Transcript my Work 4 U

ATOMIC STRUCTURE
Presented by Naveed
Nawaz
Outline
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Schrodinger Model of the Atom
Quantum Numbers
Periodic Table
X-Rays
Auger Electrons
Schrodinger Model
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Erwin Schrodinger determined that
an electron can exist as a particle
and a wave. With this view of an
electron a new model was
formulated.
This model was based on math and
used Heisenberg's uncertainty
principle which says that one can not
determine the exact position of an
electron and it's momentum at the
same time.
Schrodinger Model [cont’d]
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Unlike Bohr's atom that had very
well defined quantized orbits,
Schrodinger's model shows the orbits
very undefined.
The orbits represents regions of the
space where electrons are likely to
be found.
Schrodinger Model [cont’d]
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Although Bohr's model
explained somehow the line
spectra observed when atoms
emitted energy, it couldn’t
explain the behavior of the
ionization energy with Z.
If the Bohr model was accurate,
ionization energies of elements
would increase with increasing
atomic number.
Schrodinger Model [cont’d]
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Experimentally was found that
the ionization energy drops at
certain values of Z.
Schrodinger Model [cont’d]
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This led to the idea that there may
be sub-shells within each energy
level.
Further evidence for this comes from
the lines in the emission spectra of
atom. Some of the lines consist of
two or more lines close together
(i.e., at very similar energy levels).
Schrodinger Model [cont’d]
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These lines differ in brightness and
width. This is taken to suggest that
each shell has closely-related subshells that have slightly different
properties.
Quantum Numbers
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Schrödinger proposed that the
electrons in an atom were governed
by four quantum numbers.
The first of these is called the
principal quantum number, and is
given the symbol n. This corresponds
to the electron orbit in the Bohr
model.
n = 1, 2, 3, ...., denoting energy
Quantum Numbers [cont’d]
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The second quantum number
corresponds to the sub-shell and is
called the orbital angular
momentum, or l. This can have one
or more values, given by l = 0, 1, 2,
..., (n-1)
Therefore, if n = 1, l can be 0 or 1.
This means that the shell 1 has two
sub-shell.
Quantum Numbers [cont’d]
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Possible electrons orbits for n=3
l=1
l=2
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l=0
These sub-shells are named after the
type of line they produce in the
emission spectra of atom.
Quantum Numbers [cont’d]
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The s sub-shell gives a sharp
line, the p sub-shell gives a very
bright line (the principal line),
the d sub-shell gives a diffuse
line, and the f sub-shell gives a
line described as the
fundamental line.
s for l = 0, p for l = 1, d for l =
2, f for l = 3, g for l = 4 and
then on alphabetically.
Quantum Numbers [cont’d]
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The third quantum number is
called the orbital magnetic
quantum number, ml.
This number tells the way an
orbital aligns itself if one apply a
magnetic field, hence the name.
The values of ml are given by
ml = -l, (-l-1), (-l-2), ..., -1, 0,
1, ..., (l-1), l
Quantum Numbers [cont’d]
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Therefore, if l = 1, ml can be 1, 0, or +1. This means that
the p sub-shell contains three
orbitals.
Possible values of ml for l=2
ml
+2
+1
-1
-2
Quantum Numbers [cont’d]
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The final quantum number is the
spin magnetic quantum number,
ms. This describes the direction the
electron spins in a magnetic field and
can have one of two values, -½ or
+½.
ms=1/2
ms=-1/2
Quantum Numbers [cont’d]
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The differences between orbitals
within a shell can also be used
to explain the bonding between
atoms.
For this, it is useful to know
about the shapes of the orbitals.
http://mychemistrypage.future.easy
space.com/General/Atomic_Structur
e/Animations/QM_orbitals.html
Quantum
Numbers[cont’d]
n=1; l=0; ml=0
n=2; l=1; ml=0
n=3; l=2; ml=0
n=4; l=3; ml=0
n=6; l=2; ml=1
n=10; l=7; ml=5
Periodic Table
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When discussing multielectron
atoms, one speaks of "filling" the
shells.
So one can assign electrons to shells
by starting with the lowest quantum
numbers and moving up in order
until is reached the number of
electrons in the atom (equal to Z for
a neutral atom).
Periodic Table [cont’d]
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The Periodic Table is based on
the observation that an
element's chemical
properties depend on the
number of electrons in its
outer (valence) shell.
The Pauli Principle states that
no two electrons may be in
the same quantum state. This
explains the chemical properties
of the elements.
Periodic Table [cont’d]
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Pauli's Principle means that can
be only two electrons for any
given values of n, l and ml; one
has spin 1/2 and the other 1/2.
Similarly, for any given values of
n and l, there can only be 2l +1
pairs of electrons, corresponding
to the allowable values of ml.
Periodic Table [cont’d]
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First shell n = 1
l = 0, ml = 0, and ms = -½ or
+½. This shell therefore
contains only one orbital, with
one or two electrons.
Second shell n=2
l = 0 or 1;
Periodic Table [cont’d]
for l = 0, ml = 0, ms = -½ or +½.
