Atoms, Molecules, and Chemical Formulas

Empirical and Molecular Formulas

The empirical formula represents the simplest whole-number ratio of elements in a compound, while the molecular formula shows the actual number of atoms of each element in a molecule.

• Empirical Formula Calculation: Divide the number of moles of each element by the smallest number of moles present, then round to the nearest whole number.

• Molecular Formula Calculation: Divide the compound's molar mass by the empirical formula mass, then multiply the subscripts in the empirical formula by this factor.

• Example: For a compound with 11.34 mol C, 11.34 mol H, and 5.67 mol O, the empirical formula is CH2O.

Identifying Atoms in Compounds

To determine the number of each type of atom in a chemical formula, multiply the subscript of each element by the number of formula units if necessary.

• Example: In Al2(SO4)3, there are 2 Al, 3 × 1 S × 1 = 3 S, and 3 × 4 = 12 O atoms.

Chemical Quantities and Stoichiometry

Mole Concept and Molar Mass

The mole is a fundamental unit in chemistry representing 6.022 × 1023 entities (Avogadro's number). Molar mass is the mass of one mole of a substance, expressed in g/mol.

• Formula:

• Example: If 1.85 × 10-3 moles of a hormone weigh 0.581 g,

Stoichiometry and Limiting Reactants

Stoichiometry involves using balanced chemical equations to calculate quantities of reactants and products. The limiting reactant is the substance that is completely consumed first, limiting the amount of product formed.

• Example: For 2Rb(s) + I2(g) → 2RbI(s), if 45.2 g Rb and 92.3 g I2 are mixed, calculate moles of each, determine the limiting reactant, and find the excess remaining after reaction.

Percent Yield

Percent yield compares the actual yield to the theoretical yield:

• Formula:

• Example: If the theoretical yield is 310.12 g and the actual yield is 153 g,

Atomic Theory and Structure

Dalton's Atomic Theory

Dalton's atomic theory postulates that:

• Atoms are indivisible and indestructible particles.

• Atoms of the same element are identical in mass and properties.

• Atoms combine in simple whole-number ratios to form compounds.

• Atoms cannot be created or destroyed in chemical reactions.

Inconsistencies: Statements such as atoms of a compound are fixed and cannot rearrange, or atoms can be created, are inconsistent with Dalton's theory.

Isotopes and Mass Spectrometry

Isotopes are atoms of the same element with different numbers of neutrons. Mass spectrometry can differentiate isotopes based on their mass-to-charge ratio.

• Precision: The mass spectrometer must have sufficient precision to distinguish between isotopes differing by a small mass (e.g., g).

Charge-to-Mass Ratio

The charge-to-mass ratio () is important for characterizing particles such as ions and electrons.

• Formula: , where is charge (Coulombs) and is mass (kg).

• Example: For an alpha particle with charge C and mass amu ( kg), C/kg.

Chemical Reactions and Equations

Balancing Chemical Equations

Balancing ensures the same number of each atom on both sides of the equation, reflecting the law of conservation of mass.

• Example:

Types of Chemical Compounds

Chemical compounds can be classified as:

• Ionic compounds: Composed of cations and anions (e.g., NaCl, MgO).

• Molecular compounds: Composed of nonmetals sharing electrons (e.g., H2O, CO2).

• Acids and bases: Special classes of compounds with characteristic properties.

Naming and Formulas of Compounds

Naming Ions and Compounds

Compounds are named based on the ions or elements present:

• Cations: Named as the element (e.g., sodium ion, magnesium ion).

• Anions: Named with an -ide, -ate, or -ite suffix depending on the type (e.g., chloride, sulfate, nitrite).

• Hydrates: Compounds with water molecules attached (e.g., Ga(NO3)3 · 5H2O).

Common Polyatomic Ions

Acid Nomenclature

• Binary acids: H + nonmetal, named as "hydro-___-ic acid" (e.g., HCl = hydrochloric acid).

• Oxoacids: Contain H, O, and another element. The name depends on the polyatomic ion (e.g., H2SO4 = sulfuric acid, H2SO3 = sulfurous acid).

• Incorrect pairings: H2CrO4 is chromic acid, not H2CrO4 as "chromic acid" if the formula is wrong; H2PO3 is not phosphoric acid.

Classification of Substances

Types of Elements and Compounds

• Monoatomic element: Single atoms (e.g., Ar, Cu).

• Diatomic element: Molecules with two atoms (e.g., O2, Cl2).

• Polyatomic element: Molecules with more than two atoms of the same element (e.g., S8).

• Molecular compound: Nonmetal atoms bonded together (e.g., C12H22O6).

• Ionic compound: Metal and nonmetal ions (e.g., NH4Cl).

Additional Topics

Isotopic Abundance and Identification

Given the percent composition by mass, the identity of an element in a compound can be deduced by comparing the calculated mass percent to known atomic masses.

Hydrocarbons

Hydrocarbons are classified as:

• Alkanes: Saturated hydrocarbons with single bonds (general formula CnH2n+2).

• Alkenes: Unsaturated hydrocarbons with at least one double bond (CnH2n).

• Alkynes: Unsaturated hydrocarbons with at least one triple bond (CnH2n-2).

Sample Table: Types of Molecules

Key Equations and Concepts

• Mole-to-mass conversion:

• Atoms in a sample:

• Stoichiometric calculations: Use balanced equations to relate moles of reactants and products.

• Empirical formula from combustion data: Convert grams of CO2 and H2O to moles of C and H, subtract from total mass to find N or O.