1. Atomic Structure Chapter 4

2. Objectives
Recognize discoveries from Dalton (atomic theory), Thomson (the electron), Rutherford (the nucleus), and Bohr (planetary model of atom) and understand how these discoveries lead to the modern theory.
Describe Rutherford’s “gold foil” experiment that led to the discovery of the nuclear atom. Identify the major components (protons, neutrons, and electrons) of the nuclear atom and explain how they interact.

3. Objectives
Interpret and apply the laws of conservation of mass, constant composition (definite proportions), and multiple proportions.
Describe how changes in the nucleus of an atom during a nuclear reaction result in emission of radiation.

4. History of the Atom
Not the history of the atom itself, but the history of the idea of the atom.

5. Atom Definition
Atom
Smallest particle of an element that retains the chemical identity of that element

6. Subatomic particles Actual mass (g) Name Symbol Charge Electron e- -1
Relative
mass (amu)
Actual
mass (g)
Name
Symbol
Charge
Electron e- -1
1/1840
9.11 x 10-28
Proton p+ +1 1
1.67 x 10-24
Neutron n0 1
1.67 x 10-24

7. Information about Atom from Periodic Table

8. Atomic Number
Avg Atomic Mass

9. Atomic Number and Atomic Mass
Chemical Symbol: abbreviation for element name
Atomic Number (Z): number of protons in nucleus of atom (and electrons if neutral)
Mass Number: number of protons and number of neutrons in nucleus (whole number)

10. Isotopes
Isotopes: atoms with the same number of protons but different number of neutrons
Hyphen Notation:
oxygen-16 and oxygen-17
Nuclear Symbol:
168O 178O

11. Average Atomic Mass
Average Atomic Mass: weighted average mass of atoms found in nature (decimal number on periodic table)
Can calculate average atomic mass of elements if know percent abundance in nature
(WS Isotopes and Average Atomic Mass)

12. Models of the Atom

13. Dalton Model of Atom Small, indivisible spheres

14. J.J. Thompson’s Model of Atom
Plum Pudding Model, 1896
Thought an atom was like plum pudding
Dough was positively charged
Raisins scattered throughout the dough were negatively charged
Didn’t know about neutrons at this time

15. Rutherford’s Model of the Atom
Rutherford Model, 1911
Thought atom was mostly empty space
Nucleus in center is dense, positively charge
Electrons (negatively charged) are in empty space surrounding nucleus

16. Bohr’s Model of the Atom
Neils Bohr, 1913
Similar to Rutherford’s model
Thought atom was mostly empty space
Nucleus in center is dense, positively charge
Electrons move in orbits around the nucleus

17. (Modern) Quantum Mechanical Model of the Atom
Heisenberg, Schrodinger, many others, ~1926
Think atom is mostly empty space
Nucleus in center is dense, positively charge
Electrons are around the nucleus
Cannot locate location of electron at specific time

18. Ch. 25 Nuclear
Ch. 22 Nuclear General Chemistry

19. Radioactivity

20. Objectives
Describe how changes in the nucleus of an atom during a nuclear reaction results in the emission of radiation
Describe alpha, beta, and gamma particles; discuss the properties of alpha, beta, and gamma radiation; and write balanced nuclear reactions.
Compare nuclear fission and nuclear fusion.

21. Objectives
Explain the difference between stable and unstable isotopes.
Explain the concept of half-life of a radioactive element, e.g., explain why the half-life of C-14 has made carbon dating a powerful tool in determining the age of very old objects.

