Transcript The Structure of the Atom CHAPTER 4 1

The Structure of the
Atom
CHAPTER 4
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Early Theories of
Matter
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460-370 BC Democritus
 Greek
Philosopher
 Named atom
»smallest unit of matter
»means indivisible
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1807 John Dalton: Atomic
Theory
 Revived
and revised
Democritus’ ideas and began
developing the modern
atomic theory
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Dalton’s Atomic Theory




all elements are composed of tiny indivisible
particles called atoms
atoms of same element alike. Each element is
different from atoms of other elements
atoms of different elements combine in simple
whole number ratios to form compounds
chemical reactions occur when atoms are
rearranged
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Picture shows:
•Conservation of Mass
•Element combing in simple whole number ratios
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Subatomic Particles & the
Nuclear Atom
Chapter 4
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A. Discovering the Electron
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1879 William Crookes
 investigated
electrical
discharge in gases
 cathode ray tube
 Cathode rays are streams of
negatively charged particles.
 The particles are found in all
forms of matter
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1897 J.J. Thomson
determined nature of cathode ray
 determined charge to mass ratio of
electron
 Found that atoms were divisible Dalton & Democritus were wrong

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1901 J.J. Thomson
 positive
beam experiments
 plum pudding model of
atom or chocolate-chip
cookie dough model of the
atom
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1909 Robert Milliken
 determined
charge of electron
 oil drop experiment
 with Thomson’s charge to
mass ratio: able to determine
the mass of e
Mass of electron = 9.1x10-28 grams
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1911 Ernest Rutherford
 discovered
nucleus
 gold foil experiment disproved
plum pudding model
 small dense central part of atom
= nucleus
 (+) charge
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1920 Rutherford
 Refined
concept of nucleus
 Concluded that nucleus
contained positively charged
particles called protons
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1932 James Chadwick
 identified
neutron
 same mass as proton
 no charge
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How Atoms Differ
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NUCLEUS
1.
2.
3.
4.
Protons with (+) charge
Neutrons with no charge.
Protons & neutrons have about the
same mass.
(+) charge is responsible for most of
mass of atom (dense central part).
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ELECTRONS
 move
around nucleus
 responsible for most of volume
of atom
 (-) charge
 negligible mass
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ATOMIC NUMBER & MASS
NUMBER
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ATOMIC NUMBER
#
of protons in nucleus
 it identifies the element
 elements in Periodic Table are
listed in increasing order of
atomic #
 if atom is neutral: the # of
protons equals # of electrons
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MASS NUMBER
sum of protons & neutrons in
nucleus
 written as part of name: must be
given to you

Neon-20 mass #20 p + n = 20
atomic #10 p = 10
n = 10
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mass #
p+n
Symbol
atomic #
p
? # neutrons
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Oxygen-17
mass # 17
atomic # 8
p + n = 17
p =_8_
9n
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To calculate electrons for an ion you
must look at the charge written in the
upper right corner
 To determine the number of electrons:

» If the charge is positive then subtract that
number from the number of protons.
» If the charge is negative then add that
number to the number of protons
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ISOTOPES

Atoms of the same element are not all
identical - may differ in # of neutrons

Isotopes - atoms of the same
elements (same # of protons), but
different mass # (different # neutrons)
and therefore different masses
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12
13
C
6
14
C
6
? # neutrons
6
7
C
6
8
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ATOMIC MASS
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ATOM

smallest unit of an element that can
exist alone and still have the properties
of that element
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Atomic Mass

Average Atomic Mass - weighted
average of the masses of the naturally
occuring isotopes
» relative mass based on carbon-12 as the
standard
» Carbon-12 is defined as having a mass of
exactly 12 amu

atomic mass unit (amu) - 1/12 of the
mass of a carbon-12 atom
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Weighted Average Example
50% test
30% Lab
20% Daily
70
80
90
= 50%(70) + 30%(80) + 20%(90)
= .5(70) + .3(80) + .2(90)
= 35 + 24 + 18
= 77
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Isotopes of Hydrogen
H-1
H-2
H-3
protium
deuterium
tritium
1.0078
2.0140
3.0160
99.985%
0.015%
-------
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Calculating Average Atomic
Mass
Multiply the percent (as a decimal #) by the mass and
then add each together.
= 99.985% (1.0078 amu) + 0.015% (2.0140 amu)
= .99985 (1.0078 amu) + .00015 (2.0140 amu)
= 1.0076488 amu + 0.0003021 amu
= 1.00795 amu
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Example
chlorine - 35
75.8%
chlorine - 37
24.2%
Will the average atomic mass be closer to
35 or 37?
(35 because higher %)
75.8%(35) + 24.2%(37) =
35.5 amu
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Example

Calculate the average atomic mass for
boron. Be as accurate as possible. Use
values from table on p. 281. Check your
answer by looking at the periodic table.
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