Atoms, Ions, Compounds, and Nomenclature: Exam 2 Study Guide

Atoms and Their Structure

Atomic Size, Mass, and Volume

Atoms are the fundamental units of matter, consisting of a nucleus surrounded by electrons. Understanding their size, mass, and where chemical reactions occur is essential for general chemistry.

• Atomic Size: Atoms are extremely small, typically on the order of 1-5 Ångströms (1 Å = 10-10 m).

• Mass Location: Most of the atom's mass is concentrated in the nucleus, which contains protons and neutrons.

• Volume: The electron cloud occupies most of the atom's volume.

• Chemistry Location: Chemical reactions occur in the electron cloud, especially involving valence electrons.

Atomic Number, Mass Number, and Atomic Mass

Atoms are characterized by their atomic number and mass number, which relate to their composition and identity.

• Atomic Number (Z): The number of protons in the nucleus; determines the element.

• Mass Number (A): The total number of protons and neutrons in the nucleus.

• Atomic Mass: The weighted average mass of an element's isotopes, measured in atomic mass units (amu).

• Formula: $A = Z + N$ (where N = number of neutrons)

• Difference: Mass number is a whole number for a specific isotope; atomic mass is a decimal value reflecting isotope abundance.

Isotopes and Isotopic Notation

Isotopes are atoms of the same element with different numbers of neutrons. Isotopic notation helps identify these differences.

• Isotope: Atoms with the same number of protons but different numbers of neutrons.

• Notation: $\ ^{A}_{Z}X$ (A = mass number, Z = atomic number, X = element symbol)

• Example: $\ ^{14}_{6}C$ is carbon-14, with 6 protons and 8 neutrons.

Ions and Ionic Compounds

Ions: Cations and Anions

Ions are charged particles formed when atoms gain or lose electrons. They are classified as cations or anions.

• Cation: A positively charged ion formed by losing electrons.

• Anion: A negatively charged ion formed by gaining electrons.

• Example: Na+ (cation), Cl- (anion)

Polyatomic Ions

Polyatomic ions are groups of atoms covalently bonded, carrying a net charge. Memorizing common polyatomic ions is essential for nomenclature and formula writing.

Ion Name	Formula
Acetate	CH3COO-
Hydroxide	OH-
Ammonium	NH4+
Nitrate	NO3-
Carbonate	CO32-
Peroxide	O2-
Chromate	CrO42-
Phosphate	PO43-
Cyanide	CN-
Sulfate	SO42-
Hydrogen carbonate (bicarbonate)	HCO3-
Perchlorate	ClO4-

Writing Formulas for Ionic Compounds

Formulas for ionic compounds are written by balancing the charges of the cation and anion to achieve neutrality.

• Formula Unit: The simplest ratio of ions in an ionic compound.

• Example: For Na+ and Cl-, the formula is NaCl.

• Rule: The total positive charge must equal the total negative charge.

Physical Properties of Ionic Compounds

Ionic compounds exhibit characteristic physical properties due to their structure.

• High melting and boiling points due to strong electrostatic forces.

• Conduct electricity when dissolved in water or molten.

• Usually crystalline solids at room temperature.

Covalent Compounds and Bonding

Covalent Bonds and Compounds

Covalent compounds are formed by sharing electrons between atoms, resulting in discrete molecules.

• Covalent Bond: A bond formed by sharing electrons between atoms.

• Discrete Molecules: Covalent compounds exist as individual molecules.

• Example: H2O, CO2

Comparison: Ionic vs. Covalent Compounds

Ionic and covalent compounds differ in structure, bonding, and properties.

Property	Ionic Compounds	Covalent Compounds
Structure	Extended lattice	Discrete molecules
Bond Formation	Electron transfer	Electron sharing
Physical State	Usually solid	Solid, liquid, or gas
Conductivity	Conducts in solution	Usually non-conductive

Nomenclature: Naming Compounds

Naming Ionic Compounds

Ionic compounds are named by stating the cation first, then the anion. Type II cations require Roman numerals to indicate charge.

• Type I Cations: Metals with only one possible charge (e.g., Na+, Ca2+).

• Type II Cations: Metals with multiple possible charges (e.g., Fe2+, Fe3+).

• Roman Numerals: Used for Type II cations to specify charge (e.g., iron(III) chloride).