22. Radioactivity

23. Discovery of Radiation
Henri Becquerel (1896) experiment with uranium found it was emitting particles
Marie Curie (1898) discovered radioactive element Polonium and Radium
Ch. 22 Nuclear General Chemistry

24. Strong Nuclear Force Opposites attract, like charges repel
So why do protons stay together in nucleus?
Strong Nuclear Force holds nucleus together and is stronger than electrostatic repulsion between protons
Only works over small diameter
Neutrons help keep protons separated slightly to reduce repulsion between protons
Ch. 22 Nuclear General Chemistry

25. Mass Defect
You’d expect the mass of an atom to be the sum of the individual subatomic particles
42He 2 ( amu) =
2 ( amu) =
2 ( amu) =
Total = amu
Actual mass helium atom = amu
The difference between the calculated mass and the actual mass is called mass defect.
Ch. 22 Nuclear General Chemistry

26. Binding Energy
In Einstein’s equation: E=mc2 the “lost” mass can be converted into energy
Binding energy: energy released when a nucleus is formed from protons and neutrons
Could be considered as the amount of energy to break apart the nucleus
Associated with the strong nuclear force holding particles together
Ch. 22 Nuclear General Chemistry

27. Binding Energy per Nucleon
Ch. 22 Nuclear General Chemistry

28. Radiation Stable nuclei have large binding energies
High energy means it is hard for nucleus to break apart
Unstable nuclei can break apart and give off particles
Radiation: emission of energy as electromagnetic waves or subatomic particles
Ch. 22 Nuclear General Chemistry

29. Common Types of Radiation
Alpha (a) 42 He)
Helium nucleus
Weak strength : can stop with paper
Beta (b) electron 0-1 e)
Electron
Medium strength: stop with clothing
Gamma (g)
High energy
High energy: stop with lead
mass #  4, Atomic #  2
Mass # stays same, atomic #  1
EM wave so mass doesn’t change
Ch. 22 Nuclear General Chemistry

30. Other Types of Radiation
Positron (0+1 e)
Neutron (n) 10 n)
mass # stays the same,
Atomic #  1
Mass #  1, atomic # stays the same
Ch. 22 Nuclear General Chemistry

31. Nuclear Equations 23892 U  23490 Th + _________
146 C  N + _________
94 Be + _________  126 C n
Answers: alpha, beta, alpha
Ch. 22 Nuclear General Chemistry

32. Study Buddy Review What force holds the nucleus together?
What is binding energy?
What happens when a nucleus is unstable
What is an alpha particle? Beta particle? Gamma radiation?
Ch. 22 Nuclear General Chemistry

33. Nuclear Decay and Half Life
Ch. 22 Nuclear General Chemistry

34. Decay
Radioactive decay: spontaneous emission of radiation from nucleus of atom
Transmutation: change in the identity of an element due to the emission of particles from the nucleus
Ch. 22 Nuclear General Chemistry

35. Half-Life
Half-life: time required for half of a sample of an element to decay into another element
Known as rate of radioactive decay
Different for each isotope
A = Ao(½)n
Ch. 22 Nuclear General Chemistry

36. Half Life of Some Radioactive Isotopes
Ch. 22 Nuclear General Chemistry

37. Half life of Potassium-40
Ch. 22 Nuclear General Chemistry

38. Half-Life Problem
The half life of polonium-210 is days. How many milligrams of polonium-210 remain after days if you start with 2.0 mg of the isotope?
Answer: mg
Ch. 22 Nuclear General Chemistry

39. Nuclear Fission and Fusion
Ch. 22 Nuclear General Chemistry

40. Fusion
Energy of our sun and other stars is produced from nuclear fusion reactions
Fusion: light massed nuclei combine to form a heavier, more stable nucleus
Produces a lot of energy, also nuclear waste
4 11 H  42 He b + ENERGY
Ch. 22 Nuclear General Chemistry

41. Fission Nuclear power plants create energy from fission reactions
nuclear fission: a heavy nucleus splits into a more stable nuclei of intermediate mass
energy produced
nuclear power plants
Nuclear waste produced
23592 U + 10 n  9336 Kr Ba n + ENERGY
Ch. 22 Nuclear General Chemistry

42. Study Buddy Review What is half-life? What is radioactive decay?
Compare and contrast fusion and fission.
Ch. 22 Nuclear General Chemistry

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