1. Chapter 3 section 2 Structure of atoms Ms.Briones

2. Subatomic particles
Experiments by several scientists in the mid-1800s led to the first change to Dalton’s atomic theory.
Scientists discovered that atoms can be broken into pieces.

3. The most important particles:
Electron
Proton
Neutron

4. Electrons
Electrons were discovered by using cathode rays (just like in a TV)
J.J. Thomson, scientist, created the cathode rays.

5. What was Thomson’s creation?
He pumped the air out of glass tube.
He then applied a voltage to 2 metal plates, called electrodes which were placed at each end of the tube (positive and negative charges).

6. What was Thomson’s creation?
Thomson observed a glowing beam that came out of the cathode and struck the anode (+).
He called these the cathode rays.
Now it is used for many devices.

7. Electrons They are part of an atom. They are negatively charged.
Common charge notation: -1
Symbol: e
Located in the outer energy level.

8. Rutherford
Rutherford, also a scientist, studied Thomson’s theory and carried out the experiment.

9. The Nucleus
Rutherford discovered the nucleus in his model called the plum-pudding model.
Also called alpha particles

10. Rutherford Gold Foil Experiment
Conclusion: mass of nucleus must be larger than the alpha particle

11. The Nucleus The nucleus is the dense, central portion of the atom.
Contains most of the mass and a small fraction of the volume

12. Protons and Neutrons Protons: + charge (+1)
Neutrons: neutral charge (0)
In the nucleus

13. Protons and Neutrons can form a stable nucleus.
Coulomb’s law: the closer two charges are, the greater the force between them. (magnets)

14. Atomic number and mass number
All atoms consist of protons and electrons.
Most atoms also have neutrons.
Elements differ from each other in the number of protons their atoms contains.
To know the atomic number of an atom you look at the number of protons.
Example: Oxygen = 8 protons = 8 atomic numbers.

15. Atomic number and mass number

16. Mass number
The mass number is equal to the total number of particles of the nucleus (the total number of protons and neutrons).
Atomic mass – atomic number: number of neutrons.

17. Useful notes Atomic number = number of protons = number of electrons.
Mass number – atomic number = number of neutrons.

18. Example:
Neon: 10 protons, 10 neutrons, and 10 electrons. What is the mass number:_____
Activity page 86,
Questions 1-4 (pg.86)

19. Isotopes of an element have the same atomic number.
Isotopes: atoms of the same element that have different numbers of neutrons are called isotopes.
Example:
2 neutrons
1 neutron

20. Two standard ways to identify isotopes
1st: to write the mass number with a hyphen after the name of an element.
Example: helium-3, helium-4
2nd: shows the composition of a nucleus as the isotope’s nuclear symbol.
Example: 3/2 He

21. Did you know?
The image on a TV screen or a computer is produced when cathode rays strike the special coating on the inside of the screen.