Download atoms. molecules, and ions

Atoms, Molecules, and Ions
Development of Atomic Theory
o
It took over 2000
years from the first
proposal of an
“atomic” universe
before actual
experimental
evidence had been
accumulated!
History of the Atom
o
o
Original idea (400 B.C.) came from
Democritus, a Greek philosopher
Democritus expressed the belief that all matter
is composed of very small, indivisible particles,
which he named atomos.
Dalton’s Atomic Theory
o
John Dalton proposed explanations
for the laws of mass conservation,
definite & multiple proportions:
n
n
An element is composed of tiny
particles called atoms.
All atoms of a given element are
identical and atoms of different
elements are different.
Dalton’s Atomic Theory
n
n
Atoms of different elements
combine in ratios of small whole
numbers when forming
compounds.
Chemical reactions only rearrange
the way atoms are combined; the
atoms themselves are not
changed.
1897 J.J. Thomson
J.J. Thomson is studied
mysterious “cathode
rays” in a cathode ray
tube (CRT).
o He discovered that these
cathode rays carried a
negative charge, and that
they had mass.
His conclusion: cathode rays are really streams
of negatively charged particles, “electrons”
o
o
Thomson’s Conclusions
o
o
Thomson saw that the
cathode ray beam could
be deflected using
either an external
magnetic or electric
field
Based on the direction
of the beam deflection,
he concluded it carried
negative charge
J.J. Thomson
o
He determined the “charge
to mass ratio” for an
electron:
e/m = 1.758819 x 108 C/g
o
This is important because
if either charge or mass
can be determined then the
other can be calculated.
Thomson’s Atomic Model
Thomson’s discovery
showed that there was
matter even smaller than
atoms.
o After discovering the
electron, J.J. Thomson
proposed a “plum
pudding” or “raisin bun”
model of the atom.
He envisioned a positively charged “dough” with
negatively charged electrons scattered throughout.
o
o
1909 Robert Millikan
Determined the charge
on a electron using an oil
drop experiment.
o After he charge the oil
drop then he balanced it
between a negative and a
positive plate.
o e = 1.602177 x 10-19 C
o m = 9.101390 x 10-28 g
o
1911 Earnest Rutherford
o
o
o
Rutherford studied the “new”
phenomenon of radioactivity,
discovered by Henri Bequerel.
In 1898, Rutherford discovered
3 different types of
radioactivity: a, b, g
He later used the alpha
particles in a famous
experiment that ended with the
discovery of the “nuclear
atom”
Next: Shockwave Animation
Rutherford’s Gold Foil Experiment
The Gold Foil Experiment
Thomson’s Model?
Nuclear Model?
"It was as if you fired a 15-inch shell at a sheet of
tissue paper and it came back to hit you."
- E. Rutherford
Gold Foil
Rutherford’s Nuclear Model
If an atom was as big as a
football stadium, the nucleus
would be the size of an ant on
the 50-yard line!
1932 James Chadwick
o
o
The last of the three main subatomic particles
was discovered in 1932 by James Chadwick.
Worked with Rutherford looking for an
uncharged subatomic particle, but failed.
1932 James Chadwick
o
In 1932, he revisited some
earlier experiments and
was successful – he
discovered the neutron!
Thanks,
Mr. Chadwick!
Three Important “Subatomic” Particles
Three Important “Subatomic” Particles
o
o
o
The electron was discovered first by Thomson –
it is negatively charged and the lightest of these
three subatomic particles.
The proton was deduced by Rutherford. It is
positively charged and has a mass of
approximately 1 amu.
The neutron was the last particle to be
discovered (by Chadwick) in 1932. It has about
the same mass as a proton, but is neutral.
Important Vocabulary…
o
o
Atomic Number (Z)
n The number of protons in the nucleus of
an atom
n Also equal to the number of electrons
around the nucleus if the atom is neutral
n Determines the identity of an element
Mass Number (A)
n The number of protons & neutrons (i.e.
“nucleons”) within the nucleus of an atom
Important Vocabulary…
o
o
Isotopes
n Atoms of the same element (with the same #
of protons), with different numbers of
neutrons (i.e. different mass numbers)
Atomic Mass
n The mass of an atom.
n May be measured in “grams”
n More conveniently expressed in “atomic
mass units”, amu
Three Isotopes of Hydrogen
Two Isotopes of Sodium
Understanding Isotopes
o
How many protons,
electrons and
neutrons are there in
a neutral atom of
phosphorus-32?
n 15 Protons (Z)
n 17 Neutrons
(32 – 15)
n 15 Electrons
(neutral)
o
How many protons,
electrons, and
neutrons are in a
neutral atom of
potassium-39?
n 19 protons (Z)
n 20 neutrons
(39 – 19)
n 19 electrons
(neutral)
The Mass Spectrometer
An instrument that separates and analyzes particles
based on their masses. Pictured below is the
separation of neon gas into its three isotopes.
The Mass Spectrum of Neon
o
o
o
A computer displays a
graph of Abundance vs
Mass Number for neon gas.
The three peaks suggest
three different isotopes:
20Ne, 21Ne and 22Ne.
The area under each peak
represents the “abundance”
or the “fraction” of the
neon gas contributed by
each isotope.
Average Atomic Mass & Natural Abundances
o
Chromium exists as four stable isotopes. Use
the information below to determine the
approximate average atomic mass of chromium
rounded to a whole number.
Mass
49.946 amu
51.941 amu
52.941 amu
53.939 amu
Abundance
4.35%
83.79%
9.50 %
2.36%
~ 52 amu
Atomic Mass Units
o
o
o
The atomic mass unit is
DEFINED as one-twelfth
the mass of the nucleus of
an atom of carbon-12.
A proton or neutron has a
mass of approximately 1
amu
1 amu = 1.66054 x 10-27 kg
1922 Niels Bohr
Electrons closer to the nucleus
have lower energy and are more
stable due to strong electrostatic
attraction with the nucleus.
o The letter “n” is used to designate
energy levels (orbits), with lower
whole number values of n
representing lower energy orbits
closer to the nucleus.
o
1922 Niels Bohr
The key to Bohr’s Quantum Model was that
electrons are restricted to certain
“allowed” energy levels in atoms.
o Whereas Rutherford suggested electrons
orbit the nucleus, Bohr’s model required
that they occupy only certain orbits!
o
The Bohr Atom
o
o
o
o
The first electron orbit
holds a maximum of 2
electrons
The second orbit holds a
maximum of 8 electrons
The third orbit may hold
up to 18 electrons
The nth orbit can hold up
to 2n2 electrons!
Draw a Bohr diagram for Carbon-14
6p
8n
Draw a Bohr diagram for Oxygen-15
8p
7n
Evolution of the Atomic Model
o
o
Bohr’s model of the atom is very useful for
understanding elementary concepts in bonding and
chemical reactions
However, it is fundamentally flawed and he soon
helped replace it with the “Quantum Mechanical
Model” of the atom which no longer views the
electron as simply a “particle”, but acknowledges
and depends on the wave-nature of the electron also.