Fundamental Concepts in General Chemistry: Atoms, Molecules, and Chemical Nomenclature

Atoms, Molecules, and Chemical Formulas

Empirical and Molecular Formulas

Chemical formulas represent the composition of substances using element symbols and numerical subscripts. There are two main types of formulas: empirical formulas and molecular formulas.

• Empirical Formula: The simplest whole-number ratio of atoms of each element in a compound. For example, the empirical formula of hydrogen peroxide (H2O2) is HO.

• Molecular Formula: The actual number of atoms of each element in a molecule of the compound. For example, the molecular formula of glucose is C6H12O6.

• Some compounds have the same empirical and molecular formulas (e.g., H2O), while others differ.

Example: Among the formulas NaCl, Na2O, NaCl, and NH4, only NH4 (ammonium) must be a molecular formula, as it cannot be reduced to a simpler whole-number ratio.

Counting Atoms and Molecules: The Mole Concept

Avogadro's Number and Moles

The mole is the SI unit for the amount of substance. One mole contains exactly 6.022 × 1023 entities (Avogadro's number).

• Avogadro's Number: $6.022 \times 10^{23}$ particles/mol

• Converting Moles to Atoms: $\text{Number of atoms} = \text{moles} \times 6.022 \times 10^{23}$

• Converting Atoms to Moles: $\text{Moles} = \frac{\text{Number of atoms}}{6.022 \times 10^{23}}$

Example: To find the number of F atoms in 12.15 mol of C2H8BrF2:

• Each molecule contains 2 F atoms.

• Total F atoms = $12.15 \text{ mol} \times 2 \times 6.022 \times 10^{23}$

Percent Composition and Empirical Formulas

Calculating Percent Composition

Percent composition expresses the mass percentage of each element in a compound.

• Formula: $\%\,\text{Element} = \frac{\text{mass of element in 1 mol of compound}}{\text{molar mass of compound}} \times 100\%$

Example: For a compound C9H12N2O2:

• Calculate the mass of each element in one mole, sum to get molar mass, then apply the formula above for each element.

Chemical Nomenclature

Naming Ionic and Molecular Compounds

Chemical nomenclature is the system for naming chemical substances. The rules differ for ionic and molecular compounds.

• Ionic Compounds: Composed of cations (positive ions) and anions (negative ions). Name the cation first, then the anion. For transition metals, indicate the charge with Roman numerals.

• Molecular Compounds: Composed of nonmetals. Use prefixes (mono-, di-, tri-, etc.) to indicate the number of each atom.

• Acids: If the anion ends in -ide, the acid name begins with hydro- and ends with -ic acid. If the anion ends in -ate, the acid name ends with -ic acid; if -ite, ends with -ous acid.

Example Table: Naming Compounds

Formula	Name
NaCl	Sodium chloride
FeCl2	Iron(II) chloride
CO2	Carbon dioxide
H2SO4	Sulfuric acid
NO2	Nitrogen dioxide

Periodic Table and Element Classification

Structure of the Periodic Table

The periodic table organizes elements by increasing atomic number and similar chemical properties. Elements are arranged in rows (periods) and columns (groups or families).

• Groups: Vertical columns; elements in the same group have similar properties.

• Periods: Horizontal rows; properties change progressively across a period.

• Main Group Elements: Groups 1, 2, and 13-18.

• Transition Metals: Groups 3-12.

• Lanthanides and Actinides: Two rows below the main table.

Example: Sodium (Na) is in group 1 (alkali metals), period 3.

Practice Problems and Applications

Sample Calculations and Naming

• Calculate the number of atoms or moles given mass or number of particles.

• Determine percent composition from a chemical formula.

• Name compounds from formulas and write formulas from names.

Example: How many Fe atoms are present in 6 L of blood if the concentration is known? Use the molar mass and Avogadro's number to convert between grams and atoms.

Summary Table: Types of Chemical Formulas

Type	Description	Example
Empirical Formula	Simplest whole-number ratio of elements	CH2O (for glucose)
Molecular Formula	Actual number of atoms in a molecule	C6H12O6
Structural Formula	Shows arrangement of atoms	H–O–H (for water)

Additional info: The periodic table image in the file is a standard reference for element properties and classification. The questions provided are typical of introductory General Chemistry, focusing on formula types, mole calculations, percent composition, and nomenclature.