L2 - Atomic Structure and Moles

Introduction to Atomic Structure

• The lecture series will progress from understanding the components of an atom to exploring properties, bonding, models, and shapes.

• Today's focus: atomic structure, differentiating between physical and chemical changes, and quantification in chemistry.

Representing Chemical Reactions

• A familiar reaction: combustion of methane (CH4) with oxygen (O2) to form carbon dioxide (CO2) and water (H2O).

• Three ways to represent reactions:

  • Worded explanations: Describing reactants and products.

  • Chemical symbols: Condensed representations using element symbols (e.g., CH4 for methane).

  • Simplified stick and ball drawings: Showing relative sizes and connections between atoms.

States of Matter and Physical Changes

• Water exists in three states: gas (water vapor), liquid (water), and solid (ice).

• Change of state is a physical change: Chemical composition (H2O) remains the same.

• Only the forces holding molecules together change, influenced by temperature and pressure.

States of Matter: Microscopic View

• Solid: Packed in rigid lattice-like structures.

• Liquid: Takes the shape of the vessel, molecules held together.

• Gas: Occupies the entire vessel with even distribution.

Physical vs. Chemical Changes

• Physical Change: Alters physical form but not chemical composition; no bonds broken or formed (e.g., phase change of water; H2O remains H2O).

• Chemical Change: Requires breaking and formation of bonds (e.g., combustion of methane). To form CO_2, carbon-hydrogen bonds must break, and carbon-oxygen bonds must form.

Terminology for Phase Changes

• Liquid to Gas: Boiling.

• Gas to Liquid: Condensation.

• Solid to Liquid: Melting or Fusion.

• Liquid to Solid: Freezing.

• Solid to Gas: Sublimation (e.g., dry ice).

• Gas to Solid: Deposition.

Allotropes and Physical Properties

• Allotropes: Different forms of the same element with varying arrangements and properties (e.g., carbon as diamond, graphite, carbon 60).

• Graphite consists of weakly held sheets of carbon atoms, allowing it to leave a coating on surfaces.

• Diamond has a strong 3D network of bonds, making it hard.

• Physical properties: color, melting point, conductivity, density, surface tension. None relate to a change in chemical nature.

Chemical Properties

• Properties that relate to a change in chemical structure.

• Examples include flammability, corrosiveness, and reactivity with acid.

• These properties involve forming something new and breaking bonds.

Subatomic Particles

• Protons: Positive charge.

• Neutrons: Neutral charge.

• Electrons: Negative charge.

• Nucleons: Collective term for protons and neutrons.

Defining Elements: The Role of Protons

• The number of protons defines an element (atomic number, Z).

• Changing electrons alters charge.

• Changing neutrons alters mass (isotopes).

• The charge of the nucleus is determined by the number of protons.

• The mass number is the sum of protons and neutrons.

Using the Periodic Table

• Key information:

  • Atomic number (number of protons).

  • Chemical symbol.

  • Mass number (protons + neutrons).

Isotopes

• Atoms with the same number of protons but different numbers of neutrons.

• Example: Uranium-235 and Uranium-238 (both have 92 protons).

• Isotopes can have different properties.

Atomic Mass Units (AMU)

• Carbon-12 is defined as exactly 12 amu.

• 1 amu = 1.66054 x 10^{-24} grams.

Ions, Cations, and Anions

• Neutral species: equal number of protons and electrons.

• Cation: positive charge (more protons than electrons).

• Anion: negative charge (more electrons than protons).

Isotopic Abundance and Average Atomic Mass

• The relative abundance of isotopes varies, affecting the average atomic mass.

• Average atomic mass is calculated by:
  (\text{percentage of isotope 1} \times \text{mass of isotope 1}) + (\text{percentage of isotope 2} \times \text{mass of isotope 2}) + \ldots

• Example: Calculating the average mass of hydrogen.
  (\text{99.99%} \times 1) + (\text{0.01%} \times 2) = 1.00018 \text{ amu}

• Carbon-14 is used in carbon dating to estimate the age of fossils.

Avogadro's Number and the Mole Concept

• Avogadro's number (NA): 6.022 \times 10^{23} particles per mole.

• A mole is a count of objects.

• The mass of one mole of a substance in grams is numerically equal to its atomic mass in amu.

Molar Mass

• Molar mass: The mass of one mole of a substance (grams per mole).

• Units: grams per mole (g/mol or gmol^{-1}).