Unit 2 Objectives: Atomic History and Structure

Overview

• Purpose of the Unit 2 Test: Assess students' understanding of atomic history and structure.

• Objective: Use this guide to determine study topics and practice key concepts.

Atomic History

• Important Scientists and Their Contributions:

  • Democritus:

  • Proposed the concept of the atom as indivisible particles.

  • Introduced the idea that different atoms exist for different materials.

  • John Dalton:

  • Developed the first modern atomic theory, including:

    • Atoms are indivisible and indestructible.

    • All atoms of a given element are identical.

    • Compounds are formed by a combination of different atoms.

  • Main Flaws:

    • Did not account for isotopes or the internal structure of atoms.

  • Ernest Rutherford:

  • Conducted the gold foil experiment:

    • Discovered the nucleus, showing that atoms have a central core.

    • Proposed the planetary model of the atom.

  • J.J. Thomson:

  • Discovered the electron using the cathode ray tube experiment.

  • Proposed the plum pudding model of the atom, suggesting electrons were embedded in a positively charged 'soup'.

  • Niels Bohr:

  • Developed the Bohr model of the atom:

    • Electrons travel in defined orbits around the nucleus, with quantized energy levels.

  • Erwin Schrödinger:

  • Developed the quantum mechanical model of the atom:

    • Described electrons in terms of probabilities and wave functions, leading to the concept of orbitals.

Atomic Structure

• Basic Structure of the Atom:

  • Nucleus: Central region containing protons and neutrons.

  • Subatomic Particles:

  • Protons:

    • Symbol: p

    • Charge: +1

    • Mass: Approximately 1 atomic mass unit (amu).

  • Neutrons:

    • Symbol: n

    • Charge: 0 (neutral)

    • Mass: Approximately 1 amu.

  • Electrons:

    • Symbol: e

    • Charge: -1

    • Mass: Approximately 1/1836 amu (negligible in mass calculations).

Distinctions

• Atom: Basic unit of matter, composed of protons, neutrons, and electrons.

• Ion: An atom that has gained or lost electrons, resulting in a net charge.

• Isotope: Variants of an element with the same number of protons (atomic number) but different numbers of neutrons (mass number).

Relative Masses of Subatomic Particles

• Protons and neutrons have a mass of approximately 1 amu each.

• Electrons contribute negligibly to the mass of an atom.

Isotopic Notation

• Commonly represented as: [\text{X}^{A}_{Z}]

  • Where X is the chemical symbol, A is the mass number (number of protons + neutrons), and Z is the atomic number (number of protons).

Nuclear Physics

• Types of Radiation:

  • Alpha Radiation:

  • Composition: Helium nuclei (2 protons and 2 neutrons).

  • Low penetrating power; can be stopped by a sheet of paper.

  • Beta Radiation:

  • Composition: High-energy, high-speed electrons or positrons.

  • Moderate penetrating power; can be stopped by aluminum.

  • Gamma Radiation:

  • Composition: High-energy electromagnetic waves.

  • High penetrating power; requires lead or thick concrete for shielding.

Half-Life

• Concept: The time required for half of a radioactive substance to decay.

• Example problem: If you start with 100g of substance, after one half-life, 50g remains; after two half-lives, 25g remains.

Nuclear Reactions

• Nuclear Fission: The process of splitting a heavy nucleus into smaller nuclei, releasing energy.

• Nuclear Fusion: The process of combining lighter nuclei to form a heavier nucleus, also releasing energy.

Periodic Table Use and Chemical Nomenclature

• Key Elements:

  • Identify the atomic number, mass number, and average atomic mass of an atom using the periodic table.

  • Calculate average atomic mass based on isotopes and natural abundance.

• Nomenclature:

  • Names of anions end in –ide; cation names include Roman numerals when necessary (for transition metals).

• Calculations:

  • Use the periodic table to determine:

  • Number of protons, neutrons, and electrons in atoms, ions, and isotopes.

The Mole

• Definition: A mole is a unit of measure used to express amounts of a chemical substance, containing Avogadro's number of representative particles (atoms, molecules, etc.).

  • Avogadro's number: (6.022 \times 10^{23})

• Molar Mass: The mass of one mole of a substance (in g/mol).

• Dimensional Analysis:

  • Utilize dimensional analysis to convert between mass, moles, and number of particles in a sample.