Transcript Flipped Class Notes Ch 3A

Chapter 3
Atoms : The Building Blocks of Matter
Foundations of Atomic Theory
 Several basic laws were after the 1790’s (emphasis
on quantitative analysis):
Lavosier:
 Law of conservation of mass: mass is neither created nor
destroyed during ordinary chemical or physical
processes. (Only re-arranged)
 Mass of Reactants = Mass of Products
 Law of conservation of energy: energy is neither created nor
destroyed during ordinary chemical or physical
processes.
(Only changed from one form to another form)
Foundations of Atomic Theory
Cont…
Proust:
 Law of definite proportions: chemical compounds
contain the same elements in exactly the
same proportions by mass regardless of the
size of the sample.
 Ex. NaCl always is composed of 39.34%
sodium and 60.66% chlorine by mass.
Foundations of Atomic Theory
Cont…
 Law of multiple proportions: if two or more
different compounds are composed of the
same 2 elements, the ratio of mass of the
second element combined with a certain
mass of the first is always a ratio of small
whole numbers.
 Ex. CO and CO2: For the same mass of
carbon, the mass of the O in CO to the mass
of O in CO2 will be 1:2
Development of Atomic Models
 John Dalton (1808):
1. All matter is composed of extremely small particles
called atoms (cannot be subdivided, created, nor
destroyed)
2. Atoms of the same element are identical; atoms of
different elements are different
3. Atoms combine in simple whole number ratios to form
compounds
4. In chemical reactions, atoms combine, separate, or are
rearranged.
Dalton’s Model of the Atom
Because of Dalton’s atomic theory, most
scientists in the 1800s believed that the atom
was like a tiny solid ball that could not be
broken up into parts.
* small
*indivisible
*dense
*uniform (same throughout)
Like a steel ball-bearing or marble.
J.J. Thomson (1897)
 Used cathode rays to determine that atoms
contained small negatively charged particles
called electrons.
 These particles were attracted to the positive
plate and repelled by the negative plate.
 It didn’t matter which gas was in the tube they
all had the same results.
Cathode Ray Tube
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Electron beam
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Beam deflected by
Negative plate
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Robert Millikan (1909)
 Used the Millikan Oil Drop Experiment to
prove the mass and charge of the electron
Thomson’s Model of the Atom
 The atom has electrons.
 Electrons are embedded in a
“positive goo” to keep the
atom Neutral
 CHOCOLATE CHIP
COOKIE DOUGH
MODEL (Raisin bun, Plumpudding, blueberry Muffin)
Issues Not Delt with by
Thomson and Millikan
 Atoms must also contain positive
charges to balance the negative electrons
 Other particles must account for most of
the mass of the atom
Ernest Rutherford (1911)
 Gold Foil Experiment:
 Assumed mass and charge were evenly
distributed throughout the atom (“plum-pudding”
model)
 Shot alpha particles (2 protons + 2 neutrons) at
thin sheet of gold
 Expected most of the particles to pass with only
slight deflection
 Most particles did, but some showed wide-angle
deflections (some back to the source).
 discovery of the NUCLEUS of the atom
Ernest Rutherford Cont…
 causes proton-proton, proton-neutron, neutronneutron attractions
 small, dense, positively charged center of
the atom
 number of PROTONS in the nucleus
determines the atom’s identity
Gold Foil Experiment
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Lead
Box
Rutherford Activity
New Atomic Model
Positive
Nucleus at
center
Negative
Electrons
sitting in Empty
Space
Problem with the New Model
 Electrons would be pulled into the center of
the atom by the very positive nucleus.
(Opposite charges attract)
 We know that this doesn’t happen so how
do the electrons keep from being pulled in
???
Forces in the Nucleus
 Repulsive forces should exist between
protons in the nucleus (like charges repel).
 Strong (nuclear) force:
 Attractive force that acts over very small
distances in the nucleus.
 Overcomes the repulsive forces caused by like
charges, so the protons can all stay in the
nucleus.
Atomic Dimensions
 Atomic radii: 40 to 270 pm
 Nuclear radii: about 0.001 pm
 Nuclear density: about 2 x 108 metric
tons/cm3
 1 amu (atomic mass unit) = 1.660540 x 1027 kg