Download A quick summary about atoms: Atomic masses and/or hydrogen

A quick summary about atoms:
„
Atoms were used by John Dalton (1804) to
explain the Law of Definite Proportions: if
elements combine only in definite proportions by
weight, the simplest explanation is that whole
numbers of atoms combine into specific
molecules. Atoms were considered to be
indivisible, the smallest possible pieces of
matter.
„
Atoms were not considered real by many
chemists for at least 50 years, but...
Atomic masses and/or hydrogen
„
Atomic masses were measured chemically in the
19th Century
… these
„
„
are AVERAGE masses
Atomic masses are nearly exact multiples of the
mass of hydrogen
William Prout (1815) suggested that all atomic
weights are exact multiples of hydrogen and
hydrogen is the basic stuff of matter
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Sideshow: electrons in atoms
occupy specific energy states
Becquerel and
luminescence (1890s)
„
„
„
Luminescence:
material glows for a
time after exposure to
light.
Fluorescence:
material glows while
light shines on it.
How to measure
luminescence and
fluorescence?
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Becquerel and uranium (1890s)
„
„
„
Certain uranium salts
are phosphorescent
Salts in a drawer with
wrapped film
Film shows fogging
right through the
wrapper!
„
„
Only uranium shows
this effect
NOT linked to
phosphorescence!
Becquerel and radioactivity
„
Types of radioactivity
… Alpha:
massive,
positively-charged
particles
… Beta: light, negativelycharged particles
… Gamma: light,
uncharged particles
Nobel Prize, 1903
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A quick summary about atoms:
„
In 1899, J.J. Thomson discovered the electron
(Nobel Prize, 1906) and proposed the “plum
pudding model” of the atom, in which the atom
has simpler structural components.
„
In 1905 Albert Einstein proved that atoms must
exist (he showed that Brownian Motion would
look different if there weren't really atoms and
molecules shoving particles around). This later
won the Nobel Prize for Jean Perrin in 1926.
The “Plum Pudding”
Model of the atom
„
„
„
Negative electrons
scattered through positive
“glue”
Electrons are very small,
light particles
Electrons can be knocked
out of atoms by
…
…
…
Light
Electric potential
Particle impact
e-
e-
e-
e-
e-
ee-
ee-
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Ernest Rutherford’s Gold-Foil
Experiment (1911)
Ernest Rutherford discovers
the atomic nucleus (1911)
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Subatomic particles
„
„
„
„
Electron: light, negatively
charged
Proton: heavy, positively
charged
Atomic masses,
compared to numbers of
electrons in atoms, lead
to…
Neutron: heavy,
uncharged; mass ~equal
to proton
The Strong Nuclear Force
Short-range force between nucleons
„ Very powerful: must hold protons together!
„ Shows itself as mass:
„
2 2H → 4He
… Helium
weighs 0.05% less
… This means that one 4He needs less binding
force than two 2H.
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Talking about nuclei
„
Atomic number
…
…
„
The number of protons in
the nucleus
Defines chemical element
Mass number
…
…
Total mass of the nucleus
Sum of protons + neutrons
Mass
number
12
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often
omitted
Chemical
symbol
C
Atomic
number
Isotopes
Isotope = a particular combination of
protons and neutrons
„ An element must have a particular number
of protons: Atomic Number defines
element
„ Any number of neutrons “within reason”
„
… Hydrogen: 1H, 2H, 3H
… Carbon: 11C, 12C, 13C, 14C
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Types of Radioactivity
„
Alpha-decay, 42 α
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the nucleus sheds mass as 2 He
… Changes element to another, lighter one
… Effect:
„
Beta-decay, β (three types)
… Effect:
converts neutron to proton or vice versa
… Changes element to another of the same mass
„
0
Gamma-decay, 0 γ
… Effect:
makes nucleon arrangement more stable
… Element remains the same
Types of Beta-decay
„
Beta-minus,
0
−1
β
… Changes
neutron to proton
… Emits an electron
„
Beta-plus, 1β
0
… Changes
proton to neutron
… Emits a positron
„
Electron capture, EC
… Captures
an inner-shell electron to change proton to
neutron
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Aspects of Radioactivity
„
Radiometric Dating
… Half-life:
„ 14C
the time needed for ½ to decay
(5700 yr)
„ 40K/40Ar
(1.3×109 yr)
„ 238U/206Pb
… Where
„
(4.5×109 yr)
do radioisotopes come from?
Cosmic rays
Aspects of Radioactivity
„ Health
effects – penetrating power
…alpha
…beta
…gamma
„ Radon!
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