1. Early Theories & Subatomic Particles (4.1-4.2)
The Atom
Early Theories & Subatomic Particles ( )

2. Objectives
Describe the evolution of the atom from Dalton’s time to Rutherford’s.
Define and describe the structure of an atom
Identify the subatomic particles and the charge, mass and location of each.

3. MATTER
Anything that has mass and takes up space

4. DEMOCRITUS (460-370 B.C.) First to propose:
Matter cannot be endlessly divided
An atom is a tiny particle of matter which cannot be further divided
Word “atom” came from the Greek term atomos which means “indivisible”
Many of his contemporaries criticized his theory

5. JOHN DALTON In his Atomic Theory (1808), Dalton hypothesized:
Each element made up of extremely small particles called atoms
Atoms of a given element are identical
They have the same size, mass, chemical properties
The atoms of one element are different from atoms of all other elements
Different atoms combine in simple whole-number ratios to form compounds (H2O, CaCO3, NaCl)
In chemical reactions it’s the atoms that are separated, combined or rearranged
Atoms cannot be created, destroyed, or divided into smaller particles (conservation of matter)

6. Dalton’s theory was important in understanding matter but was inaccurate on certain points:
Atom – is smallest particle of an element that retains the properties of that element, but
atoms can be divided into smaller parts!
Not all atoms of an element have identical properties--Their masses may vary slightly

7. diameter of a single copper atom
THE ATOM
An atom is the smallest particle of an element that retains the properties of the element
An atom is the basic unit of an element
world population
6,600,000,000
copper atoms in a penny
29,000,000,000,000,000,000,000
diameter of a single copper atom m

8. Rutherford’s Atom continued
Atom’s diameter is 10,000 times that of the nucleus
Atom:
Size of the entire atom: 2 football fields
Size of the nucleus: A nickel

9. SUBATOMIC PARTICLES
Dalton believed that atoms could not be divided into smaller particles
However later experiments led to the discovery that atoms were made up of even smaller particles, “subatomic particles”
electrons, protons and neutrons

10. THE ELECTRON
In 1897, through his cathode ray experiments, JJ Thomson was credited with identifying the electron
When he applied high voltage to an empty glass tube a ray he called a “cathode ray” was produced
he proposed that these rays are streams of particles much smaller than atoms-they are in fact pieces of atoms
We now know these particles to be electrons

12. THOMSON CONTINUED
Thomson imagined the atom as a “plum pudding” with negative charges distributed throughout a uniform positive charge

14. MILLIKAN’S OIL DROP EXPERIMENT (1909)
Determined the charge and calculated the mass of the electron
Charge
x C
Mass
9.11 x grams

15. THE PROTON AND THE NUCLEUS

16. DISCOVERING THE NUCLEUS: RUTHERFORD’S GOLD FOIL EXPERIMENT (1919)
Carried out his gold foil experiment to test Thomson’s “plum pudding” model of the atom
Directed alpha particles (+ charged) at a thin sheet of gold foil
He expected that if Thomson’s model was correct and the positive charge of an atom was so diffuse, the alpha particles would mostly pass through gold foil

17. RUTHERFORD’S GOLD FOIL EXPERIMENT (1919)

18. GOLD FOIL EXPERIMENT
With Thomson’s Plum Pudding Model in mind Rutherford expected the massive alpha (+ charged) particles to mostly pass through gold foil
Instead, he found that although most particles went through the foil a few particles deflected back at large angles

21. RUTHERFORD’S CONCLUSION
Most of the atom is empty space (that’s why most of the a-particles passed through foil)
The atom’s positive charge is concentrated in the nucleus
the nucleus is the atom’s dense central core
The positively charged particles in the nucleus are called protons
Nuclear model of the atom
Dalton’s Atom -
Thomson’s Atom +
Rutherford’s Atom

22. RUTHERFORD CONTINUED In other experiments found the proton had
the same quantity of charge as an electron (equal but opposite) and
a mass of x10-24 g, 1840 times the mass of an electron

23. THE NEUTRON Rutherford & Chadwick 1932 Neutral particle in nucleus
Mass nearly equal to that of a proton (slightly greater)

24. SUBATOMIC PARTICLES particle symbol absolute charge (coulombs)
absolute mass
(grams)
relative charge
relative mass
(U)
Electron
e or e- x
9.11 x 10-28 -1
Proton p x
1.673 x 10-24 +1
1.0073
Neutron n
1.675 x 10-24
1.0087

25. THE ATOM IN SUMMARY
The atom is a neutral particle composed of electrons, protons and neutrons:
Nucleus contains:
(+) charged protons & neutral neutrons (Exception: H atom has 1 proton & 0 neutrons)
Most (99.7%) of the atom’s mass
Most of the volume of an atom is made up of a cloud of fast moving (-) charged electrons that travel through the empty space surrounding the very small, very dense nucleus
Electrostatic forces hold electrons (-) to the nucleus (+)
Since atoms are neutral
Number protons = number electrons