BackChapter 5: Molecules and Compounds – Structured Study Notes
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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 compounds are often very different from the properties of the elements that compose them.
Example: Sucrose (table sugar) is composed of carbon, hydrogen, and oxygen, but its properties differ greatly from those of graphite (carbon), hydrogen gas, or oxygen gas.
Example: Sodium chloride (table salt) is formed from sodium (a reactive metal) and chlorine (a poisonous yellow gas), yet salt is safe and stable.
Compounds vs. Mixtures
Most substances in nature are compounds, not elements. Free atoms are rare. Compounds differ from mixtures in that their constituent elements combine in fixed, definite proportions, while mixtures can have variable proportions.
Compound: Elements combine in fixed ratios (e.g., water always has a 2:1 ratio of hydrogen to oxygen).
Mixture: Proportions of elements can vary (e.g., a balloon filled with hydrogen and oxygen gas).
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 principle was formally stated by Joseph Proust.
Example: Decomposing 18.0 g of water yields 16.0 g of oxygen and 2.0 g of hydrogen, giving a mass ratio of 8.0:1.
Example: Decomposing 17.0 g of ammonia yields 14.0 g of nitrogen and 3.0 g of hydrogen, giving a mass ratio of 4.7:1.
Chemical Formulas
Chemical formulas represent compounds by indicating the elements present and the relative number of atoms of each. Subscripts show the number of atoms; a subscript of 1 is omitted by convention.
Example: H2O for water (2 hydrogen, 1 oxygen).
Example: NaCl for table salt (1 sodium, 1 chlorine).
Example: CO2 for carbon dioxide (1 carbon, 2 oxygen).
Order of Elements in Chemical Formulas
Formulas list the most metallic element first. For compounds without metals, the more metal-like element is listed first. Among nonmetals, elements to the left and lower in the periodic table are listed before those to the right and higher.
Example: NaCl, not ClNa; NO2, not O2N.
Polyatomic Ions in Chemical Formulas
Some compounds contain groups of atoms that act as a unit, called polyatomic ions. When several groups are present, parentheses and subscripts indicate their number.
Example: Mg(NO3)2 contains one magnesium atom and two nitrate groups.
To find the total number of each atom, multiply the subscript outside the parentheses by the subscripts inside.
Types of Chemical Formulas
There are three main types of chemical formulas:
Empirical formula: Shows the simplest ratio of elements (e.g., HO for hydrogen peroxide).
Molecular formula: Shows the actual number of atoms (e.g., H2O2 for hydrogen peroxide).
Structural formula: Shows how atoms are connected.
Molecular models (ball-and-stick and space-filling) visually represent the three-dimensional structure of molecules.
Representations of Matter
Chemists use macroscopic, molecular, and symbolic representations to connect the observable world with the atomic and molecular world.
Classification of Elements and Compounds
Pure Substances: Elements and Compounds
Pure substances are classified as either elements or compounds. Elements can be atomic or molecular, while compounds can be molecular or ionic.
Atomic vs. Molecular Elements
Atomic elements exist as single atoms (e.g., mercury), while molecular elements exist as diatomic molecules (e.g., chlorine, O2).
Atomic element: Mercury (Hg) exists as individual atoms.
Molecular element: Chlorine (Cl2) exists as pairs of atoms.
Diatomic Elements
Some elements naturally occur as diatomic molecules. These include hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine.
Name of Element | Formula of Basic Unit |
|---|---|
hydrogen | H2 |
nitrogen | N2 |
oxygen | O2 |
fluorine | F2 |
chlorine | Cl2 |
bromine | Br2 |
iodine | I2 |
Molecular vs. Ionic Compounds
Molecular compounds are formed from two or more nonmetals and consist of molecules. Ionic compounds contain cations (usually metals) and anions (usually nonmetals), forming a three-dimensional array called a formula unit.
Example: Dry ice (CO2) is a molecular compound.
Example: Table salt (NaCl) is an ionic compound.
Ionic Compounds
Formation and Structure
Ionic compounds form when metals lose electrons and nonmetals gain electrons, resulting in positive (cation) and negative (anion) ions that are attracted to each other. The basic unit is the formula unit, not a molecule.
Writing Formulas for Ionic Compounds
To write the formula for an ionic compound:
Write the symbol and charge for the metal and nonmetal.
Use the magnitude of each ion's charge as the subscript for the other ion.
Reduce subscripts to the smallest whole-number ratio.
Ensure the sum of charges is zero (charge-neutral).
Polyatomic Ions
Polyatomic ions are groups of atoms with an overall charge. Familiarity with common polyatomic ions is essential for writing and naming compounds.
