcium Sulfide
Li3N	Li+ and N3-	Lithium Nitride
Na3N	Na+ and N3-	Sodium Nitride
CsBr	Cs+ and Br-	Cesium Bromide
MgO	Mg2+ and O2-	Magnesium Oxide

Binary Ionic Compounds (Type II)

Some metals (usually transition metals) can form more than one type of cation with different charges. To distinguish between these, Roman numerals are used to indicate the charge on the metal.

• The cation with the higher charge has a name ending in -ic.

• The cation with the lower charge has a name ending in -ous.

Alternatively, the charge is indicated in parentheses using Roman numerals.

Examples:

Compound	Traditional Name	Stock Name
FeCl2	Ferrous chloride	Iron(II) chloride
FeCl3	Ferric chloride	Iron(III) chloride
CuCl	Cuprous chloride	Copper(I) chloride
CuCl2	Cupric chloride	Copper(II) chloride
Hg2Cl2	Mercurous chloride	Mercury(I) chloride
HgCl2	Mercuric chloride	Mercury(II) chloride

This Roman numeral method is only used when there is more than one ionic compound that can form between two elements (usually with transition metals).

• Some metals only form one cation (e.g., Zn2+, Ag+, Sc3+, Al3+), so Roman numerals are not used for these.

Ionic Compounds with Polyatomic Ions

Polyatomic ions are ions composed of more than one atom. These must be memorized as they are commonly encountered in chemistry.

Examples:

• NH4OH

• NH4Cl

• PH4Br

• NH4NO3

• KNO3

• Na2SO4

Oxyanions

Oxyanions are polyatomic ions that contain oxygen. The names of oxyanions often reflect the number of oxygen atoms present:

Ion	Name	Notes
SO42-	Sulfate
SO32-	Sulfite
ClO3-	Chlorate
ClO4-	Perchlorate	"Per-" means more oxygen than chlorate

When only two oxyanions exist for an element, the one with more oxygen ends in -ate and the one with less ends in -ite (e.g., sulfate vs. sulfite).

Key Formulas and Equations

• Empirical formula: $\text{Empirical formula} = \text{smallest whole-number ratio of elements}$

• Molecular formula: $\text{Molecular formula} = n \times \text{Empirical formula}$, where $n$ is an integer

Summary Table: Types of Chemical Formulas and Models

Type	Description	Example
Empirical Formula	Smallest whole-number ratio of elements	CH (for benzene)
Molecular Formula	Actual number of atoms in a molecule	C6H6 (benzene)
Structural Formula	Shows how atoms are bonded	H–C–C–H (for ethane)
Ball-and-stick Model	3D orientation of atoms and bonds	See ethane model above
Space-filling Model	Relative size and orientation of atoms	See ethane model above

Additional info: The above notes provide a foundation for understanding chemical bonding, molecular representation, and the systematic naming of inorganic compounds, which are essential topics in General Chemistry.