CHEM 115: Atoms, Molecules, and Ions

The Atomic Theory

• Ancient Ideas: Democritus proposed "atomos," small indivisible particles.

• Dalton's Atomic Theory (1803-1808):

  • Elements are composed of atoms; all atoms of an element are identical, distinct from others.

  • Compounds are formed from atoms of multiple elements in fixed ratios (Law of Definite Proportions).

  • Chemical reactions rearrange atoms, neither creating nor destroying them (Law of Conservation of Mass).

• Law of Definite Proportions: A compound always contains elements in the same mass ratio.

• Law of Multiple Proportions: If two elements form multiple compounds, a fixed mass of one combines with masses of the other in small whole-number ratios.

• Law of Conservation of Mass: Matter is neither created nor destroyed.

The Structure of the Atom

• Atom: Basic unit of an element that enters chemical combinations; consists of subatomic particles.

• Discovery of the Electron (J.J. Thomson, 1897): Cathode rays are negatively charged particles (electrons) with a specific mass-to-charge ratio. Developed the "Plum Pudding Model."

• Millikan’s Oil Drop Experiment (1909): Determined the charge of an electron ($1.6022 \times 10^{-19} C$) and its mass ($9.109 \times 10^{-28} g$).

• Radioactivity (Becquerel, Curie): Spontaneous emission of alpha ($\alpha$), beta ($\beta$), and gamma ($\gamma$) rays.

  • $\alpha$ rays: positive particles.

  • $\beta$ rays: electrons.

  • $\gamma$ rays: high-energy, no charge.

• Rutherford’s Gold Foil Experiment (1911): Most alpha particles passed through, some deflected, leading to the Nuclear Model.

  • Nuclear Model: Atom is mostly empty space; a dense, positively charged nucleus contains most of the atom's mass.

• Proton: Positively charged particle ($1.67262 \times 10^{-24} g$), found in the nucleus.

• Neutron (James Chadwick, 1932): Electrically neutral particle, slightly heavier than a proton, found in the nucleus. Explains observed atomic masses.

• Atomic Dimensions: Atomic radius $\approx 100 pm$; nucleus radius $\approx 5 \times 10^{-3} pm$ ($1 pm = 1 \times 10^{-12} m$).

Atomic Number, Mass Number, and Isotopes

• Atomic Number (Z): Number of protons; defines the element. In neutral atoms, \text{Z} = \text{# electrons}.

• Mass Number (A): Total number of protons and neutrons (\text{A} = \text{Z} + \text{# neutrons}).

• Nucleons: Protons and neutrons.

• Isotopes: Atoms of the same element (same Z) with different mass numbers (different number of neutrons).

  • Exhibit similar chemical properties.

• Isobars: Nuclides of different elements (different Z) with the same mass number (A).

The Periodic Table

• Elements are cataloged by increasing atomic number.

• Periods: Horizontal rows.

• Groups (Families): Vertical columns; elements in the same group share similar properties.

• Categories:

  • Metals: Good conductors, malleable, ductile, lustrous, high density/melting point; tend to form cations; left side of table.

  • Nonmetals: Poor conductors, brittle, dull, lower density/melting point; tend to form anions; right side of table.

  • Metalloids: Intermediate properties (e.g., B, Si, Ge, As, Sb, Te, Po).

• Key Groups: Alkali metals (Group 1A), Alkaline earth metals (Group 2A), Halogens (Group 7A), Noble gases (Group 8A).

The Atomic Mass Scale and Average Atomic Mass

• Atomic Mass Unit (amu): Defined as $\frac{1}{12}$ the mass of a carbon-12 atom (C-12 = $12.00 amu$).

• Average Atomic Mass (Atomic Weight): Weighted average of isotopic masses based on their natural abundances.

  • $\text{atomic mass} = \sum (\text{isotope fractional abundance})(\text{exact isotope mass})$

Ions and Ionic Compounds

• Atomic (Monatomic) Ion: Single atom with a charge.

  • Cation: Positive charge (loss of electrons).

  • Anion: Negative charge (gain of electrons).

• Ionic Compound: Formed from cations and anions (e.g., $NaCl$).

• Naming Monatomic Ions:

  • Cations: Element name (+ Roman numeral for charge if variable, e.g., Iron(II) ion); older system uses -ous (lower) and -ic (higher) suffixes.

  • Anions: Element stem + "-ide" + "ion" (e.g., Chloride ion).

• Polyatomic Ions: Ions composed of multiple atoms (e.g., Sulfate, $SO_4^{2-}$).</p></li><li><p><strong>Formulas of Ionic Compounds:</strong> Empirical formulas, where total positive charge balances total negative charge for electrical neutrality.</p></li><li><p><strong>Oxoanions:</strong> Polyatomic anions containing oxygen.</p><ul><li><p>Suffix -ate (reference)</p></li><li><p>One more O: per-…-ate</p></li><li><p>One less O: -ite</p></li><li><p>Two less O: hypo-…-ite</p></li></ul></li><li><p><strong>Hydrates:</strong> Ionic compounds associated with a specific number of water molecules (e.g.,$BaCl2 \cdot 2H2O$barium chloride dihydrate).</p></li></ul><h3 id="10ddd091-dc88-4131-a8fc-c924d0046d9b" data-toc-id="10ddd091-dc88-4131-a8fc-c924d0046d9b" collapsed="false" seolevelmigrated="true">Molecules and Molecular Compounds</h3><ul><li><p><strong>Molecule:</strong> Two or more atoms covalently bonded.</p><ul><li><p><strong>Diatomic Molecule:</strong> Two atoms (e.g.,$H2, Cl2$).</p></li><li><p><strong>Polyatomic Molecule:</strong> More than two atoms.</p></li></ul></li><li><p><strong>Chemical Formulas:</strong></p><ul><li><p><strong>Molecular Formula:</strong> Exact number of atoms (e.g.,$C6H{12}O_6$).</p></li><li><p><strong>Empirical Formula:</strong> Simplest whole-number ratio of atoms (e.g.,$CH_2O$for glucose).</p></li><li><p><strong>Structural Formula:</strong> Shows atom arrangement.</p></li></ul></li><li><p><strong>Allotrope:</strong> Different structural forms of the same element (e.g., diamond and graphite for carbon).</p></li><li><p><strong>Naming Binary Molecular Compounds:</strong> Use Greek prefixes for number of atoms; first element name, second element root + "-ide." (e.g.,$CO_2$carbon dioxide).</p></li><li><p><strong>Simple Acids:</strong> Binary compounds producing$H^+$in water; named hydro- + element stem + -ic acid (e.g.,$HCl$hydrochloric acid).</p></li><li><p><strong>Oxoacids:</strong> Contain hydrogen, oxygen, and another element; name derived from oxoanion.</p><ul><li><p>-ate ion yields -ic acid (e.g.,$HNO_3$nitric acid).</p></li><li><p>-ite ion yields -ous acid (e.g.,$HClO_2$$ chlorous acid).

• Prefixes (per-, hypo-) are retained.

Compounds in Review

• Distinguishing Compounds: Ionic compounds are typically metal/polyatomic cation with nonmetal/polyatomic anion. Molecular compounds are typically two nonmetals.

• Naming Conventions: Ionic compounds do not use prefixes; molecular compounds use Greek prefixes to indicate atom ratios. Compare Table 2.10 for common and systematic names.