History of the Atom Notes

History of the Atom

John Dalton

  • Atomic Theory:

    • Elements are composed of tiny, indivisible particles called atoms.
    • Atoms of a given element are identical in mass and properties.
    • Atoms of different elements are different.

  • Compounds:

    • Compounds are formed through combinations of different types of atoms.

J.J. Thomson

  • Discovery of Electrons:

    • Atoms contain negatively charged particles called electrons. This discovery was made using a cathode ray tube.

  • Charge-to-Mass Ratio of Electrons:

    • Thomson determined the charge-to-mass ratio of electrons to be approximately 1.76×108Cg1.76 \times 10^8 \frac{C}{g}.

Ernest Rutherford

  • Rutherford's Nucleus Theory:

    • The positive charge of an atom is concentrated in a small, dense region called the nucleus, rather than being uniformly distributed (as in the plum pudding model).
    • Most of the atom is empty space.

  • Gold Foil Experiment:

    • Rutherford conducted the gold foil experiment, where positively charged alpha particles were fired at a thin sheet of gold foil.
    • Most of the alpha particles passed through the foil with little or no deflection.
    • A small fraction of alpha particles were deflected at large angles.
    • This led to the conclusion that the atom is mostly empty space with a concentrated positive charge (the nucleus) at the center.

Niels Bohr

  • Bohr's Orbit Model:

    • Electrons move around the nucleus in specific orbits or energy levels. These orbits have fixed sizes and energies.

  • Electron Orbits:

    • Electrons orbit the nucleus in defined paths.

Erwin Schrödinger

  • Quantum Mechanical Model:

    • Electrons do not move in fixed paths around the nucleus. Their behavior is described by wave functions.

  • Orbitals and Probability:

    • It is impossible to know the exact location of an electron at a given time; instead, electrons exist in clouds of probability called orbitals.

Summary of Atom Structure

  • Atom Definition:

    • The smallest part of an element that retains the properties of that element is called an atom.

  • Atomic Diversity:

    • Each element has different atoms with distinct structures and properties.

  • Subatomic Particles:

    • Atoms can be divided into smaller subatomic particles: protons, electrons, and neutrons.

  • Nucleus Composition:

    • The nucleus is at the center of the atom and contains protons and neutrons.

  • Electron Orbitals:

    • Electrons orbit the nucleus in specific energy levels or orbitals.

  • Energy Levels:

    • Once an energy level is full, electrons begin to occupy the next energy level.

  • Orbital Capacity:

    • Each orbital can contain a maximum of two electrons.