This document provides an overview of atomic structure and theory. It begins by discussing how an understanding of atomic structure relates to materials properties. It then reviews the atomic models of Democritus, Dalton, Thomson, Rutherford, Bohr, and others. Key concepts covered include the nucleus, protons, neutrons, electrons, atomic number, mass number, and energy levels. The document also discusses atomic bonding mechanisms like ionic bonding, covalent bonding, and metallic bonding. Finally, it introduces topics like energy bands and the quantization of energy and light.
Measures of Dispersion and Variability: Range, QD, AD and SD
Mat ch-1.pptx
1. Electrical Materials and Technology
PCE3302
Chapter One
Review of atomic theory of matters
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2. Introduction
Selection of appropriate material for the intended purpose depends on the
knowledge of properties of materials. i.e.,
Physical properties
Electrical
Mechanical
Thermal and
Chemical
These properties of materials depend on the atomic structure of that material.
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3. Atomic Structure
All matter is composed of atoms.
Understanding the structure of atoms is critical to understanding the
properties of matter
Atom – smallest particle of an element that can exist alone
Two regions of an atom
Nucleus
Center of atom
Protons and neutrons
Electron “cloud”
Area surrounding nucleus containing electrons
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4. cont’d…………
Proton – Positive charge (+), 1 atomic mass unit (amu); found in the nucleus
amu -Approximate mass of a proton or a neutron
Neutron – Neutral charge (0), 1 amu; found in the nucleus
Electron – Negative charge (-), mass is VERY small
In a neutral atom, the number of protons equals the number of electrons, so the
overall charge is zero (0)
Example/ Helium, with an atomic number of 2, has 2 protons and 2 electrons
when stable
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5. Counting Atoms
Atomic Number
Number of protons in nucleus
The number of protons determines identity of the element!!
Mass Number (Atomic Mass)
Number of protons + neutrons
Units are g/mol
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6. Atomic model
Democritus 460 BC
Greek Philosopher
Suggested world was made of two things – empty space and “atomos”
Atomos – Greek word for uncuttable
2 Main ideas
Atoms are the smallest possible particle of matter
There are different types of atoms for each material
His ideas did agree with later scientific theory, but did
not explain chemical behavior, and was not based
on the scientific method – but just philosophy
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7. Cont’d……
John Dalton’s Atomic Theory (1766 - 1844)
Dalton was an English school teacher who performed
many experiments on atoms.
All matter is made of atoms.
Atoms of one element are all the same.
Atoms cannot be broken down into smaller parts
Compounds form by combining atoms
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8. Dalton’s Early Atomic Mode
“Billiard Ball” model
He envisioned atoms as solid, hard spheres, like billiard(pool) balls
So he used wooden balls to model them
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9. J.J. Thomson (1856 - 1940)
Discovered the electron
He was the first scientist to show the atom was made of even smaller things
Used the Cathode ray tube to discover electrons
1897
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16. cont’d………
Adding an electric field cause the beam to move toward the positive plate.
Thomson concluded the beam was made of negative moving pieces.
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+
-
Voltage source
17. Thomson’s “Plum Pudding” Atom Model
Plum and pudding atom modeling
..DownloadsCathode Rays Lead to Thomson's Model of the Atom.mp4
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18. Ernest Rutherford (1871 - 1937)
Discovered the nucleus of a gold atom
with his “gold foil” experiment
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19. cont’d………
Using J.J Thomson’s Plum Pudding atomic model, Rutherford
predicted the alpha particles would pass straight though the gold foil.
That’s not what happened.
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20. cont’d………
Gold Foil Experiment Results
Most alpha particles go straight through the gold foil A few alpha
particles are sharply deflected
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21. Rutherford’s Conclusion
The atom is mostly empty space.
There is a small, dense center with a positive charge.
Rutherford discovered the nucleus in atoms
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23. Rutherford’s Contribution to the Atomic Theory
The atom is mostly empty space.
The nucleus is a small, dense core with a positive charge.
..DownloadsRuther's Alpha Scattering Experiment2.mp4
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24. Bohr Model (1871 - 1937)
In 1913, the Danish scientist Niels Bohr proposed an improvement.
In his model, he placed each electron in a specific energy level.
According to Bohr’s atomic model, electrons move in definite orbits
around the nucleus, much like planets circle the sun.
These orbits, or energy levels, are located at certain distances from the
nucleus.
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25. Cont’d…….
Additionally, the electrons can jump from a path in one level to a path in
another level (depending on their energy).
