Essential Chemistry Concepts and Atomic Theory Timeline

Posted on Oct 16, 2025 in Chemistry

Fundamental Chemistry Concepts and Definitions

Physical and Chemical Properties

  • Physical Property: A description that helps identify a substance.
  • Quantitative Characteristics: Characteristics that are measured. They always include a number and a unit.
  • Qualitative Characteristics: Characteristics observed with the senses that are described but not measured.
  • Characteristic Physical Property: A unique physical property used specifically to identify a substance (e.g., density, melting point).
  • Chemical Properties: Describe how a substance reacts with another substance (e.g., flammability, reactivity). Indicators of a chemical change include color change, odor, temperature change, or light emission.
  • Precipitate: A solid formed when two liquids are mixed during a chemical reaction.
  • Stability: Refers to how easily a substance decomposes.
  • Toxicity (LD₅₀ Value): The dose required to kill 50% of the exposed population.

Classification of Matter: Pure Substances and Mixtures

  • Pure Substance: Includes elements and compounds.
  • Impure Substance (Mixture): Includes mechanical mixtures and solutions.

Mixture Types

  • Homogeneous Mixture (Solution): Appears uniform; consists of a single phase.
  • Heterogeneous Mixture: Two or more phases are visible. Can be separated by physical means and has variable composition.

Compounds and Isotopes

  • Compounds: Two or more elements chemically combined. Can be separated only by chemical means.
  • Isotopes: Atoms of the same element that have a different number of neutrons in their nucleus.

Essential Chemical Tests

These tests are used to identify specific gases produced during reactions.

  1. Hydrogen Gas Test (H₂)

    Place a lit splint at the mouth of the reaction vessel. If positive, it will give a squeaky pop sound. This combustion reaction occurs due to the presence of oxygen, and the result of the reaction is water.

  2. Oxygen Gas Test (O₂)

    Use a glowing splint (blow out the flame). Hold it near the gas. If the splint relights (rekindles), the gas is oxygen. Oxygen is essential for combustion and is often tested at concentrations near 100%, rather than the 21% found in air.

  3. Carbon Dioxide Test (CO₂)

    Bubble CO₂ into lime water. If carbon dioxide is present, the water will turn a milky, cloudy color. This change is due to the formation of a precipitate (calcium carbonate).

Atomic Structure and Notation

Subatomic Particles and Ions

  • Electrons: Are very light and contribute negligible mass to the atom.
  • Standard Atomic Notation: Represents the mass number and atomic number (proton number) alongside the element symbol.
  • Cation: A positive ion (has more protons than electrons).
  • Anion: A negative ion (has more electrons than protons).

The Periodic Table and Bonding

  • Dmitri Mendeleev created the periodic table. It was initially ordered by atomic mass, revealing predictable patterns in element properties.
  • Nonmetals are generally located to the right of the metalloids (the staircase), while metals are generally located to the left of the metalloids.
  • Atoms can share electrons with other atoms to form compounds (covalent bonding).
  • The Octet Rule: Most atoms strive to achieve eight valence electrons for stability.
  • Metals tend to lose electrons, forming cations. Nonmetals tend to gain electrons, forming anions.
  • Smaller atoms (e.g., Hydrogen) are stable with a duet (two electrons) in their first shell.

Historical Development of Atomic Theory

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  1. Democritus (c. 400 BC)

    Founded the concept of the atom (atomos, meaning indivisible). He used simple examples like a rock or an apple to illustrate the idea of fundamental, uncuttable particles.

  2. John Dalton (1807)

    Created the basis of modern particle theory. His atomic model was often visualized as a solid sphere, similar to a billiard ball.

  3. J.J. Thomson (1897)

    Discovered the electron. His model, where negative electrons were embedded in a positive mass, was known as the plum pudding model.

  4. Ernest Rutherford (1909 & 1920)

    Discovered the nucleus and the proton through the gold foil experiment (using refracted light/alpha particles). He proposed the nuclear model of the atom.

  5. James Chadwick (1932)

    Discovered the neutron. He used the solar system model as inspiration for his diagram (though this inspiration is often attributed to Bohr).

  6. Niels Bohr

    Discovered the electron shells (energy levels) of an atom that hold the electrons. He used a prism and light to study atomic spectra. He incorporated Rutherford’s nuclear model into his own planetary model.

Diagram Efficiency

The Lewis Dot Diagram is generally considered more efficient and practical than the Bohr model for representing valence electrons.

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