• No prefixes are used for ionic compound names.

Naming Covalent Compounds

Covalent compounds use prefixes to indicate the number of each atom present.

• Prefixes: mono-, di-, tri-, tetra-, penta-, etc.

• Example: CO2 is carbon dioxide; N2O4 is dinitrogen tetroxide.

• Order: The element with the lower group number is named first.

Writing Names and Formulas

Be able to write the formula from the name, the name from the formula, and determine the correct order and subscripts for elements and ions.

• Example: Magnesium chloride: MgCl2

• Example: Dinitrogen monoxide: N2O

Valence Electrons and Ionization Energy

Valence Electrons and Shells

Valence electrons are the outermost electrons involved in chemical bonding.

• Valence Electron: Electron in the outermost shell.

• Valence Shell: The highest energy shell containing electrons.

• Determination: For main group elements, the group number indicates the number of valence electrons.

Ionization Energy

Ionization energy is the energy required to remove an electron from an atom. It is influenced by the attraction between the nucleus and electrons.

• Definition: Energy needed to remove an electron from a gaseous atom.

• Factors: Nuclear charge, distance from nucleus, electron shielding.

• Trend: Increases across a period, decreases down a group.

• Formula: $F = \frac{Z_{eff} \cdot e^2}{r^2}$ (force of attraction; Zeff = effective nuclear charge, r = distance)

Periodic Table and Classification of Elements

Periodic Table Structure

The periodic table organizes elements by increasing atomic number and groups elements with similar properties.

• Periods: Horizontal rows.

• Groups: Vertical columns; include alkali metals, alkaline earth metals, halogens, noble gases, transition elements, main group elements.

• Metals, Metalloids, Nonmetals: Classified by physical and chemical properties.

Symbols and Formulas

Elements are represented by symbols; compounds by formulas.

• Element Symbol: One or two letters (e.g., H, O, Na).

• Compound Formula: Shows the types and numbers of atoms (e.g., H2O).

Diatomic Elements

Seven elements exist naturally as diatomic molecules.

• Diatomic Elements: H2, N2, O2, F2, Cl2, Br2, I2

Classification of Matter

Pure Substances vs. Mixtures

Matter is classified as pure substances or mixtures, which can be homogeneous or heterogeneous.

• Pure Substance: Has a fixed composition; can be an element or compound.

• Mixture: Combination of two or more substances; composition can vary.

• Homogeneous Mixture: Uniform composition (e.g., salt water).

• Heterogeneous Mixture: Non-uniform composition (e.g., sand and water).

Bulk vs. Particulate Matter

Bulk matter refers to observable substances; particulate matter refers to the atoms, molecules, and ions that compose them.

• Bulk Matter: Compounds, mixtures, elements.

• Particulate Matter: Atoms, molecules, ions.

Dimensional Analysis and Units

Dimensional Analysis

Dimensional analysis is a method for converting units using conversion factors.

• Process: Multiply by conversion factors to cancel units.

• Example: Convert 10 cm to meters: $10 \text{ cm} \times \frac{1 \text{ m}}{100 \text{ cm}} = 0.1 \text{ m}$

Typical Units

Common units in chemistry include mass (grams), length (meters), volume (liters), etc.

• Mass: grams (g)

• Length: meters (m)

• Volume: liters (L)

Significant Figures

Significant Figures in Calculations

Significant figures reflect the precision of measurements and must be considered in calculations.

• Rules: All nonzero digits are significant; zeros between nonzero digits are significant; leading zeros are not significant; trailing zeros are significant if there is a decimal point.

• Example: 0.00450 has three significant figures.

Summary Table: Polyatomic Ions to Memorize

Ion Name	Formula
Acetate	CH3COO-
Hydroxide	OH-
Ammonium	NH4+
Nitrate	NO3-
Carbonate	CO32-
Peroxide	O2-
Chromate	CrO42-
Phosphate	PO43-
Cyanide	CN-
Sulfate	SO42-
Hydrogen carbonate (bicarbonate)	HCO3-
Perchlorate	ClO4-

Additional info: This study guide covers foundational topics in general chemistry, including atomic structure, ions, compounds, nomenclature, periodic table, classification of matter, dimensional analysis, and significant figures. For exam preparation, review lecture notes, homework, and lab materials for detailed examples and practice problems.