Atoms, Elements, and Compounds

Types of Atoms in Chemical Formulas

Understanding the composition of chemical formulas is fundamental in chemistry. Each formula represents the types and numbers of atoms present in a compound.

• Atoms: The smallest unit of an element, retaining its chemical properties.

• Example: In the formula (NH4)2HPO4, count each type of atom:

  • Nitrogen (N): 2 atoms

  • Hydrogen (H): 9 atoms (8 from NH4 and 1 from HPO4)

  • Phosphorus (P): 1 atom

  • Oxygen (O): 4 atoms

Writing and Interpreting Chemical Formulas

Chemical formulas indicate the elements and their ratios in a compound.

• Example: A compound with ten oxygen atoms and four phosphorus atoms: P4O10

• Ionic Compounds: Formed from metals and nonmetals. Example: Aluminum and oxygen form Al2O3; potassium and oxygen form K2O.

Naming Compounds

Compounds are named according to specific rules based on their composition.

• CaCO3: Calcium carbonate

• Al2(SO4)3: Aluminum sulfate

• KOH: Potassium hydroxide

Matching Names and Formulas

It is important to correctly match compound names with their formulas.

• Copper(I) nitrate: CuNO3

• Copper(II) nitrate: Cu(NO3)2

• Copper(II) nitrite: Cu(NO2)2

Naming Molecular Compounds

Molecular compounds are named using prefixes to indicate the number of atoms.

• SO3: Sulfur trioxide

• SF5: Sulfur pentafluoride

• Heptaphosphorus octafluoride: P7F8

Classification of Substances

Types of Elements and Compounds

Substances can be classified as atomic elements, molecular elements, or molecular compounds.

• Atomic element: Exists as single atoms (e.g., Krypton).

• Molecular element: Exists as molecules of the same element (e.g., O2).

• Molecular compound: Molecules composed of different elements (e.g., SO2).

Mass, Moles, and Stoichiometry

Counting Atoms Using Mass and Moles

The mole is a fundamental unit for counting atoms and molecules in chemistry.

• Mole: entities (Avogadro's number).

• Example: One mole of nitrogen gas contains nitrogen atoms (since N2).

Empirical Formulas

The empirical formula shows the simplest whole-number ratio of atoms in a compound.

• Example: C2H3O2 could be an empirical formula.

Calculating Number of Atoms and Moles

• Number of atoms:

• Example: Number of mercury atoms in 5.8 mol:

Calculating Molar Mass

Molar mass is the mass of one mole of a substance, expressed in grams per mole.

• Potassium nitrate (KNO3):

• Diboron tetrachloride (B2Cl4):

• Calcium nitrate (Ca(NO3)2):

Mass Percent and Empirical Formula Calculations

• Mass percent:

• Empirical formula: Use masses of elements to determine the simplest ratio.

• Example: If decomposition produces 1.78 g N and 4.05 g O, calculate moles and find the ratio.

Chemical Reactions and Equations

Evidence of Chemical Reactions

Chemical reactions can be identified by observable changes.

• Formation of a new solid or gas

• Color change

• Emission of light

• Gas formation

Balancing Chemical Equations

Balanced equations have equal numbers of each atom on both sides.

• Example:

• Example:

Types of Chemical Reactions

• Double displacement:

• Single displacement:

Electrolytes and Solubility

Electrolytes are substances that dissociate into ions in solution.

• Strong electrolytes: Completely dissociate in water.

• Solubility: Soluble compounds dissolve completely; insoluble compounds do not.

Precipitation Reactions

Precipitation occurs when an insoluble solid forms from mixing solutions.

• Example:

• Spectator ions: Ions that do not participate in the reaction (e.g., K+, NO3-).

Complete Ionic and Net Ionic Equations

Complete ionic equations show all ions present; net ionic equations show only those involved in the reaction.

• Example: In the reaction above, Pb2+ and Cl- form the precipitate; K+ and NO3- are spectator ions.

Identifying Double Displacement Reactions

Double displacement reactions involve the exchange of ions between two compounds.

• Example:

  • KCl(aq) + AgNO3(aq) → AgCl(s) + KNO3(aq)

  • Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)

  • H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l)

Tables

Classification Table: Types of Substances

Solubility Table (Sample)

Common Types of Chemical Reactions

Additional info: Some explanations and tables have been expanded for clarity and completeness, based on standard introductory chemistry curriculum.