Molecules and Compounds

Introduction to Molecules and Compounds

The diversity of substances in nature arises from the ability of elements to combine and form compounds. While there are about 91 naturally occurring elements, their combinations yield an almost limitless number of compounds, each with unique properties. Understanding molecules and compounds is essential for grasping the molecular basis of life and matter.

Hydrogen, Oxygen, and Water

Properties and Comparison of Elements and Compounds

Hydrogen (H2) is an explosive gas, and oxygen (O2) is essential for combustion. Both are gases at room temperature with low boiling points. When combined, they form water (H2O), a liquid at room temperature with a much higher boiling point and very different properties from its constituent elements. This illustrates how compounds have properties distinct from the elements that compose them.

• Hydrogen: Explosive, gas at room temperature, boiling point: -253°C

• Oxygen: Supports combustion, gas at room temperature, boiling point: -183°C

• Water: Extinguishes flames, liquid at room temperature, boiling point: 100°C

In compounds, elements combine in fixed, definite proportions, unlike mixtures where proportions can vary.

Chemical Bonds

Ionic and Covalent Bonds

Atoms in compounds are held together by chemical bonds, which arise from electrostatic forces between charged particles (electrons and protons). 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 (positively charged ions) and anions (negatively charged ions). These ions are held together in a lattice by electrostatic attraction.

• Covalent Bonds: Formed by the sharing of electrons between two or more nonmetals. The shared electrons are attracted to the nuclei of both atoms, resulting in a stable molecule.

Representing Compounds: Chemical Formulas and Molecular Models

Types of Chemical Formulas

Chemical formulas indicate the elements present in a compound and the relative number of atoms or ions of each element. The main types are:

• Empirical Formula: Shows the simplest whole-number ratio of atoms in a compound (e.g., CH2 for C4H8).

• Molecular Formula: Shows the actual number of atoms of each element in a molecule (e.g., H2O2).

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

Molecular models, such as ball-and-stick and space-filling models, provide visual representations of molecular geometry and relative atom sizes.

An Atomic-Level View of Elements and Compounds

Classification of Pure Substances

Pure substances are classified as either elements or compounds:

• Atomic Elements: Exist as single atoms (e.g., Ne, Al, Fe).

• Molecular Elements: Exist as molecules composed of two or more atoms of the same element (e.g., H2, N2, Cl2).

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

• Ionic Compounds: Composed of a lattice of cations and anions (e.g., NaCl).

Molecular and Ionic Compounds

Molecular compounds are composed of discrete molecules, while ionic compounds are composed of extended networks of ions. The smallest unit of a molecular compound is a molecule; for an ionic compound, it is a formula unit.

Ionic Compounds: Formulas and Names

Writing Formulas for Ionic Compounds

Ionic compounds are charge-neutral and their formulas reflect the smallest whole-number ratio of ions. The sum of the charges of the cations must equal the sum of the charges of the anions.

• Example: Na+ and Cl- combine to form NaCl.

• Example: Ca2+ and Cl- combine to form CaCl2.

Naming Ionic Compounds

• Type 1: Metal forms only one type of cation (e.g., Na+, Ca2+). Name: [metal] [base name of nonmetal]-ide (e.g., sodium chloride).

• Type 2: Metal forms more than one type of cation (often transition metals). Name: [metal](Roman numeral) [base name of nonmetal]-ide (e.g., iron(III) chloride).

• Polyatomic Ions: Use the name of the polyatomic ion (e.g., sodium nitrate, ammonium sulfate).

• Hydrates: Include the number of water molecules (e.g., copper(II) sulfate pentahydrate).

Molecular Compounds: Formulas and Names

Naming Molecular Compounds

Molecular compounds are named using prefixes to indicate the number of each type of atom:

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

• Example: CO2 is carbon dioxide; N2O5 is dinitrogen pentoxide.

Acids: Binary and Oxyacids

Naming Acids

• Binary Acids: Composed of hydrogen and a nonmetal. Name: hydro-[base name of nonmetal]-ic acid (e.g., hydrochloric acid).

• Oxyacids: Contain hydrogen, oxygen, and another element (usually a nonmetal). Name depends on the oxyanion:

  • If the oxyanion ends in -ate: [base name of oxyanion]-ic acid (e.g., HNO3: nitric acid).

  • If the oxyanion ends in -ite: [base name of oxyanion]-ous acid (e.g., HNO2: nitrous acid).

Formula Mass and the Mole Concept for Compounds

Formula Mass and Molar Mass

The formula mass of a compound is the sum of the atomic masses of all atoms in its chemical formula. The molar mass (g/mol) is numerically equivalent to the formula mass (amu) and is used to convert between mass and moles of a compound.

• Example: Formula mass of CO2 = 12.01 + 2(16.00) = 44.01 amu

• Molar mass of CO2 = 44.01 g/mol

Composition of Compounds

Mass Percent Composition

The mass percent composition of an element in a compound is the percentage of the compound's total mass contributed by that element. It is calculated as:

• Example: In CCl2F2, mass percent Cl =

Determining Chemical Formulas from Experimental Data

Empirical and Molecular Formulas

The empirical formula is determined from the relative masses of elements in a compound, usually obtained from experimental data. The molecular formula is a whole-number multiple of the empirical formula and requires knowledge of the compound's molar mass.

To determine the empirical formula:

1. Convert masses of elements to moles.

2. Write a pseudoformula with mole values as subscripts.

3. Divide all subscripts by the smallest number of moles.

4. If necessary, multiply subscripts to get whole numbers.

To determine the molecular formula:

Molecular formula = (empirical formula) × n

Organic Compounds

Introduction to Organic Chemistry

Organic compounds are primarily composed of carbon and hydrogen, often with oxygen, nitrogen, or sulfur. The simplest organic compounds are hydrocarbons (only C and H). Hydrocarbons are classified as:

• Alkanes: Only single bonds (suffix -ane)

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

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

Functionalized hydrocarbons contain additional characteristic groups of atoms (functional groups), such as alcohols (-OH), ethers (R-O-R'), and carboxylic acids (-COOH), which impart specific chemical properties.

Summary Table: Types of Compounds and Their Representation

Key Terms

• Chemical bond: Force holding atoms together in compounds

• Ionic bond: Transfer of electrons, metal + nonmetal

• Covalent bond: Sharing of electrons, nonmetal + nonmetal

• Empirical formula: Simplest whole-number ratio of atoms

• Molecular formula: Actual number of atoms in a molecule

• Structural formula: Shows connectivity of atoms

• Formula mass: Sum of atomic masses in a formula

• Mass percent composition: Percent by mass of each element in a compound

• Hydrocarbon: Compound of only C and H

• Functional group: Characteristic group of atoms in organic compounds