1. Chapter Two: Atoms, Molecules and Ions
Part I: Atomic structure

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.
Figure 2.1 John Dalton ( )
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3. Dalton's Postulates
Each element is composed of extremely small particles called atoms.
Figure 2.1 John Dalton ( )
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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
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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6. 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.
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7. 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.
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8. 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.
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9. Historical Aspect of Atomic Structure

10. The Electron
Streams of negatively charged particles were found to emanate from cathode tubes.
J. J. Thompson is credited with their discovery (1897).
Thompson measured the charge/mass ratio of the electron to be 1.76  108 coulombs/g.
Figure 2.4
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11. 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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12. Millikan Oil Drop Experiment
Robert Millikan (University of Chicago) determined the charge on the electron in 1909.
Figure 2.5
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13. The Atom, circa 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.
Figure 2.9
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14. Discovery of the Nucleus
Ernest Rutherford shot  particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.
Figure 2.10
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15. The Nuclear Atom
Since some particles were deflected at large angles, Thompson’s model could not be correct.
Figure 2.11
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16. 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.
Figure 2.12
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17. Other Subatomic Particles
Protons were discovered by Rutherford in 1919.
Neutrons were discovered by James Chadwick in 1932.
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18. 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.
Table 2.1
Atomic mass unit (amu) is a unit of mass equal to x kg
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19. Symbols of Elements Elements are symbolized by one or two letters.
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20. Atomic Number
All atoms of the same element have the same number of protons:
The atomic number (Z)
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21. Atomic Mass
The mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom. The mass number (A).
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22. 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
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23. Atomic Mass
Atomic and molecular masses can be measured with great accuracy with a mass spectrometer.
Figure 2.13
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24. Masses of Isotopes determined with a mass spectrometer

25. Isotope Abundance
The mass spectrometer gives information on the mass and relative abundance of each element’s isotopes.
Each isotope is represented by a Relative Abundance.

26. Isotope Abundance

27. Average Mass
Because in the real world we use large amounts of atoms and molecules, we use average masses in calculations.
Average mass is calculated from the isotopes of an element weighted by their relative (percent) abundances.
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28. Percent Abundance
The percent abundance of each element’s isotopes are found by:
Percent
Abundance =
number of atoms of an individual isotope
total number of atoms of all isotopes of that element
 100
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29. Atomic Weight (Mass)
Atomic
mass
The atomic masses on the periodic table are “weighted averages” of the all of an element’s individual isotope masses.

30. Isotope Problem Example: Chlorine has two isotopes: &
75.53 % u
Cl-35
24.47 % u
Cl- 37
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