Chapter 5: Molecules and Compounds – Study Notes

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Chapter 5: Molecules and Compounds

Introduction to Molecules and Compounds

This chapter explores the nature of molecules and compounds, their differences from elements, and the principles governing their composition and representation. Understanding these concepts is fundamental to the study of chemistry.

Sugar’s Properties Differ from Those of the Elements C, H, and O

Ordinary table sugar is a compound called sucrose.
A sucrose molecule contains carbon, hydrogen, and oxygen atoms.
The properties of sucrose are very different from those of its constituent elements (e.g., graphite for carbon, hydrogen gas, and oxygen gas).
Example: Sucrose is sweet and edible, while graphite is a solid, hydrogen is a flammable gas, and oxygen is essential for respiration.

Salt’s Properties Differ from Those of the Elements Na and Cl

Elemental sodium (Na): A highly reactive metal that tarnishes quickly in air.
Elemental chlorine (Cl): A yellow, poisonous gas with a pungent odor.
Sodium chloride (NaCl): The compound formed by sodium and chlorine is table salt, which is safe to eat and essential for life.
The properties of a compound are generally different from the properties of the elements that compose it.

Many Natural Substances Are Compounds

Most substances encountered in everyday life are compounds, not elements.
Free atoms are rare in nature.
A compound is different from a mixture of elements.
In a compound, elements combine in fixed, definite proportions; in a mixture, proportions can vary.

Compounds Display Constant Composition

Mixture: The relative amounts of components can vary (e.g., a balloon filled with hydrogen and oxygen gas).
Chemical compound: The ratio of elements is fixed (e.g., water always has a 2:1 ratio of hydrogen to oxygen).

Proust’s Law of Constant Composition

Joseph Proust (1754–1826) stated that elements combine in fixed proportions to form compounds.
Law of constant composition: All samples of a given compound have the same proportions of their constituent elements.

Mass Ratio of Elements in Compounds

Water (H2O): Decomposing 18.0 g of water yields 16.0 g of oxygen and 2.0 g of hydrogen.
Mass ratio: or
This ratio is constant for any sample of pure water.
Ammonia (NH3): Decomposing 17.0 g yields 14.0 g N and 3.0 g H. or
Atoms combine in whole-number ratios, but mass ratios are not necessarily whole numbers.

Chemical Formulas: How to Represent Compounds

Chemical formula: Indicates the elements present in a compound and the relative number of atoms of each.
Example: for water (2:1 ratio of H:O).
Subscripts indicate the number of atoms; a subscript of 1 is omitted by convention.
Examples:
- for table salt (1:1 ratio of Na:Cl)
- for carbon dioxide (1:2 ratio of C:O)
- for sucrose (12:22:11 ratio of C:H:O)
Changing a subscript changes the compound entirely (e.g., CO vs. CO2).

How to List the Elements in Order in Compounds

Formulas list the most metallic element first.
Metals are on the left side of the periodic table; nonmetals are on the upper right.
Among nonmetals, those to the left are more metal-like and listed first.
Within a group, elements lower in the column are more metal-like.
Order for nonmetals (see Table 5.1): C, P, N, H, S, I, Br, Cl, O, F.
Historical exceptions exist (e.g., OH− for hydroxide).

C	P	N	H	S	I	Br	Cl	O	F

How to Represent Compounds with Polyatomic Ions

Some formulas contain groups of atoms acting as a unit, called polyatomic ions.
When multiple groups are present, use parentheses and a subscript (e.g., ).
To find the total number of each atom, multiply the subscript outside the parentheses by the subscripts inside.
Example: contains 1 Mg, 2 N, and 6 O atoms.

Types of Chemical Formulas

Empirical formula: Gives the relative number of atoms (simplest ratio).
Molecular formula: Gives the actual number of atoms in a molecule.
Structural formula: Shows how atoms are connected by bonds.
Molecular models: 3D representations (ball-and-stick, space-filling models).
Ball-and-stick models show atoms as balls and bonds as sticks; space-filling models show the actual space occupied by atoms.

Comparison of Formulas and Models for Methane, CH4

Molecular formula: CH4 (1 C, 4 H).
Structural formula: Shows each H bonded to the central C.
Ball-and-stick and space-filling models: Illustrate the 3D geometry of the molecule.

Connecting the Macroscopic and Molecular Worlds

Macroscopic world: What we see (e.g., a glass of water).
Atomic and molecular world: The particles that compose matter.
Symbolic representation: Chemical formulas and models used by chemists.

A Molecular View of Elements and Compounds

Pure substances can be elements or compounds.
Elements can be atomic (single atoms) or molecular (diatomic molecules).
Compounds can be molecular or ionic.

Classification of Elements and Compounds

Type	Example
Atomic Element	Ni (Nickel)
Molecular Element	O2 (Oxygen gas)
Molecular Compound	H2O (Water)
Ionic Compound	NaCl (Table salt)

Elements May Be Atomic or Molecular

Atomic elements: Exist as single atoms (e.g., mercury, nickel).
Molecular elements: Exist as diatomic molecules (e.g., O2, Cl2).