Internal Structure of Atoms

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 5.485799 x 10-4amu • 2000 times lighter than an atom • The charge was found to be -1.602189 x 10-19 C • C = Coulombs – SI unit

6. 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. 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 = 1.602189 x 10-19 C • Mass = 1.0073 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 1.0087 amu

14. Coulomb’s Law • States that the force between two charged particles is inversely proportional to the square of the distance between them.

15. Coulomb’s Law Force = 0.58 N Distance = 0.020 pm Force = 2.32 N Distance = 0.010 pm

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

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. Section Review • Complete numbers 1-6 in the section review on page 89.