1. Chapter 3
History of the Atom

2. Element Symbols Rule 1 First letter is capitalized
H Hydrogen C Carbon O Oxygen
Rule 2
Second letter is lower case
Cl Chlorine He Helium Ne Neon
Rule 3
Some are Latin names
Na Sodium (natrium) Fe Iron (ferrum)
Rule 4
Some elements are named for places, scientists, greek gods…
Eu Europium Am Americium

3. Period
Group

4. Foundations of Atomic Theory
Law of conservation of mass: Antoine Lavoisier
Mass is neither created nor destroyed.
HgO  Hg + O
433.2 g g g

5. Dalton’s Atomic Theory (1808)
All matter is made of indivisible and indestructible atoms.
All atoms of the same element are identical in their physical and chemical properties.
Atoms of different elements have different properties.
Atoms of different elements combine in simple whole-number ratios to form compounds.
Atoms cannot be subdivided, created, or destroyed when they are combined, separated, or rearranged in chemical reactions.

6. Changes to Dalton’s Theory
Are atoms actually indivisible?
Are all atoms identical?
No, atoms are made of protons, neutrons, and electrons
No, different electrons prodcues ions, different neutrons produces isotopes

7. The Atom
the smallest unit of an element that retains that elements properties
Made of protons, neutrons, and electrons
Can you see inside an atom?
How did they know it contains protons, neutrons and electrons?
Scientists needed to experiment in order to learn of atoms and that they were made of protons, neutrons and electrons

8. Obscertainer Activity

9. Ob-scertainer Solutions#1 #2 #3 #4 #5 #6 #7 #8 #9
#10
#11
#12

10. Cathode Ray Tube
When connected to electric current the remaining the gas forms a BEAM OF LIGHT.
The beam always started at the NEGATIVE electrode and flowed to the POSITIVE electrode.
The electrode is named by what type of particle it attracts
Cathode: Negative (-)
Anode: Positive (+)

11. Cathode Ray Tube Experiment

13. Cathode Ray Conclusions
1. The beam was attracted to a POSITIVE magnet.
2. The beam must be made of NEGATIVE particles.

14. JJ THOMSON DISCOVERED A NEGATIVE PARTICLE CALLED THE:
ELECTRON!

15. Plum Pudding Model

16. Plum Pudding OR Chocolate Chip Cookie

17. Radioactivity Becquerel Curie
discovered RADIATION
uranium would expose photographic plates in the dark
The properties of an element changed as it gave off radiation
Curie
Discovered radium and polonium
The radioactive emissions of alpha, beta and gamma rays were identified.

18. (Uranium compound)

19. Types of Radioactive Decay
Decay Type
Symbol
Charge
Reaction Particle
Strength
Can be stopped by…
Alpha
Beta
Gamma

20. Radioactive Decay

21. Ernest Rutherford’s Gold Foil Experiment
Set up Gold Foil with a detection sheet around it.
Set up radioactive source emitting alpha particles.
ALPHA PARTICLES shot at gold foil.

22. Rutherford’s Gold Foil Experiment

23. MOST particles went through the gold foil
Rutherford: It’s like shooting a cannon at a piece of tissue paper and having it bounce back at you!
MOST particles went through the gold foil
SOME particles BOUNCED back

24. Gold Foil Conclusions The atom is made up of mostly EMPTY SPACE
The center of the atom contains a POSITIVE CHARGE
Rutherford called this positive bundle of matter the NUCLEUS

25. Rutherford’s Model of the Atom NUCLEUS EMPTY SPACE – +
If the atom is the size of Giants Stadium
Then the nucleus is a marble on the 50 yard line

26. Other scientists later confirmed that the nucleus was made up of protons and neutrons

27. **DISCOVERED ENERGY LEVELS!!
Niels Bohr – 1913
Developed a new diagram of the atom
Electrons can only be at certain energies
Electrons must gain a specific amount of energy to move to a higher level, called a quantum
**DISCOVERED ENERGY LEVELS!!

28. Bohr’s Model of the Atom

29. SUBATOMIC PARTICLES

30. Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons X A Z
Mass Number
Element Symbol
Atomic Number
Isotopes: atoms of the same element (X) with different numbers of neutrons in their nuclei H 1
H (D) 2
H (T) 3 U
235 92
238

31. Isotopes

32. Do You Understand Isotopes?
How many protons, neutrons, and electrons are in C 14 6 ?
6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons, and electrons are in C 11 6 ?
6 protons, 5 (11 - 6) neutrons, 6 electrons

33. Hyphen Notation
The mass number is written with a hyphen after the name of the element.
Element – Mass#
Uranium – 235
Calcium – 40

34. cation – ion with a positive charge
An ion is an atom, or group of atoms, that has a net positive or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation. Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl-
17 protons
18 electrons Cl
17 protons
17 electrons

35. Do You Understand Ions? Al How many protons and electrons are in ?27 13 ? 3+
13 protons, 14 neutrons, 10 (13 – 3) electrons
How many protons and electrons are in Se 78 34 2- ?
34 protons, 44 neutrons, 36 (34 + 2) electrons

36. Average Atomic Mass

37. Relative Atomic Mass Atomic Mass Unit
Carbon is the standard for all masses on the periodic table.
Carbon: 6 p and 6 n = 12 amu
Atomic Mass Unit
Periodic table lists weighted average atomic masses of elements (like a GPA or final grade calculation)

38. Calculation AVERAGE Atomic Mass
Steps: 1. Percent to a decimal 2. Multiply by mass 3. ADD IT UP!
75% 133Cs
20% 132Cs
5 % 134Cs