1. Internal Structure of Atoms
Chapter 3-Section 2

2. Subatomic Particles Electrons, protons, and neutrons
Electricity – J.J.Thomson – Electrons
Metal plates – electrodes
Cathode -
Anode +

3. Cathode Ray Tube

4. Electrons
The wheel in the cathode ray tube-mass
Originate at negatively charged side – negatively charged electrons
Magnet

5. Mass
Further experiments found the mass of an electron to be x 10-4 amu
2000 times lighter than an atom
The charge was found to be x C
C = Coulombs – SI unit

6. An atom is not the simplest form of matter
Charges
An atom is not the simplest form of matter
Since electrons (e-) are negative, how is an atom neutral?

7. The Nucleus
Thompson’s plum pudding model
This is no longer a valid model

8. Gold foil experiment Alpha particles
Rutherford’s Nucleus
Gold foil experiment
Alpha particles
Some passed through and some were repelled away from the foil

9. Rutherford’s Nucleus
2 years later…
Must be a concentrated tiny space with a mass larger than the α particle
The rest of the gold foil would be empty space.

10. The modern nucleus
The nucleus is a dense central portion of the atom that is positively charged.
The nucleus has most of the mass of the atom, but is very small compared to the entire atom.
Marble and football

11. Protons and Neutrons
Protons = +
Charge = x C
Mass = amu (2000 times the mass of an electron)

12. Protons and Neutrons
A neutral atom has equal numbers of protons and electrons
Masses of all atoms (except H) were greater than the mass of e- and p+ combined so where does this mass come from?

13. Neutrons Joliot-Curie and Chadwick experiment with beryllium
Alpha particles
Beam of power not deflected by electric or magnetic fields
These particles must be neutral
Mass of amu

14. Coulomb’s Law
States that the force between two charged particles is inversely proportional to the square of the distance between them.
The closer two charges come together, the greater the force between them.
If the signs are the same, they repel each other and if they are different, they attract each other.

15. Coulomb’s Law Force = 0.58 N Distance = 0.020 pm Force = 2.32 N

16. Strong Force
The force that holds the protons and neutrons in a nucleus.
It is greater than the repulsive forces at close distances.
Must be neutrons in nucleus if there is more than one proton.

17. Atomic Number
Atomic number – the number of protons in the nucleus of an atom
# of protons =#of electrons
Always whole numbers
Same for all atoms of the same element.

18. Mass Number
The total number of protons and neutrons in the nucleus of an atom.
# of Neutrons=Mass# - Atomic#
Can vary for atoms of the same element.

19. Boron The atomic number of boron is 5. The mass number of boron is 11.
The number of protons and electrons are equal to the atomic number.

20. Boron
Atomic Number
Mass Number
Element Symbol

21. Boron
Electrons = 5
Protons = 5
Neutrons = 11 – 5 = 6

22. Copper
How many protons, neutrons, and electrons are present in copper which has an atomic number of 29 and a mass number of 64?

23. Copper
Electrons = 29
Protons = 29
Neutrons = 35

24. 94Be Atomic and mass numbers can be written beside the elements symbol
Atomic Structure
Atomic and mass numbers can be written beside the elements symbol
94Be
Mass Number
Atomic Number

25. Concept Check
What three subatomic particles are important to chemistry and what are their electrical charges?

26. Isotopes
Isotope – An atom that has the same # of protons as other atoms of the same element but has a diff. # of neutrons
Many elements exist as mixtures of isotopes.

27. Lead Isotopes Name # of Neutrons Mass (amu) Amt (%) Lead - 204 122
1.4
-206
124
24.1
-207
125
22.1
-208
126
52.4

28. Radioisotopes
Some elements have only one stable isotope and will degrade and are called radioisotopes.
Unstable nuclear configurations.
Unstable atoms that undergo radioactive decay.

29. Unstable in Nature
Some elements have no stable isotopes.
Technetium has no stable nucleus.
Was the first element to be made artificially in a laboratory.

30. Complete numbers 1-6 in the section review on page 89.