1. 4.1 & 4.2 Early Theories & Subatomic Particles

2. 4.1 Early Theories of Matter
Democritus
( B.C.)
Atom is a tiny indivisible particle of matter which can’t be further divided.
Word “atom” came from the Greek term atomos which means indivisible.

3. John Dalton (1766-1844) Atomic Theory Stated That:
1) Matter composed of small particles (atoms).
2) Atoms of a given element are identical.
(Ex. Size, mass, chemical properties)
3) Atoms can’t be divided into smaller particles.
4) Different atoms combine in simple whole-number ratios to form compounds.
5) Chemical reaction – atoms are rearranged.

4. Atom – smallest particle of an element that retains the properties of that element.
There are 2.9 x 1022 atoms of copper in a penny.
Dalton’s theory was important in
understanding matter but was
inaccurate.

5. 4.2 Subatomic Particles 1) Thomson (late 1890’s)
Worked with cathode ray tubes to find the charge : mass ratio for the particles.
Plum pudding atomic model
Believed that (-) charges distributed throughout a uniform positive charge.
Credited for discovering
the electron

6. 2) Robert Millikan (1909) Oil Drop Experiment
Determined the charge and calculated the mass of the electron.
Charge = -1
Mass =
9.11 x g
1/1840 mass of hydrogen

7. Nucleus Rutherford’s Gold Foil Experiment (1911)
Shot alpha particles through thin gold foil.
Particles much more massive than electrons.

8. (Gold Foil Experiment)
Rutherford expected alpha particles to be mostly deflected by the gold foil.
(Thomson’s Plum Pudding Model).
Found that most particles went through the foil.
Matter is mostly empty space.
A few particles deflected
back at large angles
(tiny, dense nucleus).

9. Rutherford Atom:
Atom’s diameter is 10,000 X that of the nucleus.
If atom had a nucleus the size of a nickel, the electrons would occupy 2 football fields!

10. Other Subatomic Particles:
Proton – Rutherford (1919)
+ 1 charge
In nucleus
Neutron – Rutherford & Chadwick (1932)
Neutral particle in nucleus
Mass nearly equal to that of a proton

11. Properties of subatomic particles: *Know this chart!
Symbol
Location
Charge
RelativeMass (amu)
Electron e-
Outside
Nucleus
- 1
* 0
(1/1840)
Proton p+
+ 1 1
Neutron n°

12. Study These Elements for Quiz:
1-20 25 26
28-30
35, 47,
50, 53
79, 80, 82

13. The Atom:
A cloud of fast moving electrons travel through the empty space surrounding the nucleus.
Electrostatic forces hold electrons (-) to the nucleus (+).
Number protons = number electrons
since atoms are neutral
Nucleus contains protons & neutrons.
(Except: H atom has 1 proton & 0 neutrons) 
Neutrons separate (+) charged protons.

14. The Changing Atomic Model

15. Quiz 1) What are the 3 subatomic particles?
2) What particles are in the nucleus?
3) What are the charges of each?
4) What is the mass of each?
5) What is the charge of the nucleus?
6) Where is all the mass located?
7) What keeps electrons in the atom?
8) What is the symbol for each particle?
9) What is the charge of an atom?

16. The End