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Matter & Atoms
8th Grade Science
What’s a Matter?
volume & mass
Classifications of Matter
1.Substances
always the same composition (makeup)
Elements (1 kind of atom) see Periodic Table
Examples: Gold (Au), Lead (Pb), Mercury (Hg)
Molecules (2 or more kinds of atoms chemically
bonded that act as a unit)
Examples: Hydrogen (H), Bromine (Br), Sugar
Compounds (2 or more elements chemically
joined in a specific combination)
Examples: Water (H2o),Carbon Dioxide CO2
Classifications of Matter
2. Mixtures
Matter that varies in composition. 2 or more
substances that are blended but not bonded.
Heterogeneous Mixtures - not evenly mixed
Examples: Trail Mix, Granite, Smoke
Homogeneous Mixtures - evenly mixed/no bond!
Examples: Brass, Natural Gas, Windex
Solutions – not bonded mixture
Solutions vs. Compounds
Solutions
Compounds
Composition
Substances evenly
mixed together
Changes in
composition
Solution still the
same but relative
substances differ.
Properties of parts
Substances keep
their own
properties when
mixed.
Atoms bonded
together in the
same combination
Changes
composition
makes a new
compound with
new properties.
Properties of the
compound are
different than
atoms that make it
up.
Models of the Atom
a Historical Perspective
The Greeks named these particles atoms, a
term that means “cannot be divided.”
• Early philosophers
didn’t try to prove their
theories by doing
experiments as
scientists now do.
• Their theories were the
result of reasoning,
debating, and
discussion—not of
evidence of proof.
Early Greek Theories
• 400 B.C. - Democritus thought matter
could not be divided indefinitely.
• This led to the idea of atoms in a void.
fire
Democritus
earth
air
water
• 350 B.C - Aristotle modified an earlier
theory that matter was made of four
“elements”: earth, fire, water, air.
• Aristotle was wrong. However, his
Aristotle theory persisted for 2000 years.
• During the eighteenth century, scientists,
especially the French, began debating the
existence of atoms once more. Newton
proposed held together by force.
• They found that certain substances couldn’t
be broken down into simpler substances.
• Scientists came to realize that all matter is
made up of elements.
• An element is matter made of 1 kind of atom.
Dalton’s Concept
1800 -Dalton an English schoolteacher
proposed a modern atomic model based
on experimentation not on pure reason.
Dalton pictured an atom as a hard sphere
that was the same throughout.
Dalton’s Concept
•
•
•
•
All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine in
constant ratios to form compounds.
• Atoms are rearranged in reactions.
• His ideas account for the law of conservation of
mass (atoms are neither created nor destroyed)
and the law of constant composition (elements
combine in fixed ratios).
Scientific Evidence
• In 1870, the English scientist William
Crookes did experiments with a glass tube
that had almost all the air removed from it.
• The glass tube had two pieces of metal
called electrodes sealed inside.
• The electrodes were connected to a battery
by wires. Crookes’s tube is known as a
cathode-ray tube, or CRT.
• An electrode is a
piece of metal that
can conduct
electricity.
• One electrode, called
the anode, has a
positive charge. The
other, called the
cathode, has a
negative charge.
Models of the Atom
• A shadow of the
object appeared at
the opposite end
of the tube.
• The shadow
showed Crookes
that something was traveling in a straight line
from the cathode to the anode, similar to the
beam of a flashlight. Crookes hypothesized that
the green glow in the tube was caused by
cathode rays, or streams of particles.
Models of the Atom
Cathode Rays
• Many scientists were not convinced that
the cathode rays were streams of particles.
• In 1897, J.J. Thomson, an English physicist,
tried to clear up the confusion.
• He placed a magnet beside the tube from
Crookes’s experiments.
Models of the Atom
Cathode Rays
• The beam is bent in the direction of the
magnet.
• Light cannot be bent by a magnet, so the
beam couldn’t be light.
• Thomson concluded that the beam must
be made up of (-) negatively charged
particles of matter that came from the
cathode.
Models of the Atom
Thomson’s Model
• These negatively charged particles are now
called electrons. Electrons were 1st proposed in
1874 by G. Johnstone Stoney Thomson. He also
inferred that electrons are a part of every kind of
atom.
• If atoms contain one or more negatively
charged particles, then all matter, which is
made of atoms, should be negatively charged
as well.
Thomson’s Atomic Model
• The negatively charged electrons were spread
evenly among the positive charge.
• The negatively charged electrons and the
unknown positive charge would then neutralize
each other in the atom.
• The atom is neutral.
Thomson’s Atomic Model
• Later discovered not all atoms are neutral.
#Electrons within an element can vary.
• More positive charge than negative electrons
= overall positive charge.
• More negative electrons = overall negative charge.
• Rutherford wanted to see what would happen
when they fired fast-moving, positively charged
bits of matter, called alpha particles +, at a thin
film (400nm) of a metal such as gold surrounded
by a fluorescent screen.
Models of the Atom
Rutherford’s Results Fail!
• His prediction -speeding alpha
particles would pass right
through the foil and hit the
screen on the other side.
• Rutherford reasoned - the thin,
gold film did not contain enough
matter to stop the speeding alpha
particle or change its path.
Rutherford was shocked when his
students Hans Geiger & Ernest
Marsden rushed in to tell him that
some alpha particles were veering
off at large angles.
The Model Fails
Positively charged alpha particles moving with such
high speed that it would take a large positive
charge to cause them to bounce back.
The Proton
• The actual results did not
fit this model, so
Rutherford proposed a
new one.
• He hypothesized – most of the
mass of the atom and its positive
charge is in the nucleus.
Models of the Atom
The Proton
• In 1920 scientists identified
the positive (+) charges in
the nucleus as protons.
