Chapter 5: Molecules and Compounds – Structured Study Notes

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Molecules and Compounds

Properties of Compounds vs. Elements

Compounds are substances formed from two or more elements chemically combined in fixed ratios. The properties of a compound are generally very different from the properties of the elements that compose it. For example, sodium chloride (table salt) is formed from sodium (a highly reactive metal) and chlorine (a poisonous yellow gas), yet the compound is safe for consumption.

Compound: A substance composed of two or more elements in fixed, definite proportions.
Element: A pure substance consisting of only one type of atom.
Example: Sodium chloride (NaCl) is table salt, distinct from sodium and chlorine.

Table salt Elemental sodium Elemental chlorine

Compounds vs. Mixtures

Most naturally occurring substances are compounds, not elements. Free atoms are rare in nature. Compounds differ from mixtures in that compounds have elements combined in fixed, definite proportions, while mixtures have variable proportions.

Compound: Elements combine in fixed ratios.
Mixture: Elements combine in variable ratios.

Mixture of hydrogen and oxygen Water balloon with fixed ratio of hydrogen to oxygen

The Law of Constant Composition

The law of constant composition states that all samples of a given compound have the same proportions of their constituent elements. This means that the mass ratio of elements in a compound is always constant, regardless of the sample's origin.

Example (Water): Decomposing 18.0 g of water yields 16.0 g of oxygen and 2.0 g of hydrogen. The mass ratio is:

Example (Ammonia): Decomposing 17.0 g of ammonia yields 14.0 g of nitrogen and 3.0 g of hydrogen. The mass ratio is:

Chemical Formulas: Representation of Compounds

Chemical formulas indicate the elements present in a compound and the relative number of atoms of each. The formula uses element symbols and subscripts; a subscript of 1 is omitted by convention.

Example: Water is represented as H2O, meaning two hydrogen atoms and one oxygen atom.
Subscripts: Indicate the number of atoms; never change for a given compound.

Chemical formula for water

Common Chemical Formulas

NaCl: Table salt (sodium chloride)
CO2: Carbon dioxide
C12H22O11: Sucrose (table sugar)

Changing Subscripts: Different Compounds

Changing the subscript in a chemical formula results in a different compound. For example, CO is carbon monoxide (toxic), while CO2 is carbon dioxide (relatively harmless).

CO and CO2 molecules

Order of Elements in Chemical Formulas

Formulas list the most metallic element first. In compounds without metals, the more metal-like element (leftmost or lower in the periodic table) is listed first.

Example: NaCl, not ClNa.

Polyatomic Ions in Chemical Formulas

Some chemical formulas contain groups of atoms that act as a unit, called polyatomic ions. When several groups are present, parentheses and subscripts are used. To determine the total number of each atom, multiply the subscript outside the parentheses by the subscript inside.

Example: Mg(NO3)2 contains one magnesium atom and two nitrate groups.

Mg(NO3)2 formula breakdown

Types of Chemical Formulas

Chemical formulas can be categorized as:

Empirical formula: Simplest whole-number ratio of atoms.
Molecular formula: Actual number of atoms in a molecule.
Structural formula: Shows how atoms are connected.

For hydrogen peroxide, the molecular formula is H2O2, and the empirical formula is HO.

Methane formulas and models

Molecular View of Elements and Compounds

Pure substances can be elements or compounds. Elements may be atomic (single atoms) or molecular (diatomic molecules). Compounds may be molecular (nonmetals) or ionic (metal and nonmetal).

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

Mercury atoms Chlorine molecules

Elements That Occur as Diatomic Molecules

Name of Element	Formula of Basic Unit
Hydrogen	H2
Nitrogen	N2
Oxygen	O2
Fluorine	F2
Chlorine	Cl2
Bromine	Br2
Iodine	I2

Compounds: Molecular vs. Ionic

Molecular compounds are formed from two or more nonmetals. Ionic compounds are formed from a metal and a nonmetal, involving electron transfer and resulting in positive (cation) and negative (anion) ions.

Molecular compound: Nonmetals only.
Ionic compound: Metal and nonmetal; contains cations and anions.

Dry ice composed of CO2 molecules NaCl formula units in table salt

Writing Formulas for Ionic Compounds

Ionic compounds must be charge-neutral. The sum of the charges of cations and anions must equal zero. The formula is determined by the charges of the ions.

Steps:
1. Write the symbol and charge for each ion.
2. Make the magnitude of the charge on each ion the subscript for the other ion.
3. Reduce subscripts to smallest whole-number ratio.
4. Check for charge neutrality.
Example: Mg2+ and Cl- combine to form MgCl2.

Writing Formulas for Compounds with Polyatomic Ions

For compounds containing polyatomic ions, determine the charges, ensure charge neutrality, and use parentheses for multiple groups.

Example: Ca2+ and NO3- combine to form Ca(NO3)2.

Formula Mass: Mass of a Molecule or Formula Unit

The formula mass is the sum of the atomic masses of all atoms in a chemical formula. For example, the formula mass of MgCl2 is calculated by adding the atomic masses of one magnesium atom and two chlorine atoms.