CHEM 115: Atoms, Molecules, and Ions

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.

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 (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).

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).

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})

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-}).
Formulas of Ionic Compounds: Empirical formulas, where total positive charge balances total negative charge for electrical neutrality.
Oxoanions: Polyatomic anions containing oxygen.
- Suffix -ate (reference)
- One more O: per-…-ate
- One less O: -ite
- Two less O: hypo-…-ite
Hydrates: Ionic compounds associated with a specific number of water molecules (e.g., BaCl2 \cdot 2H2O barium chloride dihydrate).

Molecule: Two or more atoms covalently bonded.
- Diatomic Molecule: Two atoms (e.g., H2, Cl2).
- Polyatomic Molecule: More than two atoms.
Chemical Formulas:
- Molecular Formula: Exact number of atoms (e.g., C6H{12}O_6).
- Empirical Formula: Simplest whole-number ratio of atoms (e.g., CH_2O for glucose).
- Structural Formula: Shows atom arrangement.
Allotrope: Different structural forms of the same element (e.g., diamond and graphite for carbon).
Naming Binary Molecular Compounds: Use Greek prefixes for number of atoms; first element name, second element root + "-ide." (e.g., CO_2 carbon dioxide).
Simple Acids: Binary compounds producing H^+ in water; named hydro- + element stem + -ic acid (e.g., HCl hydrochloric acid).
Oxoacids: Contain hydrogen, oxygen, and another element; name derived from oxoanion.
- -ate ion yields -ic acid (e.g., HNO_3 nitric acid).
- -ite ion yields -ous acid (e.g., HClO_2$$ chlorous acid).
- Prefixes (per-, hypo-) are retained.
Organic Compounds: Contain carbon and hydrogen (e.g., hydrocarbons).

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.