CHEM 1411 Chapter 2: Atoms, Molecules, and Ions

2.1 Early Ideas in Atomic Theory

• Leucippus and Democritus (5th century BC): Proposed that matter consists of indivisible particles called "atomos".
• John Dalton (1807):
  • Introduced Dalton’s Atomic Theory.
  • Matter is composed of indivisible atoms (later found not entirely true).
  • Elements consist of one type of atom with characteristic mass (isotopes exist).
  • Atoms of different elements have different properties.
  • Compounds are combinations of atoms in whole-number ratios.
  • Atoms are rearranged, not created or destroyed, during chemical changes.
• Law of Definite Proportions: Pure compounds contain the same elements in the same proportion by mass (e.g., H_2O).
• Law of Multiple Proportions: When two elements form more than one compound, a fixed mass of one element reacts with the other in a ratio of small whole numbers.

2.2 Evolution of Atomic Theory

• J.J. Thomson:
  • Experimented with cathode ray tubes.
  • Discovered electrons, which are negatively charged and much lighter than atoms.
• Robert A. Millikan:
  • Oil drop experiment determined the charge of an electron ( 1.6 x 10^{-19} C ).
  • Calculated the mass of an electron ( 9.107 x 10^{-31} kg ).
• Thomson's Model: Plum pudding model; atom is a positive sphere with embedded electrons.
• Nagaoka's Model: Saturn-like model; electrons in a ring around a positive "planet."
• Ernest Rutherford:
  • Gold foil experiment led to the discovery of the nucleus.
  • The atom has a small, dense, positively charged nucleus.
  • Electrons surround the nucleus and occupy most of the atom's volume.
  • Named the positively charged particle in the nucleus as the proton.
• Frederick Soddy: Discovered isotopes, atoms of the same element with different masses due to varying numbers of neutrons.
• James Chadwick: Discovered neutrons, neutral subatomic particles in the nucleus with mass similar to protons.

2.3 Atomic Structure and Symbolism

• Atomic Diameter: ~ 10^{-10} m. Nucleus Diameter: ~ 10^{-15} m.
• Atomic Mass Unit (amu):
  • Defined based on ^{12}C, where one atom has a mass of 12 amu.
  • 1 amu = 1.6605 x 10^{-24} g
• Subatomic Particles:
  • Proton: Mass = 1.0073 amu, Charge = +1
  • Neutron: Mass = 1.0087 amu, Charge = 0
  • Electron: Mass = 0.00055 amu, Charge = -1
• Atomic Number (Z): Number of protons in the nucleus, defining the element's identity.
• Mass Number (A): Total number of protons and neutrons in an atom.
• Ions: Atoms with unequal numbers of protons and electrons, resulting in a charge.
• Cations: Positive ions formed by losing electrons.
• Anions: Negative ions formed by gaining electrons.
• Chemical Symbols: Abbreviations for elements (e.g., Ca for calcium, Hg for mercury).
• Isotopes: Atoms of the same element with different numbers of neutrons, denoted by mass number superscripted to the left of the element symbol (e.g., ^{24}Mg, ^{25}Mg, ^{26}Mg).
• Atomic Mass: Weighted average mass of all isotopes of an element.
• Weighted Average Atomic Mass Formula: \sum(abundance \times isotopic mass)

2.4 Chemical Formulas

• Molecular Formula: Actual number of atoms of each element in a molecule.
• Empirical Formula: Simplest whole-number ratio of atoms in a compound.
• Isomers: Compounds with the same chemical formula but different structures.
• Diatomic Molecules: Elements existing as two-atom molecules (BrINClHOF: Br2, I2, N2, Cl2, H2, O2, F_2).

2.5 The Periodic Table

• Dimitri Mendeleev & Lothar Meyer: Independently recognized periodic relationships among elements.
• Periodic Law: Properties of elements are periodic functions of their atomic numbers.
• Modern Periodic Table: Elements arranged by increasing atomic number.
  • Periods: Horizontal rows.
  • Groups: Vertical columns.
• Classification of Elements:
  • Metals: Shiny, malleable, ductile, good conductors.
  • Nonmetals: Dull, brittle, poor conductors.
  • Metalloids: Intermediate properties.
  • Main Group Elements: Groups 1, 2, 13-18.
  • Transition Metals: Groups 3-12.
  • Inner Transition Metals: Lanthanides and Actinides.
  • Alkali Metals: Group 1 (except hydrogen).
  • Alkaline Earth Metals: Group 2.
  • Halogens: Group 17.
  • Noble Gases: Group 18.

2.6 Ionic and Molecular Compounds

• Ions: Electrically charged particles formed by gain/loss of electrons.
• Ionic Bond: Electron transfer forms ions; attraction between cations and anions.
• Covalent Bond: Electron sharing forms molecules; attraction between nuclei and shared electrons.
• Ionic Compounds: Formed between metals and nonmetals; electrically neutral.
• Monatomic Ions: Ions formed from one type of atom (e.g., Mg^{2+}, Na^+, Cl^-).
• Polyatomic Ions: Ions containing more than one type of atom (e.g., NH4^+, SO4^{2-}. Oxyanions contain oxygen.
• Naming Oxyanions:
  • -ate: more oxygen atoms (e.g., nitrate NO_3^-.)
  • -ite: fewer oxygen atoms (e.g., nitrite NO_2^-.)
  • per-ate: the largest number of oxygens (e.g. perchlorate ClO_4^−)
  • hypo-ite: the smallest number of oxygens (e.g. hypochlorite ClO^−)
• Molecular Compounds: Formed when nonmetal atoms share electrons.
• Formulas of Ionic Compounds with Polyatomic Ions: Treat polyatomic ions as a unit. The charges must be balanced so the ionic compound is electrically neutral.

2.7 Chemical Nomenclature

• Nomenclature: Rules for naming compounds.
• Naming Binary Ionic Compounds: Cation name (element name) + anion name (root + -ide).
• Naming Polyatomic Ionic Compounds: Cation name + polyatomic anion name.
• Ionic Compounds with Variable Charge Metal: Use Roman numerals in parentheses after the metal name to indicate charge (e.g., Iron (II) chloride).
• Ionic Hydrates: Ionic compounds with water molecules; named using Greek prefixes to indicate the number of water molecules (e.g., copper (II) sulfate pentahydrate).
• Naming Covalent (Molecular) Compounds: Use Greek prefixes to indicate the number of atoms; omit mono- for the first element (e.g., carbon dioxide).
• Naming Acids:
  • Binary Acids: hydro- + root of nonmetal + -ic + acid (e.g., hydrochloric acid).
  • Oxyacids: Root name of anion; -ate becomes -ic, -ite becomes -ous + acid, IC is more than OUS; (e.g., sulfuric acid, nitrous acid).