1. Sections 1 and 2 Atomic History and Structure
Chapter 4
Sections 1 and 2
Atomic History and Structure

2. Do Now
Of the 11 key scientists discussed, which do you believe had the most important contribution? Why?
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3. The Atom Smallest particle of an element
All matter is created of atoms
Comes from the Greek word atomos
“indivisible”
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4. Democritus (460 – 370 B.C.E.) Greek philosopher
First to propose the idea of the atom’s existence
Theorized that the atom is indivisible and indestructible, but had no experimental evidence
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5. John Dalton (1766-1844) English chemist and teacher
Pioneered the modern discovery process about atoms
Transformed Democritus’s theories on atoms into an actual scientific theory
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6. Dalton’s Atomic Theory Published 1808
All elements are made up of atoms, which are small invisible particles
Atoms of the same element are identical
Atoms of different elements can chemically combine in whole-number ratios to form compounds
Chemical reactions occur when atoms are separated, joined together, or rearranged; but atoms cannot change into another element because of a reaction
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7. How did Dalton create his theory?
Studied the way elements combined in chemical reactions and the ratio in which they combined
Ex: Liquid mercury
All drops have the same properties
All drops are made of the same atoms
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8. Is Dalton’s theory accepted today?
Mostly!
However atoms are now known to be divisible
They are comprised of smaller subatomic particles known as protons, neutrons, and electrons
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9. Dmitri Mendeleev A Russian chemist and teacher
Arranged the then 60 known elements in order by increasing atomic mass
Created the first periodic table in 1869
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10. J. J. Thomson (1856-1940) Discovered the electron in 1897
Passed electric current through a tube filled with gas and observed a glowing beam known as a cathode ray
Concluded all atoms contained electrons
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11. Cathode Ray
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Electrons pass from the negatively charged cathode to the positively charged anode
Created a glowing beam of light
Proved that negative particles (electrons!) were being attracted to the positive anode

12. “Plum-Pudding Model” of the Atom
Thomson believed electrons were evenly distributed throughout the atom
The atom was also filled with positively charged material
This model was later disproved
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13. Robert A Milikan (1868-1953) U.S. Physicist
Calculated the charge of an electron in 1909
1.60 x Coulomb
Also calculated the mass of the electron
1/1840 the mass of a hydrogen atom
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14. Eugen Goldstein ( )
Discovered positively charged subatomic particles called protons in 1886
Used a variation of the cathode ray
Calculated the mass of the proton
1840 times the mass of the electron
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15. James Chadwick ( )
Confirmed the existence of neutrally charged particles, the neutron, in 1932
Calculated that neutrons had a mass equal to the mass of a proton
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16. How do we describe the structure of the atom?
Atoms are made of protons, neutrons, and electrons
How are they put together to make an atom?
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17. Ernest Rutherford (1871-1937) Former student of J. J. Thomson
Devised the gold-foil experiment in 1911
Created a new atomic model based on his findings
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18. Gold Foil Experiment
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Alpha particles (Helium atoms without two electrons) were shot at a sheet of gold foil
Some particles passed through the foil but a few were deflected
Why??

19. L. Bernard, 2015
The nuclei of the gold atoms in the foil deflects the small alpha particles; the empty space surrounding the nuclei lets other alpha particles pass through.

20. The Rutherford Atomic Model
The atom is mostly empty space
All the positive charge and almost all the mass of the atom is contained in the nucleus
The nucleus is made of the protons and neutrons
Electrons are distributed around the nucleus
Electrons occupy almost all of the atoms volume
Rutherford’s model is still being revised today!
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21. Niels Bohr (1885-1962) Danish student of Rutherford
In 1913 he proposed that an electron is found only in specific circular paths known as orbits around the nucleus
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22. The Bohr Model
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23. Louis de Broglie (1892 – 1987) French Physicist
In 1923, he proposed that all moving particles, including electrons, had properties of waves
Influenced Schrödinger's work
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24. Erwin Schrödinger (1887-1961) Austrian physicist
Devised and solved a mathematical equation to describe the true behavior of electrons in atoms in 1926
Known as the quantum mechanical model
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25. Modern Day Model of the Atom
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26. Scientist Pamphlet Project!
Choose one of the 11 key scientists you researched today to create a pamphlet on
Do further research and include lots of information on your chosen scientist such as
Where they were born/where they lived
Family life
How they got interested in science
Any other discoveries they may have made
Prizes, awards, honors, and recognitions they got before and after death
Any fun facts!
Make sure that your information and drawings cover the entire pamphlet!
L. Bernard, 2015