1. Atomic Structure
Image courtesy of

2. Chemistry Timeline #1  2000 years of Alchemy B.C.
400 B.C. Demokritos and Leucippos use the term "atomos”
 years of Alchemy
1500's
Georg Bauer: systematic metallurgy
Paracelsus: medicinal application of minerals
1600's
Robert Boyle:The Skeptical Chemist. Quantitative experimentation, identification of
elements
1700s'
Georg Stahl: Phlogiston Theory
Joseph Priestly: Discovery of oxygen
Antoine Lavoisier: The role of oxygen in combustion, law of conservation of
mass, first modern chemistry textbook

3. Chemistry Timeline #2
1800's
Joseph Proust: The law of definite proportion (composition)
John Dalton: The Atomic Theory, The law of multiple proportions
Joseph Gay-Lussac: Combining volumes of gases, existence of diatomic molecules
Amadeo Avogadro: Molar volumes of gases
Jons Jakob Berzelius: Relative atomic masses, modern symbols for the elements
Dmitri Mendeleyev: The periodic table
J.J. Thomson: discovery of the electron
Henri Becquerel: Discovery of radioactivity
1900's
Robert Millikan: Charge and mass of the electron
Ernest Rutherford: Existence of the nucleus, and its relative size
Meitner & Fermi: Sustained nuclear fission
Ernest Lawrence: The cyclotron and trans-uranium elements

4. Dalton’s Atomic Theory (1808)
All matter is composed of extremely small particles called atoms
Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties
John Dalton
Atoms cannot be subdivided, created, or destroyed
Atoms of different elements combine in simple whole-number ratios to form chemical compounds
In chemical reactions, atoms are combined, separated, or rearranged

5. Modern Atomic Theory Dalton said:
Several changes have been made 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.

6. Modern Atomic Theory #2 Dalton said:
Atoms cannot be subdivided, created, or destroyed
Modern theory states:
Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions

7. Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

8. Thomson’s Atomic Model
J.J. Thomson
Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

9. Mass of the Electron
1909 – Robert Millikan determines the mass of the electron.
Mass of the electron is
9.109 x kg
The oil drop apparatus

10. Conclusions from the Study of the Electron
Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.
Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons
Electrons have so little mass that atoms must contain other particles that account for most of the mass

11. 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

12. 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?

13. The Answers
Target #1
Target #2

14. 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

15. Atomic Particles Particle Charge Mass (kg) Location Electron -1
9.109 x 10-31
Electron cloud
Proton +1
1.673 x 10-27
Nucleus
Neutron
1.675 x 10-27

16. The Atomic Scale Helium-4
Most of the mass of the atom is in the nucleus (protons and neutrons)
Electrons are found outside of the nucleus (the electron cloud)
Most of the volume of the atom is empty space
Image: User Yzmo Wikimedia Commons.

17. About Quarks… Protons and neutrons are NOT fundamental particles.
Protons are made of two “up” quarks and one “down” quark.
Neutrons are made of one “up” quark and two “down” quarks.
Quarks are held together by “gluons”
Images: Arpad Horvath, Wikimedia Commons.

18. Isotopes
Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.
Isotope
Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium) 1
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium) 2

19. Composition of the nucleus
Atomic Masses
Atomic mass is the average of all the naturally isotopes of that element.
Isotope
Symbol
Composition of the nucleus
% in nature
Carbon-12
12C
6 protons
6 neutrons
98.89%
Carbon-13
13C
7 neutrons
1.11%
Carbon-14
14C
8 neutrons
<0.01%
Carbon =

20. Atomic Number
Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.
Element
# of protons
Atomic # (Z)
Carbon 6
Phosphorus 15
Gold 79

21. Mass Number
Mass number is the number of protons and neutrons in the nucleus of an isotope.
Mass # = p+ + n0
Nuclide p+ n0 e-
Mass #
Oxygen - 10 - 33 42
- 31 15 18 8 8 18
Arsenic 75 33 75
Phosphorus 16 15 31