Molecules and Compounds

Introduction to Compounds

Elements combine in fixed ratios to form compounds, resulting in the vast diversity of substances observed in nature. Compounds have properties distinct from the elements that compose them, and their formation is governed by the types of chemical bonds between atoms.

• Compound: A substance composed of two or more elements chemically combined in fixed, definite proportions.

• Mixture: A physical combination of substances where the components can vary in proportion.

• Example: Water (H2O) is always composed of two hydrogen atoms and one oxygen atom, while a mixture of hydrogen and oxygen can have any ratio.

Chemical Bonds

Chemical bonds are the forces that hold atoms together in compounds. They arise from the attractions between charged particles (electrons and protons) in atoms. There are two main types of chemical bonds:

• Ionic Bonds: Formed by the transfer of electrons from a metal to a nonmetal, resulting in the formation of cations and anions held together by electrostatic forces.

• Covalent Bonds: Formed by the sharing of electrons between two or more nonmetals, resulting in the formation of molecules.

Ionic compounds in the solid phase form a lattice structure, a regular three-dimensional array of alternating cations and anions.

Representing Compounds: Chemical Formulas and Models

Chemical formulas indicate the elements present in a compound and the relative number of atoms or ions of each. There are several types of chemical formulas:

• Empirical Formula: Shows the simplest whole-number ratio of atoms in a compound.

• Molecular Formula: Shows the actual number of atoms of each element in a molecule.

• Structural Formula: Shows how atoms are connected or bonded in a molecule, often indicating geometry.

Molecular models provide three-dimensional representations of molecules:

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

• Space-filling model: Atoms fill the space between each other, approximating the actual size and shape of the molecule.

Example: Benzene (C6H6) can be represented by different models:

Classification of Elements and Compounds

Elements and compounds can be classified based on their atomic or molecular nature:

• Atomic Elements: Exist in nature as single atoms (e.g., Na, Ne, K).

• Molecular Elements: Exist as molecules composed of two or more atoms of the same element (e.g., O2, N2).

• Molecular Compounds: Composed of covalently bonded nonmetals (e.g., H2O, CO2).

• Ionic Compounds: Composed of cations and anions held together by ionic bonds (e.g., NaCl).

Naming Compounds

Naming Ionic Compounds

Ionic compounds are named based on the cations and anions they contain. There are two main types:

• Type I: Metal forms only one type of cation (charge is invariant). Name: [metal] [base name of nonmetal + -ide]. Example: KCl is potassium chloride.

• Type II: Metal forms more than one type of cation (often transition metals). Name: [metal] (Roman numeral for charge) [base name of nonmetal + -ide]. Example: FeCl2 is iron(II) chloride.

Some ionic compounds contain polyatomic ions (groups of covalently bonded atoms with an overall charge). The name of the polyatomic ion is used directly in the compound's name.

Common Polyatomic Ions

Naming Molecular Compounds

Molecular compounds are composed of two or more nonmetals. Their names use prefixes to indicate the number of each atom present:

• mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8), nona- (9), deca- (10)

• If there is only one atom of the first element, the prefix mono- is usually omitted.

• Example: CO2 is carbon dioxide; N2O4 is dinitrogen tetroxide.

Naming Acids

Acids are molecular compounds that release hydrogen ions (H+) when dissolved in water. They are named based on their composition:

• Binary acids: Composed of hydrogen and a nonmetal. Name: hydro-[base name of nonmetal]-ic acid. Example: HCl (aq) is hydrochloric acid.

• Oxyacids: Composed of hydrogen and a polyatomic oxyanion. If the ion ends in -ate, the acid name ends in -ic; if the ion ends in -ite, the acid name ends in -ous. Example: H2SO4 (sulfate) is sulfuric acid; H2SO3 (sulfite) is sulfurous acid.

Quantitative Aspects of Compounds

Formula Mass and Molar Mass

The formula mass (or molecular mass) is the sum of the atomic masses of all atoms in a molecule or formula unit. The molar mass is the mass of one mole of a substance, numerically equal to the formula mass but expressed in grams per mole (g/mol).

• Formula:

• Example: The molar mass of H2O is g/mol.

Percent Composition

The percent composition of a compound indicates the mass percentage of each element in the compound. It can be calculated from the chemical formula and the molar masses of the constituent elements.

• Formula:

Empirical and Molecular Formulas

The empirical formula gives the simplest whole-number ratio of atoms in a compound, while the molecular formula gives the actual number of atoms of each element in a molecule. The molecular formula is a whole-number multiple of the empirical formula.

• Determining Empirical Formula:

  1. Convert mass percentages to grams (assume 100 g sample).

  2. Convert grams to moles using molar mass.

  3. Write a pseudoformula using the mole values as subscripts.

  4. Divide all subscripts by the smallest number of moles to get whole numbers.

Combustion Analysis

Combustion analysis is a technique used to determine the empirical formula of compounds, especially organic compounds containing C, H, and O. The compound is burned, and the masses of the products (CO2 and H2O) are measured to deduce the original composition.

Organic Compounds

Introduction to Organic Compounds

Organic compounds are primarily composed of carbon and hydrogen, sometimes with oxygen, nitrogen, sulfur, and other elements. The key feature of organic chemistry is the versatility of carbon, which can form four covalent bonds and create chains, branches, and rings.

Hydrocarbons

Hydrocarbons are organic compounds containing only carbon and hydrogen. They are classified based on the types of bonds between carbon atoms:

• Alkanes: Only single bonds (suffix -ane).

• Alkenes: At least one double bond (suffix -ene).

• Alkynes: At least one triple bond (suffix -yne).

Base names for hydrocarbons are derived from the number of carbon atoms: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hept- (7), oct- (8), non- (9), dec- (10).

• Example: Methane (CH4), Ethane (C2H6), Propane (C3H8).

Functionalized Hydrocarbons

Functional groups are specific atoms or groups of atoms that impart characteristic chemical properties to organic compounds. Compounds with the same functional group form a family and exhibit similar reactivity.

• Example: Alcohols (–OH group), Aldehydes (–CHO group), Ketones (C=O group), Carboxylic acids (–COOH group), Amines (–NH2 group).

Additional info: The images included above are directly relevant to the structural and space-filling representations of molecules and functional groups discussed in the corresponding paragraphs.