Atoms: Development of the Atomic Theory

1. Atoms:Development of the Atomic Theory

2. Democritus • 460 BC - Greek philosopher proposes the existence of the atom • He pounded materials until he made them into smaller and smaller parts • He called them atoma which is Greek for “indivisible”.

3. Democritus • His Theory: All atoms: • Are small hard particles • Are made of a single material formed into different shapes and sizes • Are always moving, and they form different materials by joining together

4. John Dalton • 1803 - British chemist; elements combine in specific proportions to form compounds Solid Sphere Model or Bowling Ball Model Proposed by John Dalton

5. John Dalton • His Theory: • All substances are made of atoms that cannot be created, divided, or destroyed. • Atoms join with other atoms to make new substances. • Atoms of the same element are exactly alike, and atoms of different elements are different in mass and size.

6. J.J. Thomson • 1897 - English chemist and physicist; discovered 1st subatomic particles Plum Pudding Model or Raisin Bun Model Proposed by J.J. Thomson

7. J.J. Thomson • His Theory: • Atoms contain negatively charged particles called electrons andpositively charged matter. • Created a model to describe the atom as a sphere filled with positive matter with negative particles mixed in • Referred to it as the plum pudding model

8. Ernest Rutherford • 1912 - New Zealand physicist discovered the nucleus Nuclear Model Proposed by Ernest Rutherford

9. Ernest Rutherford • His Theory: • Small, dense, positively charged particle present in nucleus called a proton • Electrons travel around the nucleus,but their exact places cannot be described.

10. Niels Bohr • 1913 - Danish physicist; discovered energy levels Bohr Model or Planetary Model Proposed by Niels Bohr

11. Niels Bohr • His Theory: • Electrons travel around the nucleus in definite paths and fixed distances. • Electrons can jump from one level to a path in another level.

12. Erwin Shrodinger • 1924 - Austrian physicist; developed the electron cloud model Electron Cloud Model Proposed by Erwin Schrodinger

13. Erwin Shrodinger • His Theory: • The exact path of electrons cannot be predicted. • The region referred to as the electron cloud, is an area where electrons can likely be found.

14. James Chadwick • 1932 - English physicist; discovered neutrons • His Theory: • Neutrons have no electrical charge. • Neutrons have a mass nearly equal to the mass of a proton. • Unit of measurement for subatomic particles is the atomic mass unit (amu).

15. Modern Theory of the Atom Atomsare composed of three main subatomicparticles: the electron, proton, and neutron. Most of the mass of the atom is concentrated in the nucleus of the atom.

16. Modern Theory of the Atom The protons and neutrons are located withinthe nucleus, while the electrons exist outside of the nucleus. In stable atoms, the number of protons is equal to the number of electrons.

17. Modern Theory of the Atom The type of atom is determined by the number of protons it has. The number of protonsin an atom is equal to the atomic number.

18. Modern Theory of the Atom The sum of the number of protonsand neutrons in a particular atom is called the atomicmass. Valence electrons are the outermost electrons.