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History of the Atom – Summary Sheet – ANSWER KEY
Scientist
Summary about Theory
Reasons for it’sfailure
Democritus
•^
first to proposed an atomic model
-^ all matter could be divided into smaller and smaller pieces until a single
INDIVISIBLE
particle was reached.
•^
called this particle an
ATOM
•^
believed that different atoms are of different
SIZES
, have regular
GEOMETRIC SHAPES
and are in constant
MOTION
•^
there is
EMPTY
space between atoms.
•^
More people believedAristotle thanDemocritus
Aristotle
•^
severely criticized Democritus’s theory, arguing that the idea of atoms in continuous motion in a void is illogical.
-^ supported the
FOUR ELEMENT
theory of matter. (proposed by Empedocles a century earlier)
•^
based on the idea that all matter is made up of
FOUR
basic substances:
EARTH,
WATER,
AIR
and
FIRE
•^
each of these basic substances had different combinations of four specific qualities: dry, moist, cold, and hot
-^ was the prevailing model for almost 2000 years, including the period of
ALCHEMY
in the Middle Ages
•^
the scientific revolutionin physics and the newemphasis, in the 18thcentury, on quantitativestudies showed that toomany of theexplanations andpredictions usingAristotle’s theory werefalse.
Dalton
•^
expanded upon the atomic theory proposed by Democritus.
-^ consisted of the following statements.^
All matter is composed of tiny, indivisible particles called
atoms.
^
All atoms of an element have
IDENTICAL
properties.
^
Atoms of different elements have different properties. ^
Atoms of two or more elements can combine in constant
RATIOS
to form new substances.
^
In chemical reactions, atoms join together or separate from each other but are not destroyed.
-^ this theory explains of the law of
CONSERVATION OF MASS
•^
atoms are neither created nor destroyed in a chemical reaction.
-^ they are only
REARRANGED
during chemical reactions, (i.e. you must end up with the same number and kinds of atoms
after a chemical reaction as you had at the beginning.)
-^ Therefore, there will be no change in mass during chemical reactions.
•^
Thomson’s experimentswith cathode ray tubesdemonstrated theexistence of subatomicparticles (i.e. particlesthat make up atoms) notjust “solid indivisibleparticles”
History of the Atom – Summary Sheet – ANSWER KEY
Scientist
Summary about Theory
Reasons for it’sfailure
•^
This theory explains the
LAW OF CONSTANT COMPOSITION
•^
atoms combine to form molecules in a fixed ratio in a given chemical reaction.
-^ Since the atoms of an element have identical properties, such as mass, and combine in constant ratios, every compoundmust have a fixed, definite composition.
Thomson
•^
performed experiments using
CATHODE
ray tubes
•^
proposed that the cathode rays (the mysterious rays that where seen in a cathode ray tube) were subatomic particles
-^ called the subatomic particles
electrons.
•^
that negatively charged electrons are distributed inside the atom, which is a positively charged sphere consisting mostly ofempty space
Nagaoka
•^
represented the atom as a large, positively charged sphere surrounded by a ring of negative electrons
•^
Rutherfords “gold foil”experiment disprovedthe model that the atomswas mostly empty space(either Thomson’s orNagaoka’s model)
Rutherford
•^
famous
GOLD FOIL or ALPHA PARTICLE
experiment
•^
“shot” alpha particles (small, positively charged particles produced by radioactive decay) through very thin pieces of gold foil.
-^ Prediction
- all the alpha particles would travel through (in a straight line) the foil largely unaffected by the atoms of gold.
(based on Thomson’s model of the atom and the idea that the atom was composed mostly of empty space)
-^ Result
- most of the alpha particles did pass easily through the foil, a small percentage of particles was deflected at
LARGE
angles.
•^
Conclusion
- an atom must contain a positively charged
CORE
, which is surrounded by a predominantly empty space
containing negative electrons
-^ called the core the
nucleus
•^
coined the word
proton
for the smallest unit of positive charge in the nucleus.
•^
it could not explain thefollowing.
- if the nucleus of an
atom contained severalpositive protons thatrepelled each other,how did it staytogether?
- why didn’t the
negatively chargedelectrons rush towardand crash into thepositively chargednucleus?
3. electrons in motion
around the nucleusshould continuouslygive off radiation in theform of light and oneshould observe a
History of the Atom – Summary Sheet – ANSWER KEY
Scientist
Summary about Theory
Reasons for it’sfailure
each energy level, is associated with a
FIXED
distance from the nucleus.
an electron with a particular energy travels along a
THREE
dimensional pathway called a
SHELL
or
ORBIT.
shells are designated by the
PRINCIPLE QUANTUM
number,
n
, which can be any positive integer from 1 to infinity.
Historically, these shells have also been designated by the symbols K, L, M, N,
Theory depended on these assumptions:
an electron can travel indefinitely within an energy level without
LOSING
energy;
the
GREATER
the distance between the nucleus of the atom and the energy level, the
GREATER
the energy required
for an electron to travel in that energy level
an electron cannot exist
BETWEEN
orbits, but can move to a higher, unfilled orbit if it
ABSORBS
a specific quantity of
energy, and to a lower,
UNFILLED
orbit if it
LOSES
energy.
Transitions^ •^
if^
ADDITIONAL
energy is supplied to an electron in a given energy level, then the electron can “jump” to a higher,
UNFILLED
energy level, farther away from the nucleus
•^
this jump is called transition.
-^ the quantity of energy (a
QUANTUM)
required to cause a transition is equivalent to the
DIFFERENCE
in energy between the
energy levels.
-^ when an electron
DROPS or FALLS
back to a lower energy level, it
RELEASES
an equivalent amount of energy. (observed
as a line in the line spectrum for the element being observed)
-^ when an electron is in the lowest energy level that it can occupy, it is said to be in its
ground state
•^
when all the electrons in an atom are in the lowest possible energy levels, the
atom
is said to be in its
ground state.
Why was this theory important?^ •^
suggested that the properties of the elements could be explained by the arrangement of electrons in orbits around thenucleus. hypothesized that the stable, unreactive nature of certain elements was due to filled outermost energy levels.
-^ it explained both the stability of electrons in atoms and atomic emission spectra, and it enabled scientists to calculate theactual energies of the different energy levels within the hydrogen atom.
