1. Historical Development of an Atom

2. Democritus 470-380 B.C Democritus was a Greek philosopher (470-380 B.C.) who is the father of modern atomic thought. Democritus was a Greek philosopher (470-380 B.C.) who is the father of modern atomic thought. He proposed that matter could NOT be divided into smaller pieces forever. He proposed that matter could NOT be divided into smaller pieces forever. He believed that matter was made of small, hard, particles, that he called “atomos” He believed that matter was made of small, hard, particles, that he called “atomos”

3. John Dalton 1807 He built upon Democritus’s atomic theory. He built upon Democritus’s atomic theory. Matter is made up of atoms Matter is made up of atoms Atoms cannot be divided into smaller pieces. Atoms cannot be divided into smaller pieces. All the atoms of an element are exactly the same All the atoms of an element are exactly the same Different elements have different kinds of atoms Different elements have different kinds of atoms Compounds form by combining atoms Compounds form by combining atoms

4. John Dalton 1807 Model Drawing Model Drawing Solid throughout Solid throughout Same throughout Same throughout

5. J.J. Thomson 1898 Particles smaller than the atom exists Particles smaller than the atom exists Discovered the electron, which is negatively charged Discovered the electron, which is negatively charged If atoms have negative particles, then atoms must also have positive particles. If atoms have negative particles, then atoms must also have positive particles.

6. J.J. Thomson 1898 Model Drawing Model Drawing Positive and negative particles Positive and negative particles

7. Ernest Rutherford 1911 Rutherford performed the gold foil experiment. Rutherford performed the gold foil experiment. In his experiment a radio active beam emitted (+) alpha particles toward gold foil. In his experiment a radio active beam emitted (+) alpha particles toward gold foil. Most particles went through, some were deflected, and others bounced back! Most particles went through, some were deflected, and others bounced back! This showed that atoms have a very dense (+) center. This showed that atoms have a very dense (+) center.

8. Ernest Rutherford 1911 Rutherford showed that atoms have (+) particles in the center and are mostly empty space. Rutherford showed that atoms have (+) particles in the center and are mostly empty space. He called the center of the atoms the nucleus. He called the center of the atoms the nucleus. All the mass of an atom and its positive charge are located in the nucleus. All the mass of an atom and its positive charge are located in the nucleus.

9. Ernest Rutherford 1911 Model Drawing Model Drawing Positive nucleus Positive nucleus Negative particles surround nucleus Negative particles surround nucleus

10. Niels Bohr 1913 Niels Bohr improved on Rutherford’s model. Niels Bohr improved on Rutherford’s model. He proposed that electrons move around the nucleus in specific layers or shells. He proposed that electrons move around the nucleus in specific layers or shells. Every atom has a specific number of electron shells. Every atom has a specific number of electron shells.

11. Niels Bohr 1913 Electrons move in orbits around the nucleus Electrons move in orbits around the nucleus 1 st energy level = 2 electrons 1 st energy level = 2 electrons 2 nd energy level = 8 electrons 2 nd energy level = 8 electrons 3 rd energy level = 18 electrons 3 rd energy level = 18 electrons 4 th energy level = 32 electrons 4 th energy level = 32 electrons

12. Niels Bohr 1913 Model Drawing Model Drawing Electrons orbit the positive nucleus on energy levels Electrons orbit the positive nucleus on energy levels

13. James Chadwick 1932 Chadwick discovered neutrons Chadwick discovered neutrons He worked with Rutherford and discovered particles with no charge He worked with Rutherford and discovered particles with no charge He called those particles neutrons He called those particles neutrons Neutrons are found in the nucleus of an atom. Neutrons are found in the nucleus of an atom.

14. James Chadwick 1932 Model Drawing Model Drawing Electrons orbit the positive nucleus which contains protons and neutrons Electrons orbit the positive nucleus which contains protons and neutrons

15. Electron Cloud Model Current Electrons do not follow fixed orbits, but tend to occur more frequently in certain areas around the nucleus Electrons do not follow fixed orbits, but tend to occur more frequently in certain areas around the nucleus it is impossible to know where an electron is at any given time it is impossible to know where an electron is at any given time

16. Electron Cloud Model Current Model Drawing Model Drawing Electrons are located in clouds Electrons are located in clouds