Chapter 3: Nomenclature, Elements, and Compounds

Introduction to Compounds and Chemical Bonds

Elements combine to form compounds, which possess properties distinct from their constituent elements. Compounds are held together by chemical bonds, which are forces of attraction between atoms, primarily due to interactions between protons and electrons.

• Elements are pure substances consisting of only one type of atom.

• Compounds are substances composed of two or more different elements chemically bonded together.

• Chemical bonds can be ionic (transfer of electrons) or covalent (sharing of electrons).

Properties of Elements vs. Compounds

The properties of compounds are often drastically different from those of the elements that compose them. For example, hydrogen and oxygen are both gases with distinct properties, but when combined, they form water, a liquid with unique characteristics.

Chemical Bonds

Types of Chemical Bonds

Chemical bonds are the attractive forces that hold atoms together in compounds. The two main types are:

• Ionic bonds: Formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions that attract each other. Typically occur between metals and nonmetals.

• Covalent bonds: Formed when two atoms share electrons. Typically occur between nonmetals.

Bond Stability

The most stable arrangement of atoms in a molecule is the one with the lowest potential energy. Atoms adjust their positions to achieve this stability.

Formulas and Types of Compounds

Types of Chemical Formulas

• Empirical formula: Shows the simplest whole-number ratio of atoms in a compound. Example: The empirical formula for oxalic acid is CHO2.

• Molecular formula: Shows the actual number of atoms of each element in a molecule. Example: The molecular formula for oxalic acid is C2H2O4.

Classification of Elements and Compounds

Pure substances can be classified as elements or compounds. Elements can be atomic or molecular, while compounds can be molecular or ionic.

• Atomic elements: Exist as single atoms (e.g., Ne).

• Molecular elements: Exist as molecules composed of two or more atoms of the same element (e.g., O2).

• Molecular compounds: Composed of molecules made only of nonmetals (e.g., H2O).

• Ionic compounds: Composed of cations and anions (e.g., NaCl).

Molecular Elements

Certain elements exist naturally as molecules rather than individual atoms. The most common are the diatomic elements (H2, N2, O2, F2, Cl2, Br2, I2), often remembered by the "rule of 7's." Some elements form polyatomic molecules (e.g., P4, S8).

Naming Ionic Compounds

General Rules for Naming Ionic Compounds

Ionic compounds are named by identifying the cation (usually a metal) and the anion (usually a nonmetal or polyatomic ion).

• If the metal forms only one type of ion (invariant charge), simply use the metal name.

• If the metal can form more than one type of ion (variable charge), indicate the charge with a Roman numeral in parentheses.

• The anion is named by changing the ending of the nonmetal to -ide or by using the name of the polyatomic ion.

Examples

• KCl: potassium chloride

• MgBr2: magnesium bromide

• Fe2S3: iron(III) sulfide

Polyatomic Ions and Compounds

Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms covalently bonded, carrying a net charge. Common examples include sulfate (SO42−), nitrate (NO3−), and ammonium (NH4+).

• Names and charges of polyatomic ions do not change in compounds.

• Patterns exist for naming related ions (e.g., -ate, -ite, per-, hypo- prefixes and suffixes).

Hydrates

Definition and Naming

Hydrates are ionic compounds that include a specific number of water molecules per formula unit. The water can be removed by heating, resulting in an anhydrous compound.

• The number of water molecules is indicated by a prefix (e.g., hexahydrate for six waters).

• Example: CoCl2·6H2O is cobalt(II) chloride hexahydrate.

Naming Molecular (Covalent) Compounds

Rules for Naming Binary Molecular Compounds

Binary molecular compounds are composed of two nonmetals. Naming uses prefixes to indicate the number of each atom present.

• The first element is named in full; the second element is named with an -ide suffix.

• Prefixes (mono-, di-, tri-, tetra-, etc.) indicate the number of atoms. Mono- is not used for the first element.

Examples

• BF3: boron trifluoride

• NO2: nitrogen dioxide

• N2O5: dinitrogen pentoxide

Naming Acids

Types of Acids

Acids are molecular compounds that release H+ ions in water. They are classified as binary acids (H and one other element) or oxyacids (H, O, and another element).

Naming Binary Acids

• Prefix "hydro-" + base name of nonmetal + "-ic" + "acid"

• Example: HCl(aq) is hydrochloric acid.

Naming Oxyacids

• If the polyatomic ion ends in -ate, change to -ic acid (e.g., H2SO4: sulfuric acid).

• If the polyatomic ion ends in -ite, change to -ous acid (e.g., H2SO3: sulfurous acid).

Introduction to Organic Compounds

Carbon Bonding

Carbon atoms form four covalent bonds and can create a vast array of structures, including chains and rings. Organic compounds are primarily based on covalent bonding of carbon with other elements, especially hydrogen, oxygen, and nitrogen.

Summary Table: Common Hydrocarbons

Additional info: This guide covers the essential nomenclature and classification concepts for elements and compounds, including ionic and covalent bonding, molecular and ionic compounds, hydrates, acids, and an introduction to organic compounds. For further study, refer to the periodic table and practice naming and writing formulas for a variety of compounds.