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Quantum Physics
Dalton’s Atomic Theory
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•
Dalton’s indivisible atom has not been
disregarded—it has been modified to
explain new observations.
Two important concepts,
(1) All matter is composed of atoms and
(2) Atoms of any one element differ in
properties from atoms of another
element,
remain unchanged.
Electrons
• Discovered by Joseph John Thompson
and Robert A. Millikan
• Discovered through experiments with
cathode-ray tubes.
• Negative charge
• Very small mass 9.109 x 10-31 kg
• Plum pudding model-now had
subatomic particles
• Discovery of the electron led to 2 other
inferences about atomic structure:
– Because atoms are electrically neutral, they
must contain a positive charge to balance
the negative electrons.
– Because electrons have so much less mass
than atoms, atoms must contain other
particles that account for most of their
mass.
Atomic Nucleus
• Ernest Rutherford, Hans Geiger, and Ernest
Marsden
• Discovered through experiments with alpha
particles and gold foil.
• Except for the nucleus of the hydrogen atom,
contains protons and neutrons.
– Protons have a positive charge equal to the
negative charge of the electron.
• Atoms are electrically neutral because they contain equal
#s of protons and electrons.
– Neutrons are electrically neutral.
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
Atoms
• Defined as the smallest particle of an
element that retains the chemical
properties of the element.
• Composed of:
– Nucleus protons (+) & neutrons (no
charge)
– Electrons (-)
– Collectively referred to as subatomic
particles.
• Proton mass = 1.673 x 10-27 kg
– 1,836 times greater than the mass of the
electron
• Neutron mass = 1.675 x 10-27 kg
Nuclear Forces
• Nuclear forces are short-range protonneutron, proton-proton, and neutronneutron forces hold the nuclear
particles together.
Atomic Number
• The atomic number of an element is the
number of protons in the nucleus of
each atom of that element.
• Determines the identity of the atom.
• Look at the periodic table, what is the
atomic number for hydrogen?
Nitrogen? Sodium?
Atomic Mass
• Total number of protons and neutrons
• Therefore, if you want to know number
of neutrons, you will have to subtract
atomic # from the mass #.
• If you were not given a specific mass
for your atoms, ONLY THEN should you
use the average atomic mass on the
periodic table.
Info on the Periodic Table
Atomic #
Symbol
Mass #
Isotopes
• Isotopes are atoms of the same element that
have different masses.
– The isotopes of a particular element all have the
same number of protons and electrons but
different numbers of neutrons.
– Most of the elements consist of mixtures of
isotopes.
– Although isotopes have different masses, they do
not differ significantly in their chemical behavior.
– Nomenclature
Isotopes of Hydrogen
• The number of protons defines the
element.
• The number of neutrons varies.
1
1
H
Protium
Hydrogen-1
2
1
H
3
1
H
Deuterium
Tritium
Hydrogen-2
Hydrogen-3
Mass Number
• The total number of protons and
neutrons in the nucleus of an atom is
equal to the mass number.
mass number
atomic number
4
2
He
• Or if it is shown as
Helium-4
then it is showing element-mass #
Quantum numbers-Pauli’s exclusion
principle
•
•
•
•
N = energy level (row on periodic table)
l = sublevel (s-0, p-1, d-2, f-3)
ml = blank in sublevel
s = spin (+1/2 for up -1/2 for down)
Photoelectric Effect
• Emission of electrons from a metal
when light shines on the metal.
• This stream of electrons creates an
electric current
• For a given metal, no electrons will be
emitted IF the light’s frequency is below
a certain minimum (no matter how long
the light is shined on it)
Planck and Einsten
• Planck suggested that hot objects
release EM energy (light) as small,
specific amounts he called quanta
• Einstein said these particles of EM
energy had no mass and carry a specific
amount of energy (quantum)
• Different metals required different
minimum frequency of energy
•E=hv
• Energy = Planck’s x frequency of
constant
radiation
Joules
6.626 x 10-34
Hz
Js
What does this have to do with atoms?
• When electricity is passed through a
light bulb, the atoms are excited
• This means electrons jump to a higher
energy level… but must release energy
to return to their ground state
• If this light is passed through a prism, it
separates into specific frequencies of
light
Practice
How many protons are
in the nucleus of this
helium atom?
4
2
He
How many neutrons?
2 protons and 2 neutrons
# of protons + # of neutrons = mass number
Practice Problems 
• How many protons, electrons, and
neutrons are there in an atom of
chlorine-37?
17 protons
17 electrons
20 neutrons
More Practice
• How many protons, electrons, and
neutrons are in each of the following
atoms:
– Bromine-90
– Carbon-13
• What element contains 15 electrons and
15 neutrons?
IONS
• When an atom (or group of atoms)
gains or loses an electron(s), the atom
is no longer neutral
– Cation-+ charged ion because the atom
LOST electron(s)
– Anion-(-) charged because the atom
GAINED electron(s)
– YES, that does seem backwards, but
remember electrons are negatively charged