• Rutherford’s new model of
the atom fits the
experimental data.
• Most alpha particles could
move through the foil with
little or no interference.
Electricity is called “cathode rays” when passed
through an evacuated tube.
These rays have a small mass and are negative.
Thompson noted that these negative subatomic
particles were a fundamental part of all atoms.
1) Dalton’s “Billiard ball” model (1800-1900)
Atoms are solid and indivisible.
2) Thompson “Plum pudding” model (1900)
Negative electrons in a positive framework.
3) The Rutherford model (around 1910)
Atoms are mostly empty space.
Negative electrons orbit a positive nucleus.
The Neutron
• According to Rutherford’s model, the only
other particle in the atom was the proton.
• He proposed that another particle must be in
the nucleus to account for the extra mass.
• The mass of most atoms is at least twice as
great as the mass of its protons.
• The particle, which was later call the
neutron, would have the same mass as a
proton and be electrically neutral.
Models of the Atom
The Neutron
• 20 Years Later in 1932
-The model of the atom
was revised again to
include the newly
discovered neutrons in
the nucleus by James
Chadwick.
• The nuclear atom has
a tiny nucleus
tightly packed with
positively charged protons
and neutral neutrons.
Bohr Models
• Then, electrons would travel in
orbits around the nucleus.
• A physicist named Niels Bohr even
calculated energy levels for the hydrogen
atom.
• However, scientists soon learned that electrons
are in constant, unpredictable motion and
can’t be described try & explain the orbits.
Bohr’s Model
• Electrons orbit the nucleus in “shells”
• Electrons can be bumped up to a higher shell
if hit by an electron or a photon of light.
Be
B
Al
4 p+
5 n°
5 p+
6 n°
13 p+
14 n°
There are 2 types of spectra: continuous
spectra & line spectra. It’s when electrons fall
back down that they release a photon. These
jumps down from “shell” to “shell” account for
the line spectra seen in gas discharge tubes
(through spectroscopes).
The Electron Cloud Model
• The electrons are more
likely to be close to the
nucleus. They are
attracted to the positive
charges of the proton.
• Electrons travel in a
region surrounding the
nucleus, called the
electron cloud.
Identifying Numbers
• The atomic number of an element is the
number of protons in the nucleus.
• Atoms of an element are identified by the
number of protons because this number
never changes without changing the identity
of the element.
Number of Neutrons
• These 3 kinds of carbon atoms are called
isotopes. Isotopes are atoms of the same
element that have different numbers of neutrons,
but the same # of protons.
•
Mass Number
• The total masses of the protons and neutrons in an
atom make up most of the mass of an atom. The
mass number is the neutrons plus the protons.
• You can find the # of neutrons in an isotope by
subtracting the atomic # from the mass #.
• Electron 1/1,800 of a proton. (We don’t calculate!)
Radioactive Decay
• Many atomic nuclei are stable when they
have about the same number of protons
and neutrons. Some nuclei are unstable
because they have too many or too few
neutrons. This is especially true for
heavier elements such as uranium and
plutonium.
• The release of nuclear particles and energy
is called radioactive decay.
• In these nuclei, repulsion builds up. The
nucleus must release a particle to become
stable.
Radioactive Decay
• When the particles that are ejected from a
nucleus include protons, the atomic number of
the nucleus changes. When this happens, one
element changes into another (Transmutation).
• A smoke detector makes use of
radioactive decay. It contains
americium-241, undergoes
transmutation by ejecting
energy and an alpha particle.
Radioactive Decay
• The fast-moving alpha particles enable the air to
conduct an electric current. As long as the electric
current is flowing, the smoke detector is silent.
• The alarm is
triggered when
the flow of
electric current
is interrupted by
smoke entering
the detector.
Changed Identity Alpha Particles
• When americium expels an alpha particle,
it’s no longer americium, it becomes the
element that has 93 protons, neptunium.
Loss of Beta Particles
• During this different kind of transmutation, a
neutron becomes unstable and splits into an
electron and a proton. The proton, however,
remains in the nucleus.
• The electron, or beta particle, is released with a
large amount of energy, so the atomic # of the
element that results is greater by one..
• A beta particle is a high-energy electron that
comes from the nucleus, not from the electron
cloud.
•
Rate of Decay
• Radioactive decay is random.
• The rate of decay of a nucleus is measured
by its half-life.
• The half-life of a radioactive isotope is the
amount of time it takes for half of a sample
of the element to decay.
The radioactive decay of
unstable atoms is steady,
unaffected by conditions
(weather, pressure,
magnetic or electric
fields, and even chemical
reactions.
Radioactive
Decay
• Carbon-14 is used to determine the age
of dead animals, plants, and humans.
• In a living organism, the amount of
carbon-14 remains in constant balance
with the levels of the isotope in the
atmosphere or ocean.
• This balance occurs because living
organisms take in and release carbon.
• Geologists examine the decay of
uranium to age rocks.
Uses of Radioactive Isotopes
• Tracer elements are used to diagnose disease
and to study environmental conditions.
• The radioactive isotope is introduced into
a living system such as a person, animal,
or plant.
• It then is followed by a device that detects
radiation while it decays.
• The isotopes chosen for medical purposes have short
half-lives, which allows them to be used without the
risk of exposing living organisms to prolonged
radiation. The isotope iodine-131 has been used to
diagnose problems with the thyroid. Also used to detect
cancer, digestion problems, & circulation.
Environmental Uses
• Tracers such as phosphorus-32 are injected
into the root system of a plant.
• A detector then is used to see how the plant
uses phosphorus to grow and reproduce.
• Radioisotopes also can be placed in
pesticides or the water cycle and followed to
see the impact to the ecosystem.