Atomic Theory and Structure

Classification of Matter by Chemical Composition

Matter can be broadly classified based on its chemical composition and properties. The primary distinction is whether its composition is variable or fixed.

  • Matter

    • Variable Composition? Yes: Mixture

      • A mixture consists of two or more substances that are physically combined. Its composition is variable.

      • Uniform Throughout? Yes: Homogeneous Mixture

        • A homogeneous mixture has a uniform composition and appearance throughout. Examples include Tea with sugar.

      • Uniform Throughout? No: Heterogeneous Mixture

        • A heterogeneous mixture does not have a uniform composition and its different components can often be visibly distinguished. An example is Wet sand.

    • Variable Composition? No: Pure Substance

      • A pure substance has a fixed chemical composition and distinct properties.

      • Separable into simpler substances? Yes: Compound

        • A compound is a pure substance composed of two or more elements chemically combined in fixed proportions. An example is Pure water.

      • Separable into simpler substances? No: Element

        • An element is a pure substance consisting of only one type of atom that cannot be broken down into simpler substances by chemical means. An example is Helium.

Visualizing Atoms

Remarkably, we can visualize individual atoms. The Scanning Tunneling Microscope (STM) is a powerful tool capable of imaging matter at atomic resolution. Not only can STM image atoms, but its tip can also be used to manipulate and move individual atoms, demonstrating the tangible nature of these fundamental particles. This capability is famously illustrated in the "A boy and his atom" video.

Development of Atomic Theory: Foundational Laws

The understanding of atomic theory evolved through key observations and laws:

  • Law of Definite Proportions (Joseph Proust, 1797)

    • This law states that all samples of a given compound, regardless of their source or the method of preparation, will always have the exact same proportions of their constituent elements. For instance, pure water will always have the same mass ratio of hydrogen to oxygen.

  • Law of Conservation of Mass (Antoine Lavoisier, 1789)

    • This fundamental law in chemistry dictates that in any closed system, during a chemical reaction, matter is neither created nor destroyed. The total mass of the reactants before a chemical reaction must equal the total mass of the products after the reaction.

  • Law of Multiple Proportions (John Dalton, 1804)

    • This law applies when two elements combine to form more than one compound. It states that the masses of one element that combine with a fixed mass of the other element are in small, whole-number ratios. For example, in carbon monoxide (CO) and carbon dioxide (CO2), for a fixed mass of carbon, the mass of oxygen in CO2 is exactly twice the mass of oxygen in CO. This implies discrete units of elements combining.

Illustrating the Law of Definite Proportions

Consider two samples of the same compound:

  • Sample 1: 24.22 ext{ g} carbon and 32.00 ext{ g} oxygen

  • Sample 2: 36.22 ext{ g} carbon and 48.00 ext{ g} oxygen

To see which law this data demonstrates, we can calculate the ratio of carbon to oxygen in each sample:

  • Ratio in Sample 1: \frac{24.22 ext{ g C}}{32.00 ext{ g O}} hickapprox 0.7569 C per g O

  • Ratio in Sample 2: \frac{36.22 ext{ g C}}{48.00 ext{ g O}} hickapprox 0.7546 C per g O

The ratios are approximately the same, within measurement variability. This data explicitly demonstrates the Law of Definite Proportions, as both samples of the same compound have nearly identical proportions of their constituent elements.

Dalton's Atomic Theory (1808)

John Dalton synthesized the observations from the laws of chemical combination into his groundbreaking atomic theory, built upon five main postulates:

  1. All matter is composed of indivisible atoms. (Note: While later disproven for indivisibility, this was a crucial conceptual step).

  2. An atom is an extremely small particle of matter that retains its identity during chemical reactions.

  3. An element is a type of matter composed of only one kind of atom.

  4. A compound is a type of matter composed of atoms of two or more elements chemically combined in fixed proportions. This directly explains the Law of Definite Proportions.

  5. A chemical reaction involves the rearrangement of the atoms present in the reacting substances to give new chemical combinations present in the substances formed by the reaction. Atoms are neither created nor destroyed in this process, supporting the Law of Conservation of Mass.

Structure of the Atom and Discovery of the Electron

Early understanding of the atom's structure evolved significantly with experimental evidence, revealing that atoms are not indivisible.

  • Initial Concept: Atoms consist of a positively charged nucleus (containing most of the atom's mass) and negatively charged electrons (very small mass).

  • J.J. Thomson (1897): Discovery of the Electron

    • Thomson's cathode ray experiments provided the first evidence that atoms are not indivisible, leading to the discovery of the electron.

    • He observed a stream of rays (cathode rays) that could be visualized by a phosphorescent material like zinc sulfide.

    • Magnetic Field Effect: When a magnet was brought near the cathode ray, the stream bent, indicating that the rays were composed of charged particles.

    • Electric Field Effect: When an electric voltage was applied, the stream bent towards the positive ( ext{+}) plate, confirming that the particles carried a negative charge.

    • Through these experiments, Thomson was able to determine the mass-to-charge (m/e) ratio of these negatively charged particles, which he called