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The Structure of the Atom
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Early Theories of Matter
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460-370 BC Democritus
Greek PhilosopherNamed 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
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Discovering the Electron
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1879 William Crookes
investigated electrical discharge in gases
cathode ray tubeCathode 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 experimentsplum pudding model of
atom or chocolate-chip cookie dough model of the atom
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1909 Robert Milliken
determined charge of electronoil drop experimentwith 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 nucleusConcluded that nucleus
contained positively charged particles called protons
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1932 James Chadwick
identified neutronsame mass as protonno charge
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How Atoms Differ
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NUCLEUS
1. Protons with (+) charge
2. Neutrons with no charge.
3. Protons & neutrons have about the same mass.
4. (+) charge is responsible for most of mass of atom (dense central part).
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ELECTRONS
move around nucleusresponsible for most of volume
of atom (-) chargenegligible 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
Symbolatomic # p
? # neutrons
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Oxygen-17mass # 17atomic # 8
p + n = 17 p =_8_
9 n
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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 14
C C C 6 6 6
? # neutrons
6 7 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 70
30% Lab 80
20% Daily 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 protium 1.0078 99.985%
H-2 deuterium 2.0140 0.015%
H-3 tritium 3.0160 -------
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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