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

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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.

Isotopes and Atomic Notation

Isotopes are atoms of the same element with different numbers of neutrons. Isotopic notation helps relate atomic structure to element identity.

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

  • Isotopic Notation: Written as AZElement, where A = mass number, Z = atomic number.

  • Atomic Number (Z): Number of protons in the nucleus.

  • Mass Number (A): Total number of protons and neutrons.

  • Atomic Mass: Weighted average mass of all isotopes of an element (found on the periodic table), different from mass number (A).

Example: 146C is a carbon isotope with 6 protons and 8 neutrons.

Ions and Their Properties

Cations and Anions

Ions are charged particles formed when atoms gain or lose electrons.

  • Ion: An atom or molecule with a net electric charge.

  • Cation: Positively charged ion (loss of electrons).

  • Anion: Negatively charged ion (gain of electrons).

Example: Na+ is a cation; Cl- is an anion.

Valence Electrons and Shells

Valence electrons are the outermost electrons involved in chemical bonding.

  • Valence Electron: Electron in the outermost shell of an atom.

  • Valence Shell: The highest energy shell containing electrons.

  • Outer Shell: Synonymous with valence shell.

  • Determining Valence Electrons: For main group elements, the group number indicates the number of valence electrons.

Example: Oxygen (Group 16) has 6 valence electrons.

Ionization Energy

Ionization energy is the energy required to remove an electron from an atom.

  • Definition: The minimum energy needed to remove an electron from a gaseous atom.

  • Relation to Attractive Force: Higher attractive force between nucleus and electron means higher ionization energy.

  • Factors Influencing Attraction: Nuclear charge (number of protons), distance of electron from nucleus, shielding by inner electrons.

Example: Ionization energy increases across a period and decreases down a group.

Chemical Bonds and Compounds

Ionic vs. Covalent Bonds

Chemical bonds hold atoms together in compounds. Ionic and covalent bonds differ in their formation and properties.

  • Ionic Bond: Formed by transfer of electrons from one atom to another, resulting in oppositely charged ions held together by electrostatic attraction.

  • Covalent Bond: Formed by sharing electrons between atoms, creating discrete molecules.

  • Physical Properties of Ionic Compounds: High melting points, conduct electricity when molten or dissolved, usually crystalline solids.

Example: NaCl is an ionic compound; H2O is a covalent compound.

Writing Formulas for Compounds

Formulas represent the composition of compounds. Ionic compounds are written as formula units, while covalent compounds are written as molecular formulas.

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

  • Molecular Formula: Shows the actual number of atoms in a molecule (for covalent compounds).

  • Writing Ionic Formulas: Balance charges so the total charge is zero.

Example: Ca2+ and Cl- combine to form CaCl2.

Nomenclature (Naming Compounds)

Naming Ionic and Covalent Compounds

Systematic naming allows chemists to identify compounds unambiguously.

  • Ionic Compounds: Name cation first, then anion. No prefixes. Use Roman numerals for Type II cations (transition metals with variable charge).

  • Covalent Compounds: Use prefixes to indicate number of atoms (mono-, di-, tri-, etc.).

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

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

  • Roman Numerals: Indicate charge of Type II cations in the compound name (e.g., iron(III) chloride).

Example: CO2 is carbon dioxide; FeCl3 is iron(III) chloride.

Polyatomic Ions

Polyatomic ions are groups of atoms with a net charge, commonly found in ionic compounds.

Polyatomic Ion

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-

Classification of Matter

Pure Substances vs. Mixtures

Matter can be classified based on its composition.

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

  • Mixture: Combination of two or more substances; can be homogeneous (uniform) or heterogeneous (non-uniform).

Example: Salt water is a homogeneous mixture; sand and water is a heterogeneous mixture.

Elements, Compounds, and Mixtures

Bulk matter consists of elements, compounds, and mixtures, which are made up of atoms, molecules, and ions.

  • Element: Pure substance made of only one type of atom.

  • Compound: Pure substance made of two or more elements chemically combined.

  • Mixture: Physical combination of substances.

  • Symbols and Formulas: Elements are represented by symbols (e.g., Na), compounds by formulas (e.g., NaCl).

Example: O2 is an element; H2O is a compound.

The Periodic Table

Structure and Classification

The periodic table organizes elements by atomic number and properties.

  • Symbols: Each element has a unique one- or two-letter symbol.

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

  • Periods: Horizontal rows.

  • Groups: Vertical columns; include alkali metals (Group 1), alkaline earth metals (Group 2), halogens (Group 17), noble gases (Group 18).

  • Transition Elements: Groups 3-12.

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

Example: Sodium (Na) is an alkali metal; chlorine (Cl) is a halogen.

Diatomic Elements

Seven elements exist naturally as diatomic molecules.

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

Example: Oxygen gas is O2.

Dimensional Analysis and Units

Dimensional Analysis

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

  • Definition: Process of using conversion factors to change units.

  • Typical Units: Mass (grams), length (meters), volume (liters), etc.

  • Conversion Factors: Ratios used to convert from one unit to another.

Example: To convert 10 cm to meters:

Additional info:

  • Significant figures are important in calculations; always follow rules for rounding and reporting.

  • Review all homework, quizzes, and lab activities for comprehensive understanding.

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