History of the Atom

1. History of the Atom Scientists who shaped the modern atomic theory

2. Democritus first suggested matter was made of atoms there was no way to test these theories Aristotle suggested matter was continuous, not made of atoms this was believed 4th Century Greece

3. Alchemists • Transmutation: tried to turn all metals into gold • brought observation and experimentation to science

4. Antoine Lavoisier • In ordinary chemical reactions in a closed system, mass remained constant. • Law of conservation of mass • matter can not be created or destroyed

5. John Dalton and the Atomic Theory In the early 1800s, Dalton wrote explanations of the atom based on experiments by Lavoisier and Proust.

6. Dalton’s Atomic Theory • Atoms are indivisible. • Elements are made of atoms. • Atoms of the same element are identical. Atoms of different elements are different. • Atoms combine in simple, whole number ratios. • Reactions occur when atoms are separated or combined.

7. J.J. Thomson • 1897 • cathode rays are composed of electrons • determined the charge of an electron • “plum pudding model”

9. In the early 1900s... • Robert Millikan (American) • oil drop experiment • accurately measured the charge of an electron

10. 1896 Henri Becquerel discovered that uranium exposes photographic film. Radioactivity Early 1900s Marie and Pierre Curie discovered that rays are given off by uranium and radium. Atoms will split into smaller units Then a ray of light??

11. Albert Einstein • Explained the origin of the energy produced in nuclear reactions with E = mc2.

12. Then the modern theory began to develop • Lord Rutherford • 1913 • Proved Thomson wrong

13. Rutherford Model • Also called the planetary model • Most of the atom is empty space • Consists of a central nucleus surrounded by electrons in orbits

14. The “final” particle is discovered... • 1932 • James Chadwick • found particle with the same mass as a proton, but no charge • now known as a neutron

15. So what did this all mean to the atomic theory? • Dalton’s theory had to be revised to include subatomic particles!

16. Bohr Model • Calculated specific energy levels for electrons

17. Quantum mechanical model(Schrodinger, de Broglie, & Heisenberg) • Current model • Electron cloud model • Electrons do not have fixed positions, but exist somewhere in the electron cloud