Chapter 6.1: Particles and Forces
p 118
Atoms are made of 3 basic parts: proton, neutron, and electron
All matter in the universe can be constructed using these 3 particles
Electric Charge – property of matter that comes in two types called
positive and negative
Positive/Negative = attract
Positive/Positive = repel
Negative/Negative = repel
Elementary Charge – smallest unit of electric charge that is possible in
ordinary matter, represented by the lowercase letter e
+e = +
-e = -
+2e = ++
-2e = --
+3e = +++
-3e = ---
P119 Static Electricity
An object is neutral when its total charge is zero
Neutral – a condition where the total positive charge is cancelled by the
total negative charges. Matter is neutral most of the time.
Charged – An excess of positive or negative charge
Static Electricity – the buildup of either positive or negative charge; made
up of isolated, motionless charges
A tiny imbalance (less than 1 part per million!) of +/- charges causes static
electricity. If two neutral objects are rubbed together, the friction often
pulls some charge off one object and temporarily puts it on the other.
The forces between electric charges are very strong, that is why charged
objects do not stay charged very long.
Walk across carpet on a dry day, your body picks up excess negative
charges.
A door knob is neutral, but as you reach for it, your extra negatives repel
the doorknob’s negatives, making it slightly positive. Your negative skin is
attracted to the positive doorknob. The shock you feel is energy released
when your extra negatives jump to the doorknob.
P120 Inside the atom!
Electron: 1897 JJ. Thomson discovered that electricity flowing through a
gas gave off particles smaller than an atom and it had a negative charge.
He named it corpuscles, but eventually it was renamed electrons. Won
Nobel Prize in Physics in 1906 for discovery of the electron. He thought
atoms were just blobs of matter, and that anything could go through
them. (plum pudding theory)
Cathode Ray/Vacuum Tube
Nucleus – 1911 Ernest Rutherford, Hans Geiger, and Ernest Marsden
tested Thomson experiment. They shot positively-charged (++) helium
ions at extremely thin gold foil. They expected it to go through, but some
bounced back at them. Marsden was the person who actually did the
experiment; he was Rutherford’s student. Hans Geiger was his teaching
partner at Manchester University in England. This experiment proved
that atoms were not “blobs of matter” like Thomson thought.
Rutherford had already experimented with shooting particles at gold foil,
but asked Marsden to try it again to see if anything bounced back.
Marsden set up a screen covered with zinc sulfide (a glow-in-the-dark
powder) and if anything bounced back, it would hit the screen and cause
a poof of light to bounce off the screen. He did this in the dark, in order
to see the glowing puffs. They tried it with other elements, and the same
thing happened. Rutherford won Nobel Prize in Chemistry, and was mad,
because he considered himself a physicist.
It hit something. They realized that all the mass of a gold atom was
concentrated in a tiny, hard core at the center. Nucleus contains 99% of
mass. Positively charged proton was discovered to be part of the
nucleus, but that was only half of the mass.
Right after the electron was discovered, scientists knew that there also
had to be something positive to balance it out. After Rutherford
discovered the nucleus, he realized that this must be where the proton
was. He invented the word proton.
Neutron – 1932 James Chadwick found another particle in the nucleus
with NO Charge and it weighed the same as the proton. Rutherford was
actually the first to suggest that there was something neutral in the
nucleus, but couldn’t prove it yet.
Electron – particle with electric charge (-e) found inside an atom, but
outside the nucleus
Proton – particle with electric charge (+e) found in nucleus of atoms
Neutron – particle with zero charge found in nucleus of atoms
Nucleus – tiny core at the center of an atom containing most of its mass
and all of its positive charge
P121
Protons and neutrons are grouped together in the nucleus, which is at
the center of the atom. They are more massive than electrons.
A proton is 1,836 times heavier than an electron.
All atoms have both protons and neutrons in nuclei EXCEPT a simple
hydrogen atom, which has 1 proton and 0 neutrons.
Electrons define the volume of the atom. They take up the space outside
of the nucleus in a region called the “electron cloud.”
The diameter of an atom is really the diameter of the electron cloud.
The electron cloud is more than 10,000 times larger than the nucleus.
