History of Atomic Theory

Early Concepts of the Atom

The concept of the atom has evolved over centuries, beginning with philosophical ideas and advancing through scientific experimentation.

• Democritus (400 B.C.): Proposed that matter is composed of indivisible particles called atomos.

• Aristotle: Disagreed with Democritus, favoring the idea that matter was continuous.

• Robert Boyle (1661): Introduced the concept of corpuscles (small particles) and distinguished between elements, mixtures, and compounds.

• Antoine Lavoisier (1789): Formulated the Law of Conservation of Mass: Mass is neither created nor destroyed in chemical reactions.

• Joseph Proust (1799): Established the Law of Definite Proportions: A chemical compound always contains the same proportion of elements by mass.

• John Dalton (1803): Proposed the Law of Multiple Proportions: When elements combine, they do so in ratios of small whole numbers.

The Mole Concept

Counting Atoms and Molecules

Atoms are extremely small, making it impractical to count them individually. Chemists use the mole as a counting unit for atoms, molecules, or other particles.

• Definition: 1 mole = particles (Avogadro's number).

• Application: The mole allows chemists to relate macroscopic measurements to the number of atoms or molecules present.

• Atomic Mass: The average mass of an atom of an element, typically measured in atomic mass units (amu).

The Periodic Law and Table

Mendeleev's Periodic Law

The periodic table organizes elements based on recurring chemical properties.

• Dmitri Mendeleev (1869): Formulated the Periodic Law: "When the elements are arranged in order of increasing mass, certain sets of properties occur periodically."

• Arranged elements so those with similar properties aligned in vertical columns (groups).

• Predicted properties of undiscovered elements based on table patterns.

Structure of the Periodic Table

The periodic table is organized into periods (rows) and groups (columns or families).

• Main group elements (A) and transition elements (B)

• Metals, nonmetals, and metalloids are classified based on their properties.

• Special group names:

  • Alkali metals (group 1A)

  • Alkaline earth metals (group 2A)

  • Halogens (group 7A)

Subatomic Particles and Atomic Structure

Discovery of Subatomic Particles

Atoms consist of smaller particles: protons, neutrons, and electrons.

• Ions: Atoms or groups of atoms with a net charge. Anion (negative ion), cation (positive ion).

• Electrons: Discovered by J.J. Thompson (1897); very small compared to hydrogen atom.

• Plum Pudding Model: Early model with electrons embedded in a positive sphere.

• Rutherford's Nuclear Model: Showed the existence of a dense, positively charged nucleus at the atom's center.

• Proton: Positively charged particle in the nucleus.

Atomic Number, Mass Number, and Isotopes

Atoms are identified and classified by their numbers of protons and neutrons.

• Atomic number (Z): Number of protons in an atom; defines the element.

• Mass number (A): Sum of protons and neutrons in an atom.

• A-Z Notation:

• Isotopes: Atoms of the same element with different numbers of neutrons (and thus different mass numbers).

• Atomic mass: Weighted average of the masses of an element's naturally occurring isotopes.

Electron Arrangement: The Bohr Model

Electron Energy Levels and Spectra

Electrons in atoms occupy specific energy levels, which explains the emission of light and atomic spectra.

• Elements emit characteristic colors and line spectra when heated or excited.

• Niels Bohr: Explained that electrons exist in quantized energy levels.

• When an electron falls from an excited state to a ground state, it emits a photon of light with energy equal to the difference between the levels.

• Electron locations are not random; they are found in shells, subshells, and orbitals.

Key Equation: Energy of a Photon

The energy of a photon emitted or absorbed during an electron transition is given by:

• = energy of the photon

• = Planck's constant ( J·s)

• = frequency of the light

Summary Table: Atomic Particles

The following table summarizes the main subatomic particles:

Additional info: The notes have been expanded to include definitions, examples, and equations for clarity and completeness.