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Atoms: The Building Blocks of Matter
Ch. 3
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Dimensional Analysis *A method of problem-solving using conversion factors Conversion Factor *Is essentially an equality ex/ 12 inches = 1 foot *Is written as a ratio to cancel units 12 inches OR 1 foot 1 foot 12 inches
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Dimensional Analysis Problems
1. Convert 15 feet to inches. 2. Convert 176 feet to yards.
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1. Convert 2,500 inches to yards. 2. Convert 0.0034 miles to inches.
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25 days to seconds 1,000,000 minutes to years
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Early atomic theory Democritus (400 B.C.) Aristotle
Atoms are nature’s basic particles “atom” Greek word meaning “indivisible” Aristotle Disagreed, thought matter was continuous Aristotle's theory prevailed for next 2000 yrs.
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1700s 1st experimentation Definition of element (pure sub. made of 1 kind of atom) Elements combine compounds
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Late 1700s Quantitative studies of reactions
law of conservation of mass
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Dalton’s Atomic Theory
All matter composed of atoms Atoms… of same element are identical; atoms of diff. elements are diff. cannot be divided, created, destroyed combine in simple whole # ratios to form compounds (Law of Definite Proportions) are combined, separated or rearranged in chem. rxns.
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Modern Atomic Theory Not all of Dalton’s ideas were correct
Atoms can be divisible Atoms of same element can have diff. masses Most still holds true
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Sec. 2 - Structure of the Atom
nucleus – protons & neutrons electron cloud
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Discovery of the e- (1897) Cathode ray tube – electric current passed through gases, found neg. charged particles JJ Thomson Did more expts. and found particles identical (no matter which gas used) Named them electrons
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Choc. chip cookie model Millikan Oil drop exp.
Mass of e- extremely small, neg. charged Atom is neutral, therefore must contain + charge also Choc. chip cookie model +
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Discovery of Nucleus Rutherford Chadwick discovered
1900 Gold foil exp. Confirmed dense nucleus with + charge Chadwick discovered neutrons ~30 yrs. Later
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Modern atomic model nucleus: p+ and n0 e- cloud
All the mass, very little space e- cloud No mass, lots of space
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Nuclear forces Like forces repel, normally
BUT when protons are really close there is a strong attractive force Same with neutrons Called nuclear forces (energy, bombs)
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Size of atom Tiny Radius from center of nucleus to edge of e- cloud
Measured in pm (10-12) Atomic radii range from pm
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Sec. 3 - Counting Atoms Atomic # Ex: oxygen is 8 8 p+ and 8 e-
# of protons in nucleus Determines what element Ex: oxygen is 8 8 p+ and 8 e-
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Mass # Total # of p+ and n0 in an isotope Calculate # of neutrons: Mass # - atomic # (p + n) – (p) = n
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Isotopes Atoms of same element with diff. # neutrons Diff. masses
Ex: hydrogen H protium % H deuterium % H tritium trace, synthetic *Don’t differ much in chem. props. 1 1 2 1 3 1
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Isotope symbols Hyphen uranium-235 Nuclear symbol U 235 92
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Charges Br Ca+2 Atomic # Mass # Protons Neutrons electrons 35 80 45
36 Ca+2 20 40 18 80 35 80 35 40 20
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masses so small, came up with scale that is relative
amu’s atomic mass units masses so small, came up with scale that is relative amu = mass of 1/12 mass of carbon-12 atom Average atomic masses # on periodic table Decimals averages
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Copper – 63 53% Copper – 64 47% Arsenic – 74 13% Arsenic – 75 80% Arsenic – 76 7%
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Relating Mass to Numbers of Atoms
The mole is the SI unit for amount of substance A mole (abbreviated mol) is the amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12.
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Avogadro’s Number Avogadro’s number 6.022 1415 1023
is the number of particles in exactly one mole of a pure substance. Can find: Atoms moles Moles atoms
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Molar Mass Molar mass - The mass of one mole of a pure substance
usually written in units of g/mol numerically equal to the atomic mass of the element in amu’s.
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Examples What is the mass in grams of 3.50 mol of the element copper, Cu? A chemist produced 11.9 g of aluminum, Al. How many moles of aluminum were produced?
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How many moles of silver, Ag, are in 3.01 1023 atoms of silver?
What is the mass in grams of 1.20 108 atoms of copper, Cu?
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Chapter Review Pg. 89 # 1-3, 5-7, 11, 14-21, 27
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