Development of the Atomic Model

1. Development of the Atomic Model

2. The Ancient Alchemists • Time: 500 BC – AD 1720 • Many were dedicated to finding a substance that would transmute (change) metals into gold. • Also interested in prolonging human life. • Laid the foundation for modern chemistry.

3. Democritus • Time: 442 BC • Greek philosopher • Among the first to suggest the existence of atoms. • Believed that atoms were indivisible and indestructable. • “a tomos” = not divisible • His ideas were not scientific. • They lacked experimental support. • They did not explain chemical behavior. • Not based on the scientific method.

4. John Dalton • Time: 1803 • English chemist • Used experimental methods to transform Democritus’s ideas into a scientific theory. • Five tenets of Dalton’s atomic theory: • All elements are composed of tiny indivisible particles called atoms. • Atoms of the same element are identical, but are different from those of any other element. • Atoms of different elements can combine in whole-number ratios to make compounds. • Chem. rxns occur when atoms are separated, joined, or rearranged. • Atoms of one element never change into atoms of another element during a chemical rxn.

5. J. J. Thomson • Time: 1897 • English physicist • Discovered the electron. • “Plum pudding” model of the atom. • Imagined the atom as a positive sphere with negative electrons embedded like plums in a pudding.

6. Robert Millikan • Time: 1916 • American physicist • Measured the charge of the electron. • Calculated the mass of the electron. • 1/1840 the mass of a hydrogen atom.

7. Ernest Rutherford • Time: 1911 • New Zealand physicist working in England • Discovered the atomic nucleus. • Gold foil experiment • Developed the nuclear model of the atom. • Nucleus is tiny, dense, and positively charged. • Electrons are distributed around nucleus.

8. Rutherford’s Gold Foil Experiment To start the video, right click in the frame above and select “Play”.

9. James Chadwick • Time: 1931 • English physicist • Discovered the neutron. • Opened the door to the splitting of the atomic nucleus (nuclear fission).

10. Other Notable Names in the Development of the Modern Atomic Model • Antoine Lavoisier • Demonstrated the law of conservation of matter • Charles-Augustin de Coulomb • Formulated Coulomb’s law that describes the forces exerted by charged particles on each other • Wilhelm Conrad Roentgen • Discovered X-rays • Henri Becquerel • Discovered radioactivity • Pierre and Marie Curie • Identified and studied radioactive elements • Max Planck • Showed that radiation is quantized (can only exist in discrete amounts) • Albert Einstein • Introduced the idea that light has properties of both particles and waves and showed that mass and energy are interchangeable. • Niels Bohr • Created a model of the arrangement of electrons in atoms. • Otto Hahn • Split the uranium nucleus by bombarding it with neutrons. • Glenn Seaborg • Succeeded in creating man-made elements.