1. History of the Atom

2. Development of the Atomic Model
440 B.C. (Greek Philosopher) Democritus matter is formed of small pieces that could not be cut into smaller parts
Atomos- “uncuttable”
Remember ancient Greeks
did not perform experiments

3. Dalton’s Atomic Theory - 1803
All elements are composed of atoms that cannot be divided
All atoms of the same element are exactly alike and have the same mass. Atoms of different elements are different and have different masses

4. Dalton’s Atomic Theory cont.
An atom of one element cannot be changed into an atom of a different element.
Atoms cannot be created or destroyed in any chemical change, only rearranged
Every compound is composed of atoms of different elements, combined in a specific ratio

5. J.J Thomson’s Model
Used a Cathode Ray Tube that passes an electrical current through a vacuum tube
Found that the beam would always bend toward the positive magnet
Atom consisted of negative charges scattered throughout a ball of positive charges
Become known as electrons
Plum Pudding Model

6. Cathode Ray Tube

7. Plum Pudding Model

8. Rutherford’s Model
Ernest Rutherford was a student of Thomson who found evidence that countered Thomson’s Model
Aimed positively charged particles at a thin sheet of gold

10. Rutherford’s Model cont.
Atoms in the gold foil were mostly empty space allowing the particles to pass through but there is a small dense positively charged center of the atom causing the deflections
Proposed the Nuclear Model

11. Thompson’s Plum Pudding Model Rutherford’s Nuclear Model

12. Bohr’s Model
A student of both Thomson and Rutherford revised the atomic model again
Proposed that electrons can only travel around the nucleus in certain definite orbits - like planets around the sun
Planetary Model

13. A Cloud of Electrons - 1920’s Schrodinger and Heisenberg
Electrons do not orbit, they can be anywhere in a cloudlike region around the nucleus
Electron’s movement is related to its energy level - region at a fixed distance from the nucleus
These levels affect the way the atom reacts with other atoms
Paths electrons travel cannot be determined

14. Atoms What all elements are composed of
All atoms of the same element has the same mass
Compounds contain atoms of more than 1 element
Always NEUTRAL
Neutral - neither positive nor negative

15. Atom Comprised of three subatomic particles: Identified by a Symbol
Proton
Neutron
Electron
Identified by a Symbol
Measured in amu -
Atomic Mass Unit

16. Proton Has a POSITIVE charge +1 Located inside the nucleus
Has a mass of 1 amu

17. Neutron Has NO charge Located inside the nucleus Has a mass of 1 amu
Nucleus - dense, positively charged mass located in the center of an atom

18. Electron Has a NEGATIVE charge -1
Located outside the nucleus in the electron cloud
Has a mass of 1/1800 amu

19. Let’s look at an Element
Atomic Mass
Element Symbol
Element Name
Atomic Number

20. Atomic Number Represented by the letter Z
Equals to the number of protons
Since positive = negative; protons = electrons
Therefore Z = protons = electrons
Determines the identity of an element

21. Let’s Practice How many protons? How many electrons? Mg Li F Ne Cl ZnAg

22. More Practice What element has 11 protons?
Which element has 27 electrons?
Which element has 13 electrons?
11 = Sodium
27 = Cobalt
13 = Aluminum

23. Atomic Mass
Atomic Mass is equal to the number of protons + number of neutrons
What do you think we don’t include electrons in this number????
Electrons mass is very small, almost insignificant. <0.1% of mass

24. Isotopes
When two atoms of the same element have the same number of protons but DIFFERENT number of neutrons
Different atomic mass number
Ions have charges - +1, -1

25. Pt Symbol of an Isotope This could vary for different isotopes
The mass number
Platinum is normally 78 protons and electrons and 117 neutrons. Pt
197
The symbol of the element 78
The atomic number (Z) 78
119
This isotope has _____ protons
and ______ neutrons

26. H Hydrogen 1 1 Hydrogen (Deuterium) Hydrogen (Tritium) Hydrogen
Most common isotope
All the isotopes H 1 1
Hydrogen
(Deuterium)
Hydrogen
(Tritium)
Hydrogen
(Protium)

27. Isotopes
Deuterium
Tritium H 2 1 H 3 1

28. Review Z (atomic number) = p+ = e- Atomic Mass = p+ + n0
Isotope - different number of neutrons