This gives the 2s sub-shell with 2
electrons.
for l = 1, ml = -1, 0, or +1, and ms
for each is -½ or +½. This gives the
2p sub-shell, containing 3 orbitals
each with 2 electrons.
2nd shell contains a total of 8
electrons.
Periodic Table [cont’d]
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One can carry on with this
process to show that the 3rd
shell contains a total of 18
electrons, the 4th 32, etc.
In this way, one can deduce the
electronic structure of every
atom in the periodic table, just
by knowing its atomic number
(and so how many electrons it
contains).
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For Sodium it is (1s)2 (2s)2 (2p)6
(3s)1
Periodic Table [cont’d]
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In the filling of the fourth row, in
K atom electrons start to fill the
4s state before completing 3d
state. This happens because the
energy level corresponding to 4s
state is lower the one
corresponding to 3d state.
For many other multielectron
atoms electronic energy levels
are not filled in order.
Periodic Table [cont’d]
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Elements with the same number
of electrons in their outer
(valence) shell go in the same
column.
When the shell is full, the
element goes in the (rightmost)
"inert" column, since it does not
easily react with a filled valence
shell.
Periodic Table [cont’d]
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The next element goes into the
leftmost column (for those
elements with only one electron
in their outer shell).
Atoms interact chemically by
sharing or partially transferring
electrons.
Atoms with filled shells only, like
He and Ne, are chemically
unreactive.
Periodic Table [cont’d]
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The valency, roughly speaking, is
the number of electrons available
for transfer (so Li and Na have
valency 1) or available sites for
reception of electrons—fluorine
has an outer shell with one
vacancy, so a valency of 1.
To some extent, valency can vary
depending on the strength of
attraction of other atoms in the
chemical environment.
Periodic Table [cont’d]
X-rays Emission
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In 1901 W. C. Roentgen
discovered the X-rays.
X-rays are just like any other
kind of electromagnetic
radiation.
X rays are produced whenever a
beam of particles (electrons),
with sufficient energy collides
with a target material.
X-rays Emission [cont’d]
X-rays Emission [cont’d]
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There are two different atomic
processes that can produce x-ray
photons.
• Bremsstrahlung, which is a German
name meaning "braking radiation”
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X rays are emitted in a continuous band
• Characteristic X-ray
X-rays Emission [cont’d]
Characteristic X-rays
 When the energy of the particle
beam is above a certain
threshold value (called the
excitation potential) an electron
from inner shells of atom will be
ejected from the target atoms.
 Then valence electrons in higher
energy states of the target
atoms fill the vacancies from
inner shells and in the process
emit X-ray photons.
X-rays Emission [cont’d]
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These X-ray photons have
discrete energies that are equal
to the difference in energy
between the valence and core
energy levels.
The characteristic lines are
called K, L, M, ... and
correspond to transitions from
higher energy states to the n =
1, 2, 3, ... quantum levels,
respectively.
X-rays Emission [cont’d]
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When the two atomic energy levels
are adjacent, the transitions are
described as a lines (n = 2 to n = 1,
or n = 3 to n = 2)
When the two levels are separated
by one or more levels, the transitions
are known as b lines (n = 3 to n = 1
or n = 4 to n = 1).
X-rays Emission [cont’d]
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X-ray transitions
in an atom with
atomic number
Z=36
X-rays Emission [cont’d]
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Because all K lines arise from a loss
of electrons in the n = 1 state, the K
a and K b lines always appear at the
same.
Studing characteristic X-ray
wavelengths, Mosely found that the
square root of the frequencies of Xrays are linearly with Z.
X-rays Emission [cont’d]
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Moseley showed that the correct
ordering of the periodic table is
on the basis of the atomic
number (the number of positive
charges in the nucleus).
Before Moseley, periodic tables
were created on the basis of
increasing atomic weight (with
two exceptions).
Electron Auger
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In many cases the photon emitted in
an X-ray transition is absorbed by
another electron within the same
atom, which is therefore ejected as a
result of an internal photo-electric
effect.
This process of the internal
conversion of X-rays into photoelectrons is called the Auger effect
and the emitted photo-electrons are
called auger electrons.
Electron Auger [cont’d]
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Auger emission is the dominant
de-excitation process for low
atomic number elements such
as boron and carbon.
Auger electrons are emitted
with specific kinetic energies T
depending on the electronic
levels involved in the process.
E.g.:
T=EK-EL1-EL3
Electron Auger [cont’d]
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Schematic diagram of various twoelectron de-excitation processes.
• The KL1L1 Auger transition corresponds
to an initial K hole which is filled with L1
electron and simultaneously the other L1
electron is ejected to the vacuum.
• The LM1M1 Auger transition is the
corresponding process with an initial 2s
vacancy.
Electron Auger [cont’d]
References
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Turner, J. E., Atoms, Radiation, and
Radiation Protection, 2nd Ed.,John
Wiley&Sons , Inc.(1995)
Hunt, S.E., Nuclear Physics for
Engineers and Scientists, Ellis
Horwood Ltd. (1987)
Thank you.
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Naveed Nawaz
B.E-electrical-HUFC