General Chemistry Study Guide: Atoms, Elements, Molecules, and Compounds

Atoms & Elements

Atomic Structure and Isotopes

Atoms are the fundamental units of matter, composed of protons, neutrons, and electrons. Isotopes are variants of elements with the same number of protons but different numbers of neutrons.

• Protons: Positively charged particles found in the nucleus; the atomic number equals the number of protons.

• Neutrons: Neutral particles in the nucleus; isotopes differ in neutron count.

• Electrons: Negatively charged particles orbiting the nucleus; in a neutral atom, electrons equal protons.

• Isotope Example: 200Hg has 80 protons and 120 neutrons.

• Atomic Mass Calculation: Weighted average based on isotope abundance and mass.

Formula:

$\text{Atomic mass} = \sum (\text{isotope mass} \times \text{fractional abundance})$

Periodic Table Organization

The periodic table arranges elements by increasing atomic number and groups them by similar properties.

• Main Group Elements: Groups 1A–8A, including alkali metals, alkaline earth metals, halogens, and noble gases.

• Transition Metals: Elements in the center of the table, often forming multiple ions.

• Metalloids: Elements with properties intermediate between metals and nonmetals.

Molecules and Compounds

Naming Ionic and Molecular Compounds

Compounds are named according to their composition and the rules for ionic or molecular compounds.

• Ionic Compounds: Formed from metals and nonmetals; names include the cation and anion, with Roman numerals for variable charge metals (e.g., tin(IV) sulfate for Sn(SO4)2).

• Molecular Compounds: Formed from nonmetals; use prefixes to indicate the number of atoms (e.g., diphosphorus tetrafluoride for P2F4).

• Hydrates: Compounds with water molecules attached; indicated by prefixes (e.g., octahydrate).

Polyatomic Ions

Polyatomic ions are charged groups of covalently bonded atoms. They are common in many compounds.

• Examples: Sulfate (SO42–), Nitrate (NO3–), Ammonium (NH4+).

• Importance: Polyatomic ions are used in naming and writing formulas for compounds.

Empirical and Molecular Formulas

The empirical formula shows the simplest ratio of elements in a compound, while the molecular formula shows the actual number of atoms.

• Empirical Formula: Determined from percent composition.

• Molecular Formula: Determined from molar mass and empirical formula.

Formula:

$\text{Molecular formula} = (\text{Empirical formula})_n$

$n = \frac{\text{Molar mass}}{\text{Empirical formula mass}}$

Mathematical Operations and Functions

Mole Calculations

The mole is a fundamental unit for counting atoms and molecules. Calculations often involve converting between mass, moles, and number of particles.

• Mole Formula: $\text{moles} = \frac{\text{mass}}{\text{molar mass}}$

• Avogadro’s Number: $6.022 \times 10^{23}$ particles per mole.

• Number of Molecules: $\text{Number of molecules} = \text{moles} \times \text{Avogadro’s number}$

Mass Percent and Molar Mass

Mass percent expresses the proportion of each element in a compound. Molar mass is the mass of one mole of a substance.

• Mass Percent Formula: $\text{Mass percent} = \frac{\text{mass of element in 1 mole}}{\text{molar mass of compound}} \times 100\%$

• Molar Mass: Sum of atomic masses of all atoms in the formula.

Bonus: Lab Techniques and Procedures

Hydrates and Formula Writing

Hydrates are compounds with water molecules attached. The formula includes a dot and the number of water molecules (e.g., CaCl2·6H2O).

• Writing Hydrate Formulas: Use the compound formula followed by a dot and the number of water molecules.

• Example: Chromium(III) sulfite octahydrate is written as Cr2(SO3)3·8H2O.

Summary Table: Common Polyatomic Ions

The following table summarizes common polyatomic ions used in naming and writing chemical formulas:

Name	Formula
Acetate	C2H3O2–
Carbonate	CO32–
Hydrogen carbonate (bicarbonate)	HCO3–
Hydroxide	OH–
Nitrate	NO3–
Chromate	CrO42–
Dichromate	Cr2O72–
Phosphate	PO43–
Hydrogen phosphate	HPO42–
Dihydrogen phosphate	H2PO4–
Ammonium	NH4+
Hypochlorite	ClO–
Chlorite	ClO2–
Chlorate	ClO3–
Perchlorate	ClO4–
Permanganate	MnO4–
Sulfite	SO32–
Hydrogen sulfite (bisulfite)	HSO3–
Sulfate	SO42–
Hydrogen sulfate (bisulfate)	HSO4–
Cyanide	CN–
Peroxide	O22–