1. The History of Atomic Theory

2. Democritus 460-370 B.C. Believed the universe was made of empty space and tiny bits of stuff called atoms that couldn’t be divided into smaller pieces. The term atom comes from, atomos, which means, “cannot be divided”.

3. John Dalton-1808 Believed that matter was made of atoms of different elements. Considered the father of modern atomic theory.

4. Thomson's Model In 1897, J.J Thomson passed an electric current through a gas. He found that the gas gave off rays made up of negatively charged particles. Prior to this, atomic theory stated that atoms are neutral, there must also be positively charged particles inside an atom. Think positive, or negative

5. Thomson-1897 Thomson hypothesized that an atom was made up of a positively charged material with negatively charged particles scattered evenly throughout.

6. Thomson's model of an atom Negatively charged material Positively charged material

7. Rutherford's Model In 1911, a British scientist named Ernest Rutherford discovered that an atom is mostly empty space. He concluded that the positively charged particles are contained in a small central core called the nucleus. He also concluded that negatively charged particles were attracted to positively charged particles found in the nucleus. This attraction holds the negatively charged particles in the atom.

10. Rutherford-1911 Discovered the nucleus. Named the positively charged particles protons. Said that the atom was mostly empty space and electrons traveled around the nucleus. Not empty space

11. Rutherford‘s model of an atom

12. Bohr's Model Rutherford's model of the atom did not explain the arrangement of electrons. In 1913, Niels Bohr proposed that the electrons in an atom are found in different energy levels. Each energy level is at a certain distance from the nucleus. Electrons in different energy levels move around the nucleus in different orbits. Bohr's model explains simple atoms such as oxygen well, but it does not explain more complex atoms.

13. Niels Bohr-1913 FOUND EVIDENCE THAT ELECTRONS WERE ARRANGED IN ENERGY LEVELS. My attempt at smiling is a lower energy level.

14. Bohr's model of an atom

15. Gilbert N. Lewis - 1916 Developed dot structure diagram of atoms to explain how they bond together. Published works on how atoms combine or “bond”.

16. Chadwick & the Nucleus When chemists started looking into how much mass an atom has, they noticed that there is a mysterious source that they couldn’t figure out that made the mass bigger than they thought. James Chadwick hypothesized and tested to prove that there is another particle in the nucleus. He found that these particles have no positive or negative charge.

17. CHADWICK-1932 DISCOVERED THE NEUTRON. His ideas allowed for man-made elements to be created.

18. Modern Model According to the modern model, the location of the electrons in an atom cannot be known. Therefore, the modern model of the atom does not show any paths that electrons could be found in. Instead, energy levels are used to predict the place where an electron is most likely to be found outside of the nucleus. This area is called the electron cloud.

19. Modern model of an atom

20. The History of Atomic Theory

21. Regular microscopes cannot give you a visual of an atom using light to see it. Special electromagnetic lenses in special machines can produce an image of an atom’s nucleus magnified millions and millions of times. With this technology, scientists can manipulate the location of nuclei to produce images in sequences that look like a movie. You cannot see any electrons. They are 1836 times smaller than a proton. http://www.youtube.com/watch?v=oSCX78-8-q0 Movie Made with Atoms!