1. Section 3.3 Atomic Structure
To learn about the internal parts of an atom
To understand Rutherford’s experiment
To describe some important features of subatomic particles
To learn about the terms isotope, atomic number, and mass number
To understand the use of the symbol to describe a given atom

2. The Structure of the Atom
Dalton’s atomic theory answered many questions
It did not answer them all
What causes atoms to stick together to form compounds?
What is an atom like?

3. The Structure of an Atom
J.J. Thomson, British physicist, 1890s
First to recognize that atoms contained negatively charged particles called electrons
William Thomson (Lord Kelvin)
Plum pudding model – an atom is like plum pudding.
The atom (pudding) is positively charged. The electrons (raisins) are spread randomly throughout.
The positive and negative balance each other out so that the atom is neutral

4. Rutherford’s Experiment
New Zealand, physicist, Ernest Rutherford
Proved that plum pudding model was incorrect
He studied alpha particles
Particles containing 2 protons and 2 neutrons
Alpha particles can be emitted by radioactive material and travel through the air
Rutherford’s experiment – he bombarded a piece of gold foil with alpha particles.
The gold foil was surrounded by a detector that produced tiny flashes whenever an alpha particle collided with it

5. Rutherford’s Experiment
He was surprised by his results
He expected all of the particles to pass directly through
Some were deflected
Rutherford concluded there must be dense positively charged nucleus in the center of the gold atoms that deflected the α particles.

6. Rutherford’s Experiment
Rutherford theorized that the nucleus is made of two particles
Proton -- positive particle with an equal but opposite charge of an electron. Significantly more massive than an electron.
Neutron -- a particle with no charge but similar in mass to a proton

7. Modern Concept of Atomic Structure
Question: If all atoms are composed of the same components, why do different atoms have different chemical properties?
Number and arrangement
of electrons

8. Isotopes James Chadwick – discovered that atoms contain neutrons
Dalton’s theory had to be modified:
“All atoms of the same element contain the same number of protons and electrons, but atoms of a given element may have different numbers of neutrons”

9. Na Na Isotopes 23 11 24 11 number of protons number of neutrons
number of electrons
Atomic number
Mass number 11 Na 23 11 12 11 11 23
Sodium-23
number of protons
number of neutrons
number of electrons
Atomic number
Mass number 11 13 Na 24 11 11 11 24
Sodium-24

10. Practice
Use your periodic table. Give the number of protons, neutrons and electrons for the following:
Nitrogen-15
Lithium-9
Protons 7
Electrons 7
Neutrons 8
Protons 3
Electrons 3
Neutrons 6
Barium-136
Carbon-14
Protons 6
Electrons 6
Neutrons 8
Protons 56
Electrons 56
Neutrons 80