1. Chemistry– 11/11/2018 Objectives
3.1 Describe the crucial contributions of scientists and the critical experiments that led to the development of the modern atomic model
3.2 Describe the early models of the atom and construct Bohr Models of the atom.
 Bell Ringer – Pretest
Name 3 scientist who contributed to our understanding of the atom
Homework: Read chapter 4 and complete study guide

2. Early Greek Theories This led to the idea of atoms in a void. fire air
Democritus
400 B.C. - Democritus thought matter could not be divided indefinitely.
This led to the idea of atoms in a void.
fire
air
water
earth
Aristotle
350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air.
Aristotle was wrong. However, his theory persisted for 2000 years.

3. Alchemists (about )
Alchemists – Scientist who used chemistry and magic. Made many discoveries without knowing why..
Attempted to :
change lead and other base metals to gold
discover a universal solvent
discover a life-prolonging elixir

4. Problems with combustion
Hundreds of years went by with scientists debating why things burn.
Stahl, Georg ( ) - Suggested that a substance he called “phlogiston” flowed out of burning material.
Priestley, Joseph ( ) - Discovered oxygen (“dephlogiston air”), carbon monoxide and nitrous oxide.
Optional Demonstration: Elephant Toothpaste

5. Lavoisier, A.L. ( )
Generally regarded as the father of chemistry. Discovered nitrogen; studied acids and described composition of many organic compounds.

6. Mendeléev, D.I. ( )
Discovered periodicity of the elements and compiled the first Periodic Table.

8. John Dalton All matter is made of atoms.
1800 -Dalton proposed a modern atomic model
based on experimentation not on pure reason.
All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine in constant ratios to form compounds.
Atoms are rearranged in reactions.
His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

9. Changes to Dalton’s Theory
Dalton said: Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties
Modern theory states: Atoms of an element have a characteristic average mass which is unique to that element.
Dalton said: Atoms cannot be subdivided, created, or destroyed Atoms cannot be subdivided, created, or destroyed.
Modern theory states: However, these changes CAN occur in nuclear reactions!

10. Early Theories of Matter
Experiments around the turn of the twentieth century provided evidence that the atom consisted of smaller particles.

11. Early Theories of Matter
Day 1 - History of the Atomic Theory
Early Theories of Matter
When J.J. Thomson applied a high voltage to two electrodes sealed inside an evacuated tube, an invisible beam, or ray, was found to emanate from the negative electrode (cathode) of the tube and travel toward the positive electrode (anode). The path of the beam could be made visible by the glow it caused when it passed across a plate coated with fluorescent material. The ray, called a cathode ray, could be deflected by electric and magnetic fields.
Thomson concluded that the ray consisted of a stream of negatively charged particles that had been dislodged from atoms. These particles became known as electrons.
How did he discover an invisible beam?

12. Thomson, Sir J.J. ( )
Research on cathode rays resulted in proof of existence of electrons (1896).

13. Thomson’s Plum Pudding Model of the Atom

14. Millikan, Robert ( )
In 1909 Robert Millikan accurately determined the mass of an electron to be 1/1840 the mass of a hydrogen atom. He also determined the electron’s charge to be 1-. This value was accepted as a fundamental unit of electrical charge.

15. Early Theories of Matter
Day 1 - History of the Atomic Theory
Early Theories of Matter
In 1911, Ernest Rutherford conducted experiments in which he aimed a beam of alpha particles (positively charged helium nuclei) at a thin sheet of gold. The fact that a few of these heavy particles were deflected almost directly backwards revealed that the positive charge of an atom, along with nearly all of the atom’s mass is concentrated in a very small region in the center of the atom. This positively charged mass is called the nucleus of the atom.
How did he discover an invisible beam?

16. Rutherford, Sir Ernest (1871-1937)
Proved the existence of the nucleus.
Explained the nuclear atom, containing a dense nucleus with electrons traveling around the nucleus at a large distance.

17. Rutherford’s Gold Foil Experiment
Alpha particles are helium nuclei
Particles were fired at a thin sheet of gold foil
Particle hits on the detecting screen (film) are recorded

18. Try it Yourself!
In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

19. The Answers
Target #1
Target #2

20. Rutherford’s Findings
Most of the particles passed right through
A few particles were deflected
VERY FEW were greatly deflected
“Like howitzer shells bouncing off of tissue paper!”
Conclusions:
The nucleus is small
The nucleus is dense
The nucleus is positively charged

21. Expected and Actual Results of Rutherford’s Experiment

22. Early Theories of Matter
Day 1 - History of the Atomic Theory
Early Theories of Matter
The particles that make up the positive charge of the nucleus are called protons, and each has a charge of 1+ (equal to but opposite the charge of an electron).
How did he discover an invisible beam?

23. Early Theories of Matter
Day 1 - History of the Atomic Theory
Early Theories of Matter
In 1932, Chadwick demonstrated the existence of the neutron, a nuclear particle having no charge but with nearly the same mass as a proton.
How did he discover an invisible beam?

24. Bohr, Niels ( )
Niels Bohr – Suggested that electrons in an atom move in set paths around the nucleus in energy levels much like the planets orbit the sun in our solar system.
Developed a quantum model for the hydrogen atom.

25. NEXT DAY LESSONS IF EXTRA TIME

26. Atomic Structure
Nucleus – the center of an atom; made up of protons and neutrons
Proton – A positively charged subatomic particle in the nucleus of an atom
Neutron – A neutral subatomic particle in the nucleus of an atom
Electron – a tiny negatively charged subatomic particle moving around outside the nucleus of an atom

27. Atomic Structure
Atomic number – the number of protons in a nucleus of an atom of that element
Atomic (average) mass number – the number of protons plus the number of neutrons

28. Properties of Subatomic Particles
Symbol
Electrical
Charge
Relative
Mass
Location
Electron e- 1-
1/1840
Orbiting the nucleus
Proton p+ 1+ 1
In nucleus
Neutron n0
Quiz on this information in 2 days