If an atom were the size of a football stadium, the nucleus would be a
pea at the center, while gnats (electrons) buzzed around the stadium.
Mostly empty space!
P122 - 4 Forces of Nature Inside Atoms:
1. Electromagnetic Forces – electrons are bound to the nucleus because
they are attracted to protons.
Electrons don’t fall or crash into the nucleus because they are moving
around (kinetic energy, like Earth orbiting Sun)
2. Strong Nuclear Force – all positively charged protons in nucleus repel
each other. So what holds nucleus together?
It is actually called “Strong Nuclear Force” – the strongest force known to
science. This force attracts neutrons and protons to each other –
preventing the protons from repelling each other.
In every atom heavier than helium, there is at least ONE neutron for
every proton in the nucleus.
3. Weak Force – If you leave a single neutron outside the nucleus, the
“weak force” will cause it to break down into a proton and electron.
It doesn’t do anything in a stable atom, but plays a part when atoms
break apart or decay.
Gravity – force of gravity on atoms is the weakest force of all that acts on
them, but it does act on atoms just like any other type of matter.
P123 Atoms of Different Elements:
Atomic Number – the number of PROTONS in the nucleus. Atomic
number determines what the element is.
Hydrogen
1 proton
1 electron
Helium
2 protons
2
Lithium
3 protons
3
Carbon
6 protons
6
Nitrogen
7 protons
7
Oxygen
8 protons
8
= zero charge
The number of protons and electrons in a COMPLETE atom is always
equal!
Now for the strange, the weird, the unstable, the incomplete ATOMS, the
exceptions, the stuff about atoms that doesn’t follow the rules:
P124
ISOTOPES – Atoms of the same element that have different numbers of
NEUTRONS in the nucleus
Lithium: 3 protons, 3 electrons, but sometimes 3 neutrons, but most of
the time 4 neutrons. Called Lithium 6 or Lithium 7. Lithium 7 is naturally
occurring and is much more common than Lithium 6, which scientists
think came here from other stars.
Carbon is usually 6 protons, 6 electrons, 6 neutrons for a total atomic
mass of 12 (6protons+6neutrons)
BUT, there is:
Carbon 13 with 6 protons and 7 neutrons
Carbon 14 with 6 protons and 8 neutrons
They are ALL carbon because they all contain 6 protons, but they are
different isotopes of carbon.
P125
Radioactivity
https://www.youtube.com/watch?v=ktvTqknDobU
Almost all elements have one or more isotopes that are stable. “Stable”
means the nucleus stays together.
The nucleus of an atom becomes unstable if it contains too many or too
few NEUTRONS compared to the number of protons.
If the nucleus is unstable, it breaks apart.
Carbon has two stable isotopes: Carbon 12 and Carbon 13. Carbon 14 is
radioactive because it has an unstable nucleus. Carbon 14 eventually
becomes Nitrogen 14.
It will eventually change into a more stable form. Radioactivity is a
process in which the nucleus emits particles or energy as it changes into a
more stable isotope. Radioactivity can change one element into a
completely different element.
Alpha Decay: a type of radiation where the nucleus ejects two protons
and two neutrons (which is the nucleus of helium 4 atom. Alpha
radiation is actually fast-moving “Helium” nuclei.
When alpha decay occurs, the atomic NUMBER is reduced by two
because two protons are removed, but the atomic MASS is reduced by
four because two neutrons are ejected with the two protons.
Example: Uranium 238 goes through alpha decay to become Thorium
234.
Beta Decay: a neutron in the nucleus splits into a proton and an electron.
The proton stays in the nucleus, but the electron is ejected and is called
beta radiation.
During beta decay, the atomic number increases by 1 because one new
proton is created. The mass number stays the same because the atom
lost a neutron but gained a proton.
https://www.khanacademy.org/science/chemistry/nuclearchemistry/radioactive-decay/v/types-of-decay
Gamma Decay: This is how the nucleus gets rid of excess energy. The
nucleus emits pure energy in the form of gamma rays. The number of
protons and neutrons stays the same.
https://www.youtube.com/watch?v=OHNtSne2Uxk