1. The document discusses the subatomic particles that make up atoms, including electrons, protons, and neutrons. It defines related terms like atomic number and mass number. 2. Atoms can form isotopes that have the same number of protons but different numbers of neutrons. The forces that hold atoms together are electric forces between the positively charged protons and negatively charged electrons. 3. Oppositely charged particles attract, while similarly charged particles repel, according to the rules of electric force. This balance of forces is what allows atoms to form without collapsing in on themselves or flying apart.

1. Table of sub-atomic particles:
Particle Charge Mass Compared to
Electron
Actual Mass (kg)
Electron -1 1 9.11x10-31
Proton +1 1836 1.673x10-27
Neutron 0 1841 1.675x10-27
Nucleons

2. Definitions:
• atomic number: Number of protons in the Nucleus
• mass number: Number of protons + number of
neutrons
• atomic mass: mean mass of all isotopes (measured in
AMU – Atomic Mass Units)
Atomic number and mass number are counts.
Atomic mass has units of mass (AMU).

3. Isotopes
• For a particular type of atom (say, Iron) you
must have exactly 26 protons and 26 electrons.
• The number of neutrons may vary, resulting in
isotopes.
Example:
Hydrogen Deuterium

4. What holds an atom together?
• The electric charge of
the proton and electron
hold an atom together.
Gravity doesn’t have much sway at this
size – the electric force is much
stronger here.
What is the rule for North and South poles
of magnets?
Positive and negative charges are similar:
opposites attract, likes repel.

5. Like Charges Repel
• Given that like charges repel, why do we have
solids?
– Electrons move around, temporary polarization,
sharing of electrons between two nuclei
• Okay, so they can attract. Why don't the just
form a blob?
– The nuclei don't like each other.

6. How close can atoms get?
-0.5
0
0.5
1
1.5
0 1 2 3
Force(relative)
Distance (relative)
Force between two atoms
Above zero = repulsive force
Below zero = attractive force
There is a limit to how close two atoms can be
due to the negatively charged electron clouds.

7. Do we ever touch?
• What happens when two negative objects get close together?
• The electron clouds are negative, so what happens when two
atoms get close? Either:
– They repel each other
– They react chemically
We do not actually ‘touch’ objects in the way we usually think we do.

8. Imagining a New Force
• One billions and billions of times stronger than
gravity.
• One that, like gravity, loses strength as the
square of the distance.
• One that unlike gravity can be either attractive
OR repulsive.
This is the electric force.

9. There are two types of
particles that interact in
the electric force:
P (positive) and
N (negative)
Opposites attract, likes
repel.
P N
P Repulsive Attractive
N Attractive Repulsive

10. The Electric Force
Finally, Suppose we have equal numbers of
these two types of particles (P and N).
Well, we do. P is positively charged protons, N is
negatively charged electrons.
They clump together into atoms, which have an
overall neutral charge – thus preventing a big
rip or a big crush.

11. Coulomb’s Law
2
21 **
d
qqk
F =
• Force is proportional to charge.
• Charge is measured in Coulombs. 1C = 6.25*1018
electrons of
charge.
• Force is inversely proportional to the square of distance.
• k is a constant value (think of it as being ‘like pi’ but not 3.14).
q1 q2
d

12. Inverse Square Law
Author: Borb, GNU Free Documentation License
As distance from source increases, the area of a shell around the source
increases as the square of distance.
So if the number of ‘lines of force’ are constant, the density will decrease as the
square of distance.

13. • Outer most electrons are weakly held.
• These same outer electrons are responsible for most
of a substance’s chemical properties.
• Some substances hold electrons more weakly than
others (DEMO – hair vs. plastic).

14. Conservation of Charge
• In the processes you witness today no
electrons or protons are created or destroyed.
• Just as energy is conserved, so is charge
conserved – the universe’s net charge is a
constant.
• There are no known violations of this principle
(it’s more than a theory, we consider it a law).

15. Polarization
• Electrons are very light
(about 1/2000th
the mass of a
proton or neutron).
• They can easily be pushed
around by the electric force.
• Imagine the electron cloud
getting displaced slightly
from the nucleus at the
center...

16. Demo: Induced Polarization
• demo balloon on wall:
The wall is not charged, but the
balloon sticks – electrons in wall
get pushed around.

17. Example Problem
• When you rub a balloon on your hair, does it
become charged?
• Does your hair become charged?
• When you then stick the balloon to a wall
(assuming it is dry enough to work) is the
WALL charged?

