1. What is an atom? Smallest unit that maintains the characteristics of each element 10 23 atoms in a thimbleful of water 7.1 The Atom

2. History 450 BCE, Democritus coined the term átomos (Greek: ἄ τομος), which means "uncuttable" or "the smallest indivisible particle of matter” Coined the word Atom 7.1 The Atom

3. History In 1803, John Dalton proposed that each element consists of atoms of a single, unique type, and that these atoms can join together to form chemical compounds Atomic Theory 7.1 The Atom

4. History In 1827, botanist Robert Brown used a microscope to look at dust grains floating in water and discovered that they moved about erratically Brownian motion 7.1 The Atom

5. History in 1909, Ernest Rutherford had people bombard a sheet of gold foil with helium ions gold foil experiment positive charge of an atom and most of its mass was concentrated in a nucleus at the center of the atom, with the electrons orbiting it like planets around a sun. 7.1 The Atom

6. History In 1913,Niels Bohr modified the model Electrons confined into clearly defined, quantized orbits, and could jump between these, but could not freely spiral inward or outward in intermediate states Planetary Model 7.1 The Atom

7. Modern View In 1924, a French physicist named Louis de Broglie suggested that electrons could act as both particles and waves Erwin Schrodinger electrons confined in their orbits would set up standing waves and you could describe only the probability of where an electron could be. Orbitals could be described as electron density clouds The Atom

8. Atoms contain particles called protons, neutrons, and electrons. Protons and Neutrons are in the “nucleus”. Electrons are in the “orbits” Positive and Negative Charge 7.1 The Atom

9. Protons have positive electric charge and electrons have negative electric charge. Protons and neutrons are 1800 times larger than electrons Atom is mostly empty space Positive and Negative Charge 7.1 Electric Charge

10. An atom contains equal numbers of protons and electrons, so the positive and negative charges cancel out and an atom has no net electric charge. Objects with no net charge are said to be electrically neutral. Positive and Negative Charge 7.1 Electric Charge

11. Helium Atom 7.1

12. Review Question Who coined the term “atom”? a.Brown b.Rutherford c.Democritus d.Bohr

13. Review Question Which part of the atom is the smallest and located on the outside in the “orbits”? a.Proton b.Neutron c.Electron