Chapter 1: atoms (part 1)

Introduction to Atoms

• Definition: Atoms are the smallest units of matter.

• Atoms consist of protons, neutrons, and electrons.

• Key examples: Water (H2O), demonstrating atomic movement and kinetic energy.

Kinetic Energy in Different States of Matter

• Kinetic Energy: Movement associated with particles in various states of matter.

  • Solids:

    • Kinetic Energy: Very low, particles vibrate in fixed positions.

    • Types:

      • Crystalline Solids: Ordered arrangement (e.g., diamonds).

      • Amorphous Solids: Disordered arrangement (e.g., carbon in pencils).

  • Liquids:

    • Kinetic Energy: Moderate, particles can flow and slide past one another.

    • Properties: Definite volume, indefinite shape, not compressible.

  • Gases:

    • Kinetic Energy: High, particles move freely and rapidly.

    • Properties: No definite shape or volume; compressible.

Classification of Matter

• Pure Substances: Consist of one type of element or compound.

  • Elements: Basic substances that cannot be broken down (e.g., oxygen).

  • Compounds: Composed of two or more elements chemically combined (e.g., water - H2O).

• Mixtures: Combinations of two or more substances that retain their individual properties.

  • Homogeneous Mixtures: Uniform composition (e.g., tea, saltwater).

  • Heterogeneous Mixtures: Non-uniform composition (e.g., wet sand).

Properties of Mixtures and Pure Substances

• Definite Composition: Pure substances have a specific composition (e.g., water is always H2O).

• Variable Composition: Mixtures can vary (e.g., different samples of air).

• Physical Changes: Homogeneous mixtures can often be separated by physical means (e.g., evaporation).

Laws of Chemistry

• Law of Conservation of Mass: Matter cannot be created or destroyed in chemical reactions.

• Law of Definite Proportions: A compound consists of the same elements in the same proportions by mass.

  • Example: Water (2 parts hydrogen to 1 part oxygen by mass).

Discovery of Atomic Structure

• Historical Context: Various experiments led to the modern understanding of atomic structure.

  • Cathode Ray Experiment: Established the existence of electrons as negatively charged particles.

  • Gold Foil Experiment: Revealed that atoms consist of a small, dense nucleus surrounded by electrons.

Structure of an Atom

• Components:

  • Protons: Positively charged particles in the nucleus.

  • Neutrons: Neutral particles in the nucleus.

  • Electrons: Negatively charged particles orbiting the nucleus.

• Mass Contribution: The mass of an atom is primarily due to protons and neutrons; electrons have negligible mass.

Isotopes and Atomic Mass

• Isotopes: Atoms of the same element with different numbers of neutrons.

  • Atomic Mass: Weighted average of all isotopes of an element, factoring in their natural abundance.

Identifying Elements Using the Periodic Table

• Atomic Number: Number of protons in an atom, located above the element symbol on the periodic table.

• Mass Number: Total number of protons and neutrons.

  • Example: For Neon-20, there are 10 protons and 10 neutrons (20 - 10 = 10).

Ions and Charges

• Ions: Atoms that have gained or lost electrons, resulting in a charge.

  • Cations: Positively charged ions (loss of electrons).

  • Anions: Negatively charged ions (gain of electrons).

• Group Trends:

  • Elements in groups often share similar ionization and bonding characteristics.

Summary of Key Points

• Atoms are composed of protons, neutrons, and electrons.

• The properties of substances depend on their atomic structure and arrangement.

• Understanding atoms leads to insights about chemical reactions and the conservation of mass.