Chem 141_Ch02_FC

Chemistry - Chapter 2: Atoms, Molecules, and Ions

Exam Checklist

  • List contributions of key scientists to modern atomic theory in chronological order.

  • Classify matter:

    • Elements

    • Compounds

    • Mixtures (homogeneous & heterogeneous)

    • Substances

  • Describe subatomic particles:

    • Charges

    • Relative masses

    • Locations in an atom

  • Define and use:

    • Atomic number

    • Mass number

    • Nuclide symbols

  • Calculate atomic weight or find:

    • Abundance

    • Mass of an isotope

  • Classify elements:

    • Metals

    • Metalloids

    • Nonmetals

  • Identify groups within the periodic table:

    • Halogens, alkali metals, etc.

  • Identify and give examples of molecular elements.

  • Predict charge of ions; define cations and anions.

  • Write a chemical name from a given formula.

  • Write a chemical formula from a given name.

Early History of Chemistry

Key Concepts

  • The term "atom" originates from the Greek word "atomos" which means indivisible.

  • Cutting a piece of graphite eventually yields individual carbon atoms.

  • Understanding atoms is fundamental to understanding matter.

Brownian Motion

  • Robert Brown observed particles suspended in water showing continuous motion using a microscope.

  • Albert Einstein (1905) explained this motion quantitatively;

  • Jean Perrin (1908) confirmed Einstein’s model.

  • This work established the particulate nature of matter.

Fundamental Chemical Laws

Major Laws Leading to Atomic Theory

  1. Law of Conservation of Mass (Lavoisier): Matter is neither created nor destroyed in a chemical reaction.

  2. Law of Definite Proportions (Proust): All samples of a compound have the same proportions of elements.

  3. Law of Multiple Proportions (Dalton): When two elements form different compounds, the mass ratios can be expressed as small whole numbers.

Dalton’s Atomic Theory

  1. Matter consists of indivisible atoms.

  2. Atoms of the same element are identical.

  3. Atoms can combine in whole-number ratios to form compounds.

  4. Atoms cannot change into different elements in chemical reactions.

Experiments and Determination of Atomic Structure

J.J. Thomson

  • Conducted cathode ray experiments, discovering the electron.

  • Cathode Rays travel in straight lines and carry a negative charge.

  • Determined charge-to-mass ratio of electrons.

Rutherford’s Gold Foil Experiment

  • Ernest Rutherford directed alpha particles at gold foil.

  • Majority passed through but some were deflected, leading to the conclusion that:

    • Atoms consist of mostly empty space with a small, dense nucleus.

  • Proposed nuclear theory:

    1. The nucleus contains most of the atom’s mass and positive charge.

    2. Electrons are dispersed around the nucleus in a cloud.

    3. Atom is electrically neutral due to equal numbers of protons and electrons.

Periodic Table

Organizing Principles

  • Mendeleev’s periodic law: Properties recur periodically when elements are arranged by increasing atomic mass.

  • Modern periodic table organizes by increasing atomic number.

  • Groups include:

    • Alkali metals (1A)

    • Alkaline earth metals (2A)

    • Halogens (7A)

    • Noble gases (8A)

Elements and Their Properties

Classifications

  • Metals: Good conductors, malleable, ductile, tend to lose electrons.

  • Nonmetals: Poor conductors, tend to gain electrons, varied states at room temperature.

  • Metalloids: Exhibit properties of both metals and nonmetals, often semiconductors.

Ions and Ionic Compounds

  • Cations: Positively charged ions (e.g., Na⁺, Mg²⁺).

  • Anions: Negatively charged ions (e.g., Cl⁻, O²⁻).

  • Ionic compounds consist of cations and anions bound by ionic bonds.

  • Formula unit: Smallest ratio of ions that results in charge neutrality, e.g., NaCl.

Naming Ionic Compounds

Type I (Metal with invariant charge)

  • Name cation and anion:

    • E.g., NaCl - Sodium Chloride

    • Formula: Na⁺ + Cl⁻ → NaCl

Type II (Metal can have multiple charges)

  • Use Roman numerals to indicate charge:

    • E.g., CuCl₂ (Copper(II) chloride)

Molecular Compounds

  • Composed of two or more nonmetals.

  • Prefixes indicate the number of atoms:

    • Mono-, di-, tri-, tetra-, etc.

Acids

  • Binary acids: Composed of hydrogen and a nonmetal.

  • Oxyacids: Composed of hydrogen and oxyanion.

  • Naming:

    • Hydro- + base name of nonmetal + -ic + acid (e.g., HCl = hydrochloric acid).

    • For oxyacids: if ending in -ate, change to -ic; if -ite, change to -ous.

Practice Problems

  • Answers for various naming and writing chemical formulas, including for ionic and molecular compounds.