18. Inherent Polarization: Water
• Water is a polar molecule
• The oxygen carries a partial
negative charge and the
hydrogens carry a partial
positive charge.
• Oxygen has a stronger ‘hold’
on electrons.
• Water can ‘hydrogen bond’
through these weak partial
charges, which makes water
unusually stable...and allows
us to have some fun.
Image courtesy Qwerter, GNU Free Documentation License

19. Demo: Water and an electric force
• Water is a dipole.
– dipole means there is a slight
charge separation.
• Water, since it is charged, will
interact with another charged
object.
+
- - - -

20. Atoms: attraction and repulsion
• Repulsion close
-electrons in each atom
push against each other
• No force far away –
atoms are overall neutral
• What of the attraction
region? Polarization at
work.
-0.5
0
0.5
1
1.5
0 1 2 3
Force(relative)
Distance (relative)
Force between two atoms

21. Example Problem
• Is a polarized object charged?

22. Electric Fields
An Electric Field is similar to a
gravitational field (we live in
Earth’s gravitational field).
It is also similar to a magnetic
field (you can see magnetic
field lines by pouring iron
filings on a magnet).
• A charged particle in an
Electric Field will experience a
force.

23. Microwaves – How they Work
• Water is polar.
• Microwaves are electromagnetic fields.
• The frequency of a microwave oven is near a
resonant frequency of rotation for the water.
• The water keeps getting banged back and
forth.
• Motion = heating. Things near the water get hit
by the water and are heated.

24. Microwaves: the picture
Electric field
Direction in
electromagnetic
wave
Time
Water molecule
Orientation

25. Conductors
• Electrons in conductors are very mobile.
• Will always separate so as to cancel the
electric field inside.
• Faraday Cage: using of metal to create a
structure that shields against electric fields.
No
Electric
Field
Inside
-
-
-
-
+
+
+
+
+
+
-
-
-
+

26. Van de Graff – Demo (it was broken last time I
tried to find it so if it has been fixed you will see it else, sorry!)
Schematic view of a classical Van De Graaf
generator.
1. hollow metallic sphere (with positive
charges)
2. electrode connected to the sphere, a
brush ensures contact between the
electrode and the belt
3. upper roller (for example in plexiglass)
4. side of the belt with positive charges
5. opposite side of the belt with negative
charges
6. lower roller (metal)
7. lower electrode (ground)
8. spherical device with negative charges,
used to discharge the main sphere
9. spark produced by the difference of
potentials
Image and text by Dake, Made available under Creative Commons Attribution ShareAlike 2.5

27. The Periodic Table and Chemical
Bonding

29. Metals
• Highly conductive of
heat and electricity.
• Ductile (may be pulled
into wires)
• Malleable (may be
pounded flat)

30. Nonmetals
• Poor conductors of both
heat and electricity.
• Solids are brittle – not
malleable or ductile.
• Many nonmetals are
gasses at room
temperature.

31. Metalloids
• Properties in between metals
and nonmetals.
• Semiconductors – basis of
modern civilization.
(May be more like a metal or
more like a nonmetal
depending on position –
closer to metals, metallic and
vice-versa)

32. Semiconductors in action
• Works like a garden
hose: squeeze down (G
to right hand side),
decrease flow of
electrons from Souce (S)
to Drain (D).
• Switching! On a tiny
scale. 731 million of
these in a chip ½” on a
side (Intel’s latest chips).

33. A brief Interlude: why is the periodic table
structured the way it is?
• The periodic table is a map of electronic structure.
• First two columns represent filling simplest “orbital” – 2
electrons may fit (except He is displaced)
• Last six columns represent filling of next simplest “orbital” – 6
electrons will fit.
• Similar for the inner 10 and the odd two rows of 14 displaced to
the bottom.

34. Giving names to some parts of the
periodic table:

35. A look across the table: periods
Some properties change in a regular way as you
go across a row (natural enough, as the
valence “shell” is filling up as you go).
• Size decreases
• Electronegativity (how much the atom wants an
electron) goes up.

36. Columns: Grouping up
All elements in the same column have the same
valence (outer) electron arrangement.
• Size goes up as you go down a column
• Electronegativity goes down
• Elements in the same column tend to have similar
properties
For example: Cu, Ag, Au all in one group – and are among
the few elements to be found naturally in pure forms; they
are unusually nonreactive)

37. Size

38. Electronegativity