ATOMIC STRUCTURE

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THE TRUE MYSTERY OF THE WORLD IS THE VISIBLE, NOT
THE INVISIBLE. - Oscar Wilde -
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ATOMIC STRUCTURE

• "I think I can safely say that nobody understands
  quantum mechanics."
• - Richard Feynman-

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QUANTUM OF ENERGY THE ENERGY REQUIRED TO MOVE
AN ELECTRON FROM ONE ORBIT OR ENERGY LEVEL TO
ANOTHER. PHOTONS OF LIGHT REPRESENT DISCRETE
PACKETS OF ENERGY WITH THE WAVELENGTH OF LIGHT
DETERMINING THE AMOUNT OF ENERGY E hc/l
where h constant, c speed of light l
wavelength of light ERWIN SCHRODINGER USED
THESE RELATIONSHIPS TO DESCRIBE THE BEHAVIOR OF
ELECTRONS IN ATOMS. ATOMIC ORBITAL A REGION IN
SPACE WITH A HIGH PROBABILITY OF FINDING AN
ELECTRON.
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WHERE DO OUR CURRENT IDEAS OF ATOMIC STRUCTURE
COME FROM? de Broglie put forward that all
objects in motion have wave nature. The smaller
an object is, the greater the wave nature. This
means that the electron would behave as much like
a wave as a particle. Schrodinger came up with a
differential wave equation that would describe
the motion of an electron about a nucleus in
three dimensions. Chemists call these wave
functions orbitals.
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The math involved is very complex. However, the
results do an excellent job of describing why
atoms of different elements behave as they do,
and they are very useful in describing how atoms
interact to form molecules. They even do a good
job helping to describe the shapes of molecules.
This is very important in biochemistry and
medicine.
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From the quantum theory, there are four quantum
numbers that describe electron orbitals. n
principal quantum number - what shell the
electron goes in (n 1, 2, 3...) l angular
momentum quantum number - what subshell the
electron goes in (0 lt l lt n-1) ml magnetic
quantum number how many orbitals the subshell
is broken into (-l lt ml lt l) ms spin quantum
number (- 1/2 or 1/2)
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Principal Quantum Number (shell) Number of Subshells Type of Subshell
n 1 1 1s (1 orbital)
n 2 2 2s (1 orbital) 2p (3 orbitals)
n 3 3 3s (1 orbital) 3p (3 orbitals) 3d (5 orbitals)
n 4 4 4s (1 orbital) 4p (3 orbitals) 4d(5 orbitals) 4f (7 orbitals)
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The quantum numbers, in a sense, tell us how
many electrons can go where. There are some
rules that tell us how electrons fill shells.
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ELECTRON FILLING RULES Aufbau (build-up)
Principal - Electrons enter and fill lower energy
orbitals before higher energy orbitals. Pauli
Exclusion Principal - orbitals can contain a
maximum of two electrons which must be of
opposite spin. Hunds Rule - when there are
orbitals of equal energy, electrons will enter
the orbitals one-at-a time until all of the
orbitals are half filled and only then will
pairing occur.
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So, lets take a look at how electrons go into
orbitals as we move through the periodic
table. Hydrogen has only one electron, so it
would go in the first shell. The first shell has
only one subshell, the 1s. So, wed write that
as Hydrogen, H 1s1
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Helium has an atomic number of 2, so it has two
electrons. This second electron would also go in
the 1s subshell, but it would have opposite
spin. Hydrogen, H 1s1 Helium, He
1s2 This fills up this shell and subshell.
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Notice H and He are in the first row or first
period. Electrons are going into the first shell.
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Lithium has an atomic number of 3, so this means
that it has 3 electrons. The first shell is
filled, so the third electron has to go in the
second shell. It will go in the 2s subshell, as
this has the lowest energy. Hydrogen, H
1s1 Helium, He 1s2 Lithium, Li 1s2 2s1
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Beryllium has an atomic number of 4, so 4
electrons. Two in the first shell, and two in
the 2s subshell. Hydrogen, H 1s1 Helium,
He 1s2 Lithium, Li 1s2 2s1 Beryllium, Be
1s2 2s2 This fills the 2s subshell. The next
electrons will have to go in the 2p subshell.
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Boron has an atomic number of 5, so it has 5
electrons. B 1s2 2s2 2p1 There are actually 3
2p subshells, all of equal energy, px, py, pz.
One electron will go into each of these. After
each has an electron, the the additional three
electrons will pair up. Right now, dont be too
concerned. Just remember that 6 electrons can go
into the p subshells.
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So, for the rest of the second row (period), we
have Lithium, Li 1s2 2s1 Beryllium, Be 1s2
2s2 Boron, B 1s2 2s2 2p1 Carbon, C
1s2 2s2 2p2 Nitrogen, N 1s2 2s2 2p3 Oxygen, O
1s2 2s2 2p4 Fluorine, F 1s2 2s2 2p5 Neon,
Ne 1s2 2s2 2p6 This gives us a filled
first shell and a filled second shell.
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The order in which the third shell would fill
would be Sodium, Na 1s2 2s2 2p6
3s1 Magnesium, Mg 1s2 2s2 2p6 3s2 Aluminum, Al
1s2 2s2 2p6 3s2 3p1 Silicon, Si 1s2
2s2 2p6 3s2 3p2 Phosphorus, P 1s2 2s2 2p6 3s2
3p3 Sulfur, S 1s2 2s2 2p6 3s2
3p4 Chlorine, Cl 1s2 2s2 2p6 3s2
3p5 Argon, Ar 1s2 2s2 2p6 3s2 3p6
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There is one very important reason we are going
through this. It is the number of electrons in
the outer shell (valence electrons) that
determine the properties of the element. If you
look at the periodic table, you will see that all
of the elements in a given column have the same
number of valence electrons. So, they will have
similar properties. A column is called a family.
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A row is called a period. The period number
tells what shell is being filled. The octet rule
says that the maximum number of electrons that
can occur in any outer shell is 8, except for the
first shell, which is 2. This represents a
stable configuration, and elements will react by
losing, gaining or sharing electrons to obtain
this configuration.