Molecules and Compounds: Types, Representation, and Quantitative Composition

Chapter 3: Molecules and Compounds

Introduction to Molecules and Compounds

Molecules and compounds are fundamental chemical entities formed by the combination of atoms. Understanding their types, how they are represented, and how to quantify their composition is essential in general chemistry.

Types of Compounds

Ionic Compounds

Ionic compounds are formed when metals transfer electrons to nonmetals, resulting in the formation of oppositely charged ions (cations and anions) that are held together by electrostatic attraction known as an ionic bond. The basic unit of an ionic compound is the formula unit, which is the smallest, electrically neutral collection of ions.

• Metals lose electrons to form cations.

• Nonmetals gain electrons to form anions.

• Example: Sodium (Na) reacts with chlorine (Cl) to form sodium chloride (NaCl).

Covalent Compounds

Covalent compounds are formed when two or more nonmetals share electrons, resulting in a covalent bond. The atoms involved share electrons to achieve lower potential energy, and the resulting entity is called a molecule.

• Nonmetal + Nonmetal → Covalent compound

• Electrons are shared, not transferred.

• Example: Water (H2O) is formed by sharing electrons between hydrogen and oxygen.

Representing Compounds: Chemical Formulas and Models

Chemical Formulas

Chemical formulas indicate the elements present in a compound and the relative number of atoms or ions of each element. Subscripts denote the number of atoms; a subscript of 1 is typically omitted. The more metallic element is usually listed first.

• Molecular formula: Actual number of atoms of each element (e.g., H2O2).

• Empirical formula: Simplest whole-number ratio of atoms (e.g., HO for H2O2).

• Structural formula: Shows how atoms are connected (e.g., H–O–O–H).

Molecular Models

Models help visualize molecular structure:

• Ball-and-stick model: Atoms as balls, bonds as sticks, showing geometry.

• Space-filling model: (Not covered in detail here.)

Comparison of Formula Types

Name of Compound	Empirical Formula	Molecular Formula	Structural Formula	Ball-and-Stick Model
Benzene	CH	C6H6	Hexagonal ring with alternating double bonds	Ring structure with alternating bonds
Acetylene	CH	C2H2	H–C≡C–H	Linear structure

Classification of Elements and Compounds

Pure Substances: Elements vs. Compounds

Pure substances can be classified as elements (atomic or molecular) or compounds (molecular or ionic).

• Atomic element: Single atoms (e.g., Ne).

• Molecular element: Molecules of the same element (e.g., O2).

• Molecular compound: Molecules of different elements (e.g., H2O).

• Ionic compound: Lattice of ions (e.g., NaCl).

Practice: Classifying Substances

Classify substances as atomic element, molecular element, molecular compound, or ionic compound based on their composition and bonding.

Quantitative Composition of 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, expressed in atomic mass units (amu). The molar mass is the mass of one mole of the compound, expressed in grams per mole (g/mol).

• Formula mass = (number of atoms of element 1 × atomic mass of element 1) + (number of atoms of element 2 × atomic mass of element 2) + ...

• Molar mass (g/mol) numerically equals formula mass (amu).

The Mole Concept: Converting Between Mass and Number of Molecules

The mole is a counting unit used to relate the mass of a substance to the number of particles (atoms, molecules). Avogadro's number (6.022 × 1023) is the number of particles in one mole.

• To convert mass to moles: $\text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$

• To convert moles to number of molecules: $\text{number of molecules} = \text{moles} \times 6.022 \times 10^{23}$

Mass Percent Composition

Mass percent composition expresses the percentage by mass of each element in a compound. It is calculated as:

• $\text{Mass percent of element X} = \frac{\text{mass of element X in 1 mol of compound}}{\text{mass of 1 mol of compound}} \times 100\%$

Using Mass Percent as a Conversion Factor

Mass percent can be used to convert between the mass of an element and the mass of a compound containing that element.

• Example: If NaCl is 39% sodium by mass, the mass of Na in a given mass of NaCl can be found by multiplying by 0.39.

Determining Chemical Formulas from Experimental Data

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. Experimental data, such as mass percent composition, can be used to determine these formulas.

• Steps to determine empirical formula:

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

  2. Convert grams to moles for each element.

  3. Divide by the smallest number of moles to get subscripts.

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

Combustion Analysis

Combustion analysis is a technique used to determine the empirical formula of a compound containing carbon, hydrogen, and sometimes oxygen. The compound is burned in oxygen, and the masses of CO2 and H2O produced are measured to calculate the amounts of C and H in the original sample.

• Oxygen content is found by subtracting the masses of C and H from the total mass.

Determining Molecular Formula from Empirical Formula and Molar Mass

The molecular formula is a whole-number multiple of the empirical formula. It is determined by dividing the compound's molar mass by the empirical formula mass.

• $n = \frac{\text{molar mass}}{\text{empirical formula mass}}$

• Molecular formula = (empirical formula) × n

Summary Table: Types of Chemical Formulas

Type	Description	Example (for H2O2)
Molecular Formula	Actual number of atoms of each element	H2O2
Empirical Formula	Simplest whole-number ratio	HO
Structural Formula	Shows connectivity	H–O–O–H

Key Takeaways:

• Compounds can be ionic or covalent, with distinct bonding and properties.

• Chemical formulas and models provide different levels of information about molecular structure.

• Quantitative composition (formula mass, molar mass, mass percent) is essential for laboratory calculations.

• Empirical and molecular formulas can be determined from experimental data, including combustion analysis.