1. THE ATOMIC NUCLEUS How is the atom put together?
Originally believed that the + and Θ charges were evenly distributed throughout atom. (plum pudding model)

2. ERNEST RUTHERFORD ( )

3. ERNEST RUTHERFORD ( )
Bombarded thin gold foil with a beam of alpha particles.
With evenly-spaced protons and electrons, the alpha particles should have easily passed through the foil.

4. RUTHERFORD'S EXPT.

5. RUTHERFORD'S EXPT. Gold foil surrounded by a fluorescent screen.
As expected, most particles passed straight through.
BUT: Some particles were deflected!

6. RUTHERFORD'S EXPT.

7. CONCLUSIONS
Most particles pass through – the atom is mostly empty space.
Most of the mass and all of the + charge of an atom are concentrated in a small core.

8. CONCLUSIONS
This region has a positive charge capable of causing deflection.
This region is called the NUCLEUS.

9. Protons in the nucleus = ATOMIC NUMBER
unique for each element
equal to the number of protons in the nucleus of an atom of that element
The number of protons must equal the number of electrons.

10. MASS NUMBER whole number (not on table)
Mass of atom is concentrated in the nucleus.
equals number of protons plus neutrons

11. ATOMIC COMPOSITION Determined from mass number and atomic number
atomic no. = no. of protons
= no. of electrons
mass no. – protons = no. of neutrons

12. ATOMIC COMPOSITION Using the chemical symbol: mass # C 12 atomic # 6