Chapter 4: Basic Chemical Bonding – Compounds, Formulas, and Nomenclature

Chapter 4: Basic Chemical Bonding

4.1 Compounds and Molecules

This section introduces the fundamental concepts of molecules and compounds, their types, and how they are classified based on their composition and bonding.

• Molecule: A discrete, uncharged group of two or more atoms (usually nonmetals) held together by covalent bonds.

• Molecular elements: Substances where two or more atoms of the same element are joined by covalent bonds (e.g., O2, H2).

• Covalent molecular compounds: Compounds formed from two or more different nonmetal atoms joined by covalent bonds (e.g., H2O, CO2, C6H12O6).

• Compound: Composed of two or more different elements chemically combined in fixed, definite proportions. Can be decomposed into simpler substances by chemical means (e.g., H2O, NaCl).

• Covalent (Molecular) compound: Formed from the reaction between two or more nonmetals; atoms share electrons (covalent bond). Basic unit: molecule.

• Ionic compound: Formed by the transfer of one or more electrons from a metal to a nonmetal (ionic bond). Basic unit: formula unit (e.g., NaCl).

Classification of Elements and Compounds:

• Elements: Atomic (e.g., Ne) or Molecular (e.g., O2).

• Compounds: Molecular (e.g., H2O) or Ionic (e.g., NaCl).

Metals vs. Nonmetals:

• Ionic compounds tend to form between metals and nonmetals.

• Molecular (covalent) compounds tend to involve only nonmetals.

Example Classification:

• Aluminum (Al): Atomic element

• Chlorine (Cl2): Molecular element

• Acetone (C3H6O): Covalent molecular compound

• Sodium bromide (NaBr): Ionic compound

4.2 Chemical Formulas

Chemical formulas represent the composition of substances using element symbols and numerical subscripts to indicate the ratio of atoms.

• Structural formula: Shows the type, actual number, and arrangement of atoms in a molecule.

• Condensed structural formula: Shows connections between atoms in a molecule (e.g., CH3CH2).

• Molecular formula: Indicates the actual type and number of elemental atoms in the compound (e.g., C2H4).

• Empirical formula: Shows the simplest whole-number ratio of atoms in the molecule (e.g., CH2).

Models for Representing Molecules:

• Space-filling model: Shows relative sizes and orientations of atoms in a molecule.

• Ball-and-stick model: Atoms as balls and bonds as sticks, reflecting molecular shape.

Name of Compound	Empirical Formula	Molecular Formula	Structural Formula	Ball-and-Stick Model	Space-Filling Model
Benzene	CH	C6H6	Ring structure	Ball-and-stick	Space-filling
Acetylene	CH	C2H2	H–C≡C–H	Ball-and-stick	Space-filling
Glucose	CH2O	C6H12O6	Ring structure	Ball-and-stick	Space-filling
Ammonia	NH3	NH3	H–N–H	Ball-and-stick	Space-filling

4.3 Ionic and Covalent Compounds

Ionic and covalent compounds differ in their bonding, composition, and properties.

	IONIC	COVALENT
Bonded Name	Formula Unit	Molecule
Bonding Type	Transfer e-	Share e-
Types of Elements	Metal & Nonmetal	Nonmetals
Physical State	Solid	Solid, Liquid, or Gas
Melting Point	High (above 300°C)	Low (below 300°C)
Conductivity	Good	Poor

Properties of Ionic Compounds:

• Hard, rigid, brittle, high melting points

• Do not conduct electricity in solid state (ions fixed in lattice)

• Conduct electricity when melted or dissolved (ions free to move)

Compound	mp (°C)	bp (°C)
MgCl2	714	1412
KBr	734	1435
CaCl2	782	>1600
NaCl	801	1413
MgO	2852	3600

Ionic Lattice: Ionic compounds form a 3D lattice held together by electrostatic attractions between oppositely charged ions. The chemical formula indicates the simplest ratio of ions.

Why Ionic Compounds Crack: When an external force moves like charges near each other, repulsion causes the crystal to crack.

4.4–4.5 Nomenclature for Ionic and Covalent Compounds

Nomenclature is the system of naming chemical compounds according to standardized rules (IUPAC).

• Binary Compounds: Composed of two different elements.

• Binary Ionic Compounds: Metal + nonmetal

• Binary Covalent Compounds: Nonmetal + nonmetal

Type I Ionic Compounds: Metal forms only one cation (e.g., Na+, Ca2+).

• Name = metal ion name + nonmetal ion name (root + -ide)

• Example: NaCl = sodium chloride

Type II Ionic Compounds: Metal can form more than one cation (usually transition metals).

• Charge on metal ion specified by Roman numeral.

• Example: FeCl2 = iron(II) chloride

Type III Binary Covalent Compounds: Formed between two nonmetals.

• First element named fully; second as if anion (-ide).

• Prefixes denote number of atoms (mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-).

• Example: CO2 = carbon dioxide

Number	Greek Prefix
1	mono-
2	di-
3	tri-
4	tetra-
5	penta-
6	hexa-
7	hepta-
8	octa-
9	nona-
10	deca-

Examples:

• SO2: sulfur dioxide

• P2O5: diphosphorus pentoxide

4.4–4.5 Polyatomic Ions and Oxyanions

Polyatomic ions are charged entities composed of several atoms bound together. They have special names and must be memorized.

Name	Ion Symbol
Ammonium	NH4+
Acetate	C2H3O2-
Nitrate	NO3-
Sulfate	SO42-
Phosphate	PO43-

Oxyanions: Polyatomic anions containing oxygen. Naming depends on the number of oxygens:

• -ate: Higher number of oxygens (e.g., sulfate SO42-)

• -ite: Lower number of oxygens (e.g., sulfite SO32-)

• Prefixes: per- (more than), hypo- (less than)

Naming Ionic Compounds with Polyatomic Ions: Follows similar rules as binary compounds. Example: NH4C2H3O2 is ammonium acetate.

4.4–4.5 Naming Acids

Acids are recognized by the hydrogen that appears first in the formula (e.g., HCl). Naming depends on the presence of oxygen in the anion.

• If the anion does not contain oxygen: Use prefix hydro- and suffix -ic (e.g., HCl: hydrochloric acid).

• If the anion contains oxygen:

  • If the anion ends in -ate: Suffix -ic (e.g., HNO3: nitric acid).

  • If the anion ends in -ite: Suffix -ous (e.g., HNO2: nitrous acid).

Examples:

• H2SO4: sulfuric acid

• H2SO3: sulfurous acid

• HC2H3O2: acetic acid

Summary Table: Key Differences Between Ionic and Covalent Compounds

Property	Ionic Compounds	Covalent Compounds
Constituent Elements	Metal + Nonmetal	Nonmetals only
Bonding Mechanism	Electron transfer	Electron sharing
Basic Unit	Formula unit	Molecule
Physical State	Solid (usually)	Solid, liquid, or gas
Melting Point	High	Low
Electrical Conductivity	Good (when molten or dissolved)	Poor

Additional info: The notes also include classification exercises, periodic table references for metals/nonmetals, and flowcharts for naming compounds, which are useful for exam preparation and practical application of nomenclature rules.