1. Chapter 2 Atomic Structure

2. Early Models of an Atom
Have you ever been asked to believe in something you could not see?
You cannot see the tiny fundamental particles that make up matter
Yet, all matter is composed of such particles called atoms
An atom is the smallest particle of an element that retains its identity in a chemical reaction

3. Early Models of an Atom
Concept of an atom intrigued a number of early scholars
Scholars were unable to see the atom but this did not stop them from propose ideas on the structure of atoms
Democritus was the first!

4. Democritus’s Atomic Philosophy
Democritus was among the first to suggest the existence of atoms
Democritus believed that atoms were indivisible and indestructible
Democritus’s ideas agreed with later scientific theory, but they did not explain chemical behavior
They also lacked experimental support b/c his approach was not based on the scientific method

5. Dalton’s Atomic Theory
The real nature of atoms and the connection between observable changes and events at the atomic level were not established for more than 2000 years after Democritus
The modern process of discovery regarding atoms began with John Dalton
Dalton transformed Democritus’s ideas on atoms into a scientific theory

6. Subatomic Particles
Most of Dalton’s atomic theory is still accepted today except it is now known that atoms are now divisible
Atoms can be broken down into even smaller, more fundamental particles, called subatomic particles
Three kinds of subatomic particles
Electrons
Protons
Neutrons

7. Electrons 1897 JJ Thomson discovered the electron
Thomson found that electrons are negatively charged subatomic particles
Electrons are really small in comparison to the other two subatomic particles (1/1840 of a proton or neutron)
Thomson created the Cathode Ray Tube Experiment
Symbol=e-

8. Thomson’s Cathode Ray Tube Experiment
Thomson passed electric current through gasses at low pressure
He sealed the gases in glass tubes fitted at both ends with metal disks called electrodes
The electrodes were connected to a source of energy and one electrode (anode) became positively charged and one electrode (cathode) became negatively charged
Result: A glowing beam, or cathode ray, that traveled from the cathode to the anode

9. Thomsons Model

10. Protons
Eugen Goldstein observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays
Called them canal rays and concluded that they were composed of positive particles
He went on to call these particles protons
Proton=positively charged subatomic particle
Protons have a mass 1840 times greater than the electron
Symbol= p+

11. Neutrons
James Chadwick confirmed the existence of the last subatomic particle: the neutron
Neutrons are subatomic particles with no charge
They have a mass that is equal to that of a proton

12. Rutherford’s Gold-Foil Experiment
Rutherford shot alpha particles (helium atoms that have lost their two electrons and have a double positive charge) into a piece of gold foil
If previous model was correct, alpha particles should have passed easily through the gold with only slight deflection
Result: Most particles passed straight through without deflection, small fraction bounced off the gold at very large angles, some bounced straight back

13. Rutherford’s Atomic Model
Rutherford proposed atom is mostly empty space
Concluded that all positive charge and almost all the mass are concentrated in small region in middle
Called the nucleus
Nucleus is the tiny central core of an atom and is composed of protons and neutrons
Also called the Nuclear Atom
Nuclear atom consists of protons and neutrons that are located in the nucleus with electrons distributed around the nucleus

14. Nuclear Atom