History of the Atom

1. History of the Atom Scientists and Their Contribution to the Model of an Atom

2. Democritus(460 BC – 370 BC) • Proposed that all atoms are small, hard, indivisible and indestructible particles made of a single material formed into different shapes and sizes. • Aristotle did not support his atomic theory Image taken from: https://reich-chemistry.wikispaces.com/T.+Glenn+Time+Line+Project

3. Aristotle (350 B.C.) • modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. • Aristotle was wrong. However, his theory persisted for 2000 years!

4. John Dalton(1766 – 1844) • In 1803, proposed an Atomic Theory which states: • All matter is made of atoms. • Atoms are indestructible – can’t be divided. • All atoms of one element are exactly alike. • Atoms join with other atoms to make new substances. • Calculated the atomic weights of many various elements Image taken from: chemistry.about.com/.../John-Dalton.htm

5. J.J. Thomson (1856 – 1940) • Proved that an atom can be divided into smaller parts • While experimenting with cathode-ray tubes, discovered electrons • Stated that the atom is neutral • In 1897, proposed the Plum Pudding Model which states that atoms mostly consist of positively charged material with negatively charged particles (electrons) located throughout the positive material • Won a Nobel Prize Image taken from: www.wired.com/.../news/2008/04/dayintech_0430 Chocolate chip cookie dough model

6. Ernest Rutherford(1871 – 1937) • In 1909, performed the Gold Foil Experiment and suggested the following characteristics of the atom: • The protons are surrounded by negatively charged electrons, but most of the atom is actually empty space • It consists of a nucleus, that contains most of the mass of the atom • This nucleus is made up of particles called protons, which have a positive charge • Did extensive work on radioactivity (alpha & beta particles, gamma rays/waves) and was referred to as the “Father of Nuclear Physics” • Was a student of J.J. Thomson Image taken from: http://www.scientific-web.com/en/Physics/Biographies/ErnestRutherford.html

8. Niels Bohr (1885 – 1962) • In 1913, proposed Bohr Model-suggests electrons travel around the nucleus of an atom in orbits or definite paths. • Additionally, electrons can jump from a path in one level to a path in another level (depending on their energy) • Won a Nobel Prize • Worked with Ernest Rutherford Image taken from: commons.wikimedia.org/wiki/File:Niels_Bohr.jpg

9. History of the Atom - Timeline Antoine Lavoisier makes a substantial number of contributions to the field of Chemistry J.J. Thomson discovers the electron and proposes the Plum Pudding Model in 1897 Niels Bohr proposes the Bohr Model in 1913 James Chadwick discovered the neutron in in 1932 1766 – 1844 1871 – 1937 1887 – 1961 460 – 370 BC 1700s 1800s 1900s 0 Erwin Schrodinger describes the electron cloud in 1926 Democritus proposes the 1st atomic theory John Dalton proposes his atomic theory in 1803 Ernest Rutherford performs the Gold Foil Experiment in 1909 1891 – 1974 1743 – 1794 1885 – 1962 1856 – 1940 Click on picture for more information

10. - - + - - - - Progression of the Atomic Model The structure of an atom, according to: Democritus & John Dalton James Chadwick Neils Bohr Ernest Rutherford Erwin Schrodinger J.J. Thomson