1. Chapter 3

2. Atomic Theory of Matter
The theory that atoms are the fundamental building blocks of matter reemerged in the early 19th century, championed by John Dalton.
Atom comes from the Greek word atomos meaning “indivisible or uncuttable
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3. Dalton's Postulates
Each element is composed of extremely small particles called atoms.
As chemists learned to measured the amount o element that react with one another to form new substances, the ground was laid for an atomic theory
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4. Dalton's Postulates
All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.
Figure 2.1 John Dalton ( )
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5. Dalton's Postulates Figure 2.1 John Dalton (1766-1844)
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6. Dalton's Postulates
Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.
Figure 2.1 John Dalton ( )
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7. Dalton’s Postulates
Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms (fixed proportions).
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8. Law of Constant Composition Joseph Proust (1754–1826)
This is also known as the law of definite proportions.
It states that the elemental composition of a pure substance never varies.  +
Postulates explained several simple law or chemical combination that were known during his time
Hydrogen atom
Oxygen atom
Water Molecule
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9. Law of Conservation of Mass
The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place.
In postulate 3 – atoms retain their identity in a chemical reaction
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10. The law of Multiple O H H O H H H H O O Water Oxygen molecule
Oxygen molecule
Hydrogen molecule
Hydrogen Peroxide H H O O
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11. The Electron
When high voltage was applied to the electrodes in the tube radiation (cathode rays) was produced
Their movement was seen because they rays cause glass to give off light
When a magnetic field was applied the cathode rays are deflected
Subatomic particles which the atom is made of… proton, neutron, electron
Some atoms are positively charged, some are negatively charged
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12. The Electron
Thompson added fluorescent screen so he could quantitatively measure the effects of electrical or magnetic field on the thin stream of electrons passing through
Thompson measured the charge/mass ratio of the electron to be 1.76  108 coulombs/g.
Now measuring either the charge or mass of an electron would yield the other quantity
Coulombs = SI unit for electrical charge
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13. Millikan Oil Drop Experiment
Once the charge/mass ratio of the electron was known, determination of either the charge or the mass of an electron would yield the other.
Figure 2.5
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14. Millikan Oil Drop Experiment
Robert Millikan (University of Chicago) determined the charge on the electron in 1909.
Figure 2.5
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15. Millikan Oil Drop Experiment
Experimentally the charge of electrons was 1.602x10-19 C
The charge to mass ration 1.76x108 C/g (from cathode- ray tube)
Calculate the mass of an electron x10-28 g
Figure 2.5
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16. The Atom, 1900
The prevailing theory was that of the “plum pudding” model, put forward by Thompson.
It featured a positive sphere of matter with negative electrons imbedded in it.
Thompson predicted since electrons contributed a very small amount to the atoms mass, it probably also took a very small space
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17. Discovery of the Nucleus
Ernest Rutherford shot  particles (positive) at a thin sheet of gold foil and observed the pattern of scatter of the particles.
Figure 2.10
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18. Discovery of the Nucleus
Why did the α particles behaved the way they did?
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19. The Nuclear Atom Since some particles were deflected at large angles
Thompson’s model could not be correct.
Gave rise o the idea that most of the mass of an atom and all of its positive charge reside in a very small area which Rutherford called the nucleus.
Alpha particles are positive repel positive nucleus
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20. The Nuclear Atom
Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom.
Most of the volume of the atom is empty space, in which electrons move around the nucleus
Figure 2.12
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21. Other Subatomic Particles
Protons were discovered by Rutherford in 1919.
Neutrons were discovered by James Chadwick in 1932.
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22. Subatomic Particles
Protons and electrons are the only particles that have a charge.
Protons and neutrons have essentially the same mass.
The mass of an electron is so small we ignore it.
The list of particles in the nuclei has increased but we can just look at P N E because they are the ones that have an effect on chemical behaivior
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23. Subatomic Particles Protons have a charge of 1.602x10-19 C
Electrons have a charge of x10-19 C
Neutrons have no charge ~ they are neutral
The list of particles in the nuclei has increased but we can just look at P N E because they are the ones that have an effect on chemical behaivior
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24. Subatomic Particles
Protons and neutrons reside together in the nucleus
The vast majority of an atom’s volume is the space in which the electrons reside
The electrons are attracted to the protons in the nucleus by the electrostatic force between + and – charge
Later the strengths of attractive forces between electrons and nuclei different from element to element
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25. Subatomic Particles The mass of the heaviest known atom is 4x10-22
Atomic Mass Unit (amu)
1amu = x10-24g
1g = x1023amu
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26. Symbols of Elements Elements are symbolized by one or two letters.
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27. Atomic Number
All atoms of the same element have the same number of protons:
The atomic number (Z)
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28. Atomic Mass
The mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom.
Carbon - 12
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29. Isotopes
Isotopes are atoms of the same element with different masses.
Isotopes have different numbers of neutrons. 11 6 C 12 6 C 13 6 C 14 6 C 6 8
Protons
Electrons
Neutrons 6 5 6 6 7
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30. Isotopes How many protons, neutrons, and electros are in
138Ba atom
an atom of phosphorous-31
56 protons, 56 electrons, 82 neutrons (138-56)
15 protons, 15 electrons, 16 neutrons (31-15)
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31. Isotopes Magnesium has three isotopes, with mass numbers 24, 25, 26
Write the complete chemical symbol (super- and subscript) for each of them
How many neutrons in each isotope atom
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