He proposed the following
Protons and neutrons are in the nucleus
Electrons can only be certain distances from the nucleus
The electrons orbit the nucleus at fixed energy levels
The electrons must absorb or emit a fixed amount of energy to travel
between these energy levels
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26. Atomic bonding
Chemical bonds :-forces that hold together atoms when they combine
produce by interaction of electrons in outer levels of two or more atoms
can form by sharing or transferring electrons
All atoms will try to achieve a stable octet
Two Major Types of Bonding
Ionic Bonding
Form when electrons are transferred from one atom to another
Forms ionic compounds ,
Transfer of e- 11/13/2022 26
27. Ionic Bonding
Two neutral atoms close to each can undergo an ionization process in
order to obtain a full valence shell
Due to ionization, electrons are transferred from one atom to the
other
The atom giving up the electron becomes positively charged (cation)
The atom taking up the electron becomes negatively charged (anion)
The ions are bonded due to coulombic forces of attraction
Generally, metallic atoms donate electrons to non-metallic atoms
Examples: NaCl, KCl, MgBr2 etc.
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29. Covalent Bonding
The outer electron levels of atoms, which are close to each other, can
interact
The interaction leads to a sharing of electrons between the atoms
One pair of electrons shared => single covalent bond
Two pairs of electrons shared => double covalent bond and so on
The shared electrons are said to be delocalized i.e. they do not belong
to any particular atom
Generally, between non-metallic atoms
Examples: H2, CO2, C6H12O6 and other molecules
polar molecule :- have covalent bonds between but atoms do not share
electrons equally
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30. Cont’d….
Single hydrogen atom (left) and two hydrogen atoms forming a covalent
bond with a shared electron pair (right)
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31. Metallic bonds
Atoms come together, electrons from outer shell of atoms share space
with neighboring atoms.
The electrons can move freely within the atom orbitals.
Sharing of ‘free’ electrons among a lattice of positively charged ions
(An array of positive ions in a sea of electrons)
Electrostatic attractive forces between delocalized electrons and
positively charged metal ions.
Between metallic atoms
Examples: Ni, Fe and other metals
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32. Cont’d….
The outer electrons are so weakly bound to metal atoms that they are free
to roam across the entire metal.
This results in a lattice of positively ions in a sea of communal electrons
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33. Energy Bands
Electrons try to occupy the
lowest energy band possible
Not every energy level is a
legal state for an electron to
occupy
These legal states tend to
arrange themselves in bands
Allowed
Energy
States
Disallowed
Energy
States
Increasing
Electron
Energy
Energy Bands
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35. Band Diagrams
Band Diagram Representation
Energy plotted as a function of position
EC Conduction band
Lowest energy state for a free electron
EV Valence band
Highest energy state for filled outer shells
EG Band gap
Difference in energy levels between EC and EV
No electrons (e-) in the bandgap (only above EC or below EV)
EG = 1.12eV in Silicon
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EG
EC
EV
Increasing electron energy
Increasing voltage
36. Energy Bands In Solids
Explains and distinguish between conductors, insulators and semi
conductors
Conduction Band – small energy can remove an electron from an atom
Valence Band – Below the conduction band
Filled Band- Near the nucleus
Insulators- Conduction band is empty but valence is almost
completely filled+ wide energy gap between the two bands (Poor
conductivity)
Conductors- Overlapping of both conduction and valence bands
Conduction es move almost twice as fast as holes
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37. Example 1
Aluminum (Al) (no periodic table)
Protons = 13
Electrons =
Neutrons = 14
Atomic Number =
Atomic Mass =
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38. Quantization of Energy
Energy is quantized in some systems, meaning that the system can
have only certain energies and not a continuum of energies, unlike the
classical case
The quantum view of the atom
Observation: An atom will only absorb or release light at discrete
frequencies
Explanation:
Absorption or emission of light is caused by electron energy
transitions within the atom
The energy carried by light is connected with its frequency.
Electrons are only allowed to move between discrete energy levels in
the atom
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40. Light is also quantized
Light seems to carry energy in discrete packets
We call these packets photons
In this sense, light behaves like a particle
But light also has many wave-like properties
Particle-wave duality: it is both at the same time!
The photoelectric effect
Metal plate illuminated by a light beam
If UV light is used, electrons are emitted
If visible light is used, no electrons are emitted no matter how bright
the light
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41. Cont’d…..
Explanation:
A minimum amount of energy is needed to remove an electron from
the metal
The energy of UV light must be greater than the energy of visible light
Energy and frequency are related by
E = hn
where h is called Plank’s constant
h = 6.625 x 10–34 J s
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