Transcript the history of the atom! - Bangor Area School District

Atomic Structure
Historical look at the Atom
440 BC
Democritus
1803
John Dalton
1897
Joseph John (J.J.) Thomson
1911
1922
1932
Ernest Rutherford
Niels Bohr
James Chadwick
Democritus

DISCONTINUOUS THEORY OF MATTER
• All matter is composed of atoms, which are bits of matter too small to
be seen. These atoms CANNOT be further split into smaller portions.

Aristotle  Continuous theory of Matter
Where does the term atom come from?
In Greek, the prefix "a" means "not" and the word "tomos"
means cut. Our word atom therefore comes from
atomos, a Greek word meaning uncuttable.
Democritus
John Dalton (~1803)
• proposed the first theory of the nature of matter in
stating that all matter was composed of atoms.
• Dalton based his theory on three scientific
principles:
o the Law of Conservation of Mass (Lavoisier)
o the Law of Definite Proportions (or Law of Constant
Composition) (Proust)
o the Law of Multiple Proportion (his own)
Law of Conservation of Mass
 Established
in the 1760’s by Antoine Lavoisier
 in any chemical reaction, mass is neither created nor
destroyed (reactants will have the same mass as
products in a chemical change)
Ex. C6H12O6
180 grams
Ex.
CO2
+ O2
70 grams
+ H2O
110 grams

140 grams
CO2
+ H2O
110 grams

140 grams
C6H12O6 + O2
180 grams
70 grams
Law of Definite Proportions
Established in 1799 by the French Chemist Joseph
Louis Proust

States that in a pure compound, the elements are
always present in the same definite proportion by
mass.

Ex. H2O
2.0 g of hydrogen will combine with 16g of oxygen to
produce 18 g of H2O
and
4 g of hydrogen will combine with 32 g of oxygen to produce
36 g of H2O

Ratio of H:O is always 2:16 or 1:8 (by mass)

Law of Multiple Proportions

When two or more compounds are formed
from the same pair of elements, the masses of
one element combines with a fixed mass of the
other element to form simple whole-number
ratios of those elements
Example:
Two different compounds are formed by the elements carbon and
oxygen. The first compound contains 42.9% by mass carbon and
57.1% by mass oxygen.
The second compound contains 27.3% by mass carbon and 72.7%
by mass oxygen.
57.1 g O / 42.9 g C
=
1.33 g O per g C
72.7 g O / 27.3 g C =
2.66 g O per g C
Dividing the mass O per g C of the second (larger value)
compound:
2.66 / 1.33 = 2
John Dalton cont.
Four basic ideas in Dalton’s chemical atomic theory:
(Postulates of Atomic Theory)
Based upon 3 Laws:
 Matter is composed of tiny indivisible
particles
 all atoms of the same element are identical in
mass, but differ from atoms of other elements
 atoms only combine in definite fixed
numerical ratios such as 1:1, 1:2, 2:1
 chemical change consists of a reshuffling of
atoms, the individual atoms themselves remain
intact
John Dalton
The 1st Subatomic Particle
General Info
Electrode ==> substance that allows
electrons to enter or exit
Anode ==> positively charged electrode
Cathode ==> negatively charged
electrode
William Crookes
Determined the flow of current was in a
straight path
 The flow always originated from the
cathode (negative electrode) and flowed to
the anode (positive electrode)
 Called these “rays” cathode rays




1897, used magnetic and electric fields to
alter the cathodes path which showed the
“rays” must be negatively charged
Determined the charge to mass ratio of
this negative particle (e-/m)
Also felt there was a positive balance to
this negative charge
negative charged
particle
positive charged
matter
Robert Millikan

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American Physicist
~ 1909 determined the charge of an electron and
the mass of an electron
Oil drop experiment
Used Thomson’s charge to mass ratio (e-/m)
 1.602 x 10-19 coulombs (charge of electron)
 9.109 x 10-28 g (mass of electron)
(100 years later …within 1% of the known value)
Sir Ernest Rutherford
Positively a great scientist
Student of J.J. Thomson

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1909 conducted the alpha scattering
experiment (better known as the ‘gold foil
exp.’)
used a beam of high speed alpha
particles to study the interaction with
metal foils (gold mostly)
found that over 99% of the alpha
particles went through the foil
determined the atom was mostly empty
space with a positively charge core
(nucleus)



Deflections were a result
of electrical repulsion
Determined nearly all the
mass of an atom is located
in the nucleus (“little nut”)
This positive charged core
only occupied a small
portion of the atoms
volume
DUAL NATURE OF LIGHT
Characteristics of energy as waves

Wavelength … symbol  lambda



Amplitude



Represents the height of the crest (or depth of a trough)
Represents the intensity of the radiation (ie. visible light)
Frequency …



Distance between any point on a wave to a corresponding point on the next wave
Expressed in meters (m)… as the wave shortens  nm (10-9)
Number of cycles a wave passes a given point every second
Expressed as.. 1
or waves
or sec-1
or Hertz
sec
sec
Speed …


symbol  nu
symbol  c
Represents the speed of light in a vacuum
3.00 x 108 m
sec
EM Spectrum
Radio, Microwave, Infrared, Visible light,
Ultraviolet, X-Rays, Gamma rays
 Emission spectra



represents energy released by an electron as it
returns to ground state from excited state
Absorption spectra

represents energy absorbed by the electron as
an electron moves to an excited state
Formulas you need
l = c
u
Wavelength = speed of light
frequency
and
E=hxu
Energy = Planck’s constant x freq.
Terminology



Ground state – lowest energy level for an electron
when an atom is in its most stable energy state
Excited state – a higher energy state of an electron
(energy is absorbed by electron)
Electromagnetic spectrum – range of energy that
exhibits wave-like behavior as it travels through
space.
 Consists
of both electric and magnetic field
components.
 classified according to the frequency of its wave.
Bohr Model
Question: Why didn’t the atom collapse on itself?
ANSWER:
 electrons move within defined “orbits”
 electrons have a “fixed” energy within the orbit
and do not radiate energy as they move
(DUE TO
UNDERSTANDING OF THE ENERGY ASSOCIATED TO THE ELECTROMAGNETIC SPECTRUM)
 electrons can move from orbit to orbit, but can
not be between orbits
 the orbits are considered “energy levels”
 the Bohr model was a precursor to the “quantum
mechanical model”
Bohr Model
-
+
ORBITS
(CONCENTRIC ENERGY
LEVELS AROUND THE
NUCLEUS)
NUCLEUS
Parts of Atom

Electrons



Protons

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
negatively charged subatomic particle
mass = 9.11 x 10 -28 g
positively charge subatomic particle
mass = 1.67 x 10 -24 g
Neutron – Discovered by James Chadwick (~ 1932)


Subatomic particle with no charge
mass = 1.67 x 10 -24 g
Terminology
Atomic number
 number of protons in the nucleus
 due to atom’s electrical neutrality, also indicates the number of electrons
 on the periodic table above or below the symbol of the atom
Isotope
 atoms with the same number of protons but with different number of neutrons
Atomic mass
 weighted average of all the isotopes of a particular element
 on the periodic table above or below the symbol of the atom
Mass number
 number of neutrons and protons in the nucleus
 mass number - atomic number = neutrons