Download Ch3 notes - Midway ISD

Chapter 3
Atoms: The Building
Blocks of Matter
Sect. 3-1: The Atom: From
Philosophical Idea to
Scientific Theory
 Democritus vs. Aristotle
 Atom vs. infinitely divisible
 Aristotle’s idea won out for ~ 2000 yr.
 Problem with both ideas…
Foundations of Atomic Theory
 Law of conservation of mass
 Mass not created or destroyed
 Law of definite proportions
 Same compound always same ratio
of elements
 Water is always 2 H and 1 O
 Law of multiple proportions
 Compounds of same elements
combine in different ratios
 Carbon dioxide (1 C : 2 O) and carbon
monoxide (1 C : 1 O)
Dalton’s Atomic Theory
1) Atoms
2) Atoms of an element are the same (mass,
size, etc), atoms of different atoms are
different
3) Atoms can’t be divided
4) Atoms combine in ratios to form compounds
5) Atoms combine, separate, or rearrange
during a chemical reaction
 What’s the difference between
Democritus and Dalton???
 Experimental Data!!!
Modern Atomic Theory
 What’s no longer held as true from
Dalton’s theory?
 Atoms being divisible
 Atoms of same element having same
mass
Sect. 3-2: The Structure of
the Atom
 Atoms made up of:
 Protons
 Neutrons
 Electrons
Discovery of the Electron
 Cathode Ray Tube experiments
concluded that the “cathode rays” had
mass and a negative charge
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 J.J. Thomson measured ratio of charge
to mass for these particles
 concluded particles were the same for all
elements
 Cathode-ray particles later named
electrons
 Millikan determined exact mass of
electron (about 1/1837 mass of hydrogen
atom)
 Confirms negative charge of electron
 Inferences made:
 must be something positive to balance
negative
 Atoms must have more particles due to tiny
mass of electron and much larger mass of
atom
Discovery of Atomic Nucleus
 Rutherford’s gold foil experiment
http://www.dlt.ncssm.edu/TIGER/diagrams/structure/GoldFoilExperiment.jpg
 Significance of Rutherford’s experiment:
 Nucleus of the atom
 Very dense
 Positive charge
 Nucleus = marble, whole atom = football
field
Composition of the Atomic Nucleus
 Protons – positive charge
 Very massive compared to electrons
 Determine what type of element the atom is
 Neutrons – neutral (no charge)
 Slightly more massive than protons
 Nuclear forces – short range forces that
hold protons and neutrons together in the
nucleus
Sect. 3-3: Counting Atoms
 Atomic number = # protons
 Isotopes
 Same element, different masses
 same # protons, different # of neutrons
 Mass # = protons + neutrons
 Hyphen notation
 Hydrogen - 3
 Nuclear symbol

3
1H
 # neutrons = mass # - atomic #
 How many of each of the following are in
the carbon – 14 isotope?
 Protons
 Neutrons
 Electrons
 6, 8, and 6 respectively
Relative Atomic Masses
 1 atomic mass unit (amu) = 1/12 the
mass of a carbon-12 atom
 Isotopes differ in mass, but not
significantly different in chemical
behavior
Average Atomic Masses
of Elements
 Weighted average of atomic masses of
all naturally occurring isotopes of an
element
 (%of isotope) (mass isotope) +
(%of isotope) (mass isotope) =
weighted average
Relating Mass to Numbers
of Atoms
 Mole – SI unit for amount of substance
 Based on Carbon-12
 Abbreviated mol
 Counting unit (like dozen)
 Avogadro’s Number - # of particles in one
mol of a substance
 6.022 x 1023 particles = 1 mol
 Ex: How many moles of silver are in
3.01 x 1023 atoms of silver?
 0.500 mol
 Molar Mass – mass of one mole of a pure
substance
 Units are typically reported in g/mol
 What is the molar mass of Cu?
 63.55 g/mol
 Gram to mole conversion
 Use molar mass as conversion factor
 What is the mass in grams of 3.50 mol of
Cu?
 222g