Name | Formula | Name | Formula |
|---|---|---|---|
acetate | C2H3O2- | hypochlorite | ClO- |
carbonate | CO32- | chlorite | ClO2- |
hydrogen carbonate | HCO3- | chlorate | ClO3- |
nitrate | NO3- | perchlorate | ClO4- |
nitrite | NO2- | sulfate | SO42- |
phosphate | PO43- | sulfite | SO32- |
ammonium | NH4+ | cyanide | CN- |
Nomenclature: Naming Compounds
Naming Ionic Compounds
To name an ionic compound, first identify it as ionic (metal + nonmetal). There are two main types:
Type I: Metal forms only one type of ion (invariant charge).
Type II: Metal forms more than one type of ion (variable charge).
Binary Ionic Compounds with Metal of Invariant Charge
Names have the form: name of cation (metal) + base name of anion (nonmetal) + -ide.
Example: NaCl is sodium chloride.
Nonmetal | Symbol for Ion | Base Name | Anion Name |
|---|---|---|---|
fluorine | F- | fluor- | fluoride |
chlorine | Cl- | chlor- | chloride |
bromine | Br- | brom- | bromide |
iodine | I- | iod- | iodide |
oxygen | O2- | ox- | oxide |
sulfur | S2- | sulf- | sulfide |
nitrogen | N3- | nitr- | nitride |
Binary Ionic Compounds with Metal Whose Charge May Vary
Names have the form: name of cation (metal) + charge of cation in Roman numerals + base name of anion + -ide.
Example: FeCl3 is iron(III) chloride.
Example: Cu+ is copper(I), Cu2+ is copper(II).
Naming Ionic Compounds with Polyatomic Ions
Use the name of the polyatomic ion whenever it occurs in the compound.
Example: KNO3 is potassium nitrate.
Example: Fe(OH)2 is iron(II) hydroxide.
Example: NH4NO3 is ammonium nitrate.
Naming Oxyanions and Halo-Oxyanions
Oxyanions are anions containing oxygen. The one with more oxygen atoms gets the ending -ate, the one with fewer gets -ite. Prefixes hypo- and per- are used for series with more than two ions.
Example: NO3- is nitrate, NO2- is nitrite.
Example: ClO- is hypochlorite, ClO4- is perchlorate.
Polyatomic Ions in Everyday Chemistry
Polyatomic ions are found in many household products:
Sodium hypochlorite: Bleach
Sodium bicarbonate: Baking soda
Calcium carbonate: Antacids
Sodium nitrite: Food preservative
Naming Molecular Compounds
Rules and Prefixes
Molecular compounds are formed from two or more nonmetals. Names use prefixes to indicate the number of atoms:
mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8), nona- (9), deca- (10)
If only one atom of the first element, the prefix mono- is omitted.
Vowel is dropped when prefix ends and base name begins with a vowel (e.g., monoxide).
Naming Acids
Classification and Naming
Acids are molecular compounds that form ions in water. They are classified as binary acids (hydrogen + nonmetal) or oxyacids (hydrogen + nonmetal + oxygen).
Naming Binary Acids
Names have the form: hydro + base name of nonmetal + -ic + acid.
Example: HCl(aq) is hydrochloric acid.
Example: HBr(aq) is hydrobromic acid.
Example: H2S(aq) is hydrosulfuric acid.
Naming Oxyacids
Oxyacids contain oxyanions. The number of H+ ions depends on the charge of the oxyanion. Names depend on the ending of the oxyanion:
-ate: base name of oxyanion + -ic + acid (e.g., HNO3 is nitric acid)
-ite: base name of oxyanion + -ous + acid (e.g., HNO2 is nitrous acid)
Acid Formula | Acid Name | Oxyanion Name | Oxyanion Formula |
|---|---|---|---|
HNO2 | nitrous acid | nitrite | NO2- |
HNO3 | nitric acid | nitrate | NO3- |
H2SO3 | sulfurous acid | sulfite | SO32- |
H2SO4 | sulfuric acid | sulfate | SO42- |
HClO2 | chlorous acid | chlorite | ClO2- |
HClO3 | chloric acid | chlorate | ClO3- |
H2CO3 | carbonic acid | carbonate | CO32- |
Chemistry in the Environment: Acid Rain
Acid rain forms when rainwater mixes with air pollutants such as NO, NO2, and SO2, which react to form acids like HNO3 and H2SO4. Acid rain harms aquatic life, weakens trees, and damages buildings.
Formula Mass
Definition and Calculation
The formula mass of a compound is the sum of the atomic masses of all atoms in its chemical formula.
Formula:
Summary of Key Learning Objectives
Restate and apply the law of constant composition.
Write chemical formulas and determine the total number of each type of atom.
Classify elements as atomic or molecular, and compounds as ionic or molecular.
Write formulas for ionic compounds and name them according to their type.
Name compounds containing polyatomic ions, molecular compounds, binary acids, and oxyacids.
Calculate formula mass for compounds.
