atomictheory
A T O M I C ... S T R U C T U R E
and
The Discovery of Sub-Atomic Particles ATOMIC THEORIES:
... Democritus (Ancient)
... Dalton (Modern)
LAW OF CONSERVATION OF MASS
... The Total Mass Entering a chemical reaction = the total mass leaving the reaction. LAW OF DEFINITE PROPORTIONS (Dalton's Law)
... Every compound has a definite ratio of elements by mass. (Ah... Formulae!) LAW OF MULTIPLE PROPORTIONS
... Some compounds exhibit whole number ratios of elements.
H
2O, H
2
O,
2
NO, N
2
O, N
2
O,
3
N
3
O,
4
FeO, Fe
2
O,
3
Fe
3
O
4
THE LAW OF GAY LUSSAC
... The combining ratios in reactions are of numbers.
small whole
The Above Laws indicate that there are basic particles (atoms) that make up matter. FARADAY'S DISCOVERY--
... Electricity decomposes compounds into elements. On re-forming, electricity is re-generated.
... This shows that the binding force between atoms in a compound is electrical.
THE DISCOVERY OF PARTS OF THE ATOM(Late 19th and early 20th century action)--
... The high voltage Induction Coil, "Sparky" gave the energy needed to probe the atom.
... Discharge Tubes-- lowering air pressure in a tube allowed the to travel great distances (many meters).
Cathode Rays
THE DISCOVERY OF THE ELECTRON--
... The Tube of Sir William Crookes--
The Rays--
... travel from Cathode (negative) to Anode (positive). Hence they are NEGATIVE.
... are easily stopped by thin metal obstacles (the Iron Cross).
... travel in Rectilinear Propagation because the shadow of the cross is sharp.
... are attracted to positive electric charges and repelled by negative charges placed beside the tube
... are bent at right angles by magnet fields and they cause fluorescence on the glass and upon certain minerals. THE DISCOVERY OF THE MOMENTUM OF THE RAYS--
The Paddle Wheel Tube
(the tube of Jean Perrin) showed that the Cathode Rays have mass and velocity (momentum) because they push the paddles. Great fluorescent colors are demonstrated by paint on the paddles.
particles"Electrons".
... The rays therefore must be and were named
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THE MEASUREMENT OF THE CHARGE TO MASS RATIO OF ELECTRONS--
...
The Tube of Sir JJ Thompson--
The bent the cathode rays in a magnetic field and showed their path on a fluorescent screen. This enabled the calculation of the . Higher charge would bend the beam more, higher mass would bend the beam less.
candy cane-shaped tube Charge to Mass Ratio e/m = ratio = 1.8 X 10 coulomb/gram.
8THE DETERMINATION OF THE CHARGE OF THE ELECTRON--
Millikan Oil Drop Experiment to measure the actual charge on an electron. X-Rays would add or subtract electrons to the microscopic droplets. By varying the electric charge needed to balance the weight of the droplet, three biggies were discovered:
... 1. All electrons are identical.
... 2. The electron is a basic particle of electricity.
... 3. The charge of the electron:
e = 1.6 X 10-19coulomb (c) o
f charge.
Here is the apparatus:
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THE CALCULATION OF THE MASS OF THE ELECTRON--
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... The Mass of the electron is determined from e/m ratio and the charge:
e/m = ratio
so
m = e/ratio
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hence: m = 1.6 X 10 c /1.8 X 10 c/g
-29 WOW! That’s tiny! m = 9.11 X 10 gram/electron
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THE DISCOVERY OF IONS AND THE PROTON--
was built to see if there are rays from the anode. There were positive rays, but not from The Canal Ray Tube
the anode. They were produced by the impact of electrons with atoms of gas in the tube.
... The Canal Ray Tube
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The collisions knocked electrons off the atoms creating positive IONS. The e/m ratios of the ions (in a JJ tube) depended on which gas was used. When using the simplest gas, Hydrogen, we found the simplest ion, the Hydrogen nucleus called the "PROTON".
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THE DISCOVERY OF ISOTOPES--
The Mass Spectroscope is a refined Sir JJ tube built for accurate measurements. When ions were sent through it, they separated into several beams according to different masses (their charges were the same). These different masses for the same element were called ISOTOPES
(in the same place on the periodic table).
THE DISCOVERY OF THE NEUTRON--
ISOTOPES NEUTRAL
To explain we postulated a particle whose mass was equal to that of the Proton. This way we could account for a change in mass without changing which element was present.
The NEUTRON was discovered in 1932 by Chadwick using a piece of jam jar paraffin to react with radiation.
THE DISCOVERY OF X-RAYS--
Roentgen was experimenting with discharge tubes and found fluorescent substances were glowing around in his lab including places behind barriers. Powerful! They are created when electrons strike a metal target.
X-rays are electromagnetic waves above the Ultra-Violet on the spectrum.
X-rays are used to determine:
...The Atomic Number(the number of protons in the nucleus. The wave length of X-rays depends on number of protons).
... The Structure of crystals(X-ray diffraction studies).
THE DISCOVERY OF RADIOACTIVITY AND ITS THREE RAYS--
Becquerel, experimenting with fluorescent minerals, found that Uranium ore on his desktop exposed film in the drawers below with the shadow of a key thereon.
Marie Curie analyzed Uranium ore and discovered new radioactive elements including Radium.
Lord Rutherford, using Marie's Radium, found three rays coming therefrom:
Alpha rays-- Positive Helium ions. Small deflection due to high mass.
Beta rays-- Electrons. Large deflection due to very low mass.
Gamma rays-- Electromagnetic waves of very high energy ... WOW!
Properties of Radioactivity : Cause ionization, Discharge electroscope, Cause fluorescence, Expose photographic film, Destroy livingcells, Promote nuclear reactions.
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THE SIZE OF THE ATOM --
... For Copper, weigh out 1 mole-- 63.5 g/mol.
... Make it into a cube. Ah, it's 2 cm per side.
... This cube contains 6 X 10 atoms (1 mol)
23... To find the number of atoms per side of the cube, take the cube root of 6 X 10. It's about 10atoms per side.
238... Now divide the 2 cm per side by the 10 atoms per side, and you get about 10 cm per atom.
8-8... This is called the (the size of an atom).
Angstrom unit
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THE SIZE OF THE NUCLEUS--
Lord Rutherford shoots Alpha Particles (Helium nuclei) through a thin piece of gold foil.
SHOCK! most Alphas go right on through. Hence the atom must be mostly empty space!
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A very few Alphas are deflected however. From the statistics of the deflection patterns, the size of the nucleus is calculated.
-13
About 10 cm diameter!
-8
Compared to the size of the atom (10 cm), the nucleus is "The flea in Yankee Stadium".
DISCOVERY OF THE ORBITALS OF THE ELECTRONS:
Spectroscopy--
When electrical discharge is sent through gases in a tube, and the light emittted passed through a prism, we get a spectrum of bright lines. These lines are the spectral "finger prints of the atom"
From the line spectra we learn
..... Which elements are present (like in the stars).
..... The arrangement of electrons around the nucleus, called the
Electronic Configuration (the Goose Chart).
NUCLEAR PARTICLES --
Proton mass = 1 g/mol (amu)!charge = +1.
Neutron mass = 1 g/mol charge = 0.
Electron mass = 1/2000 g/mol charge = -1
Isotopes have a different number of neutrons.
p + n = mass
Atomic Weight is the average of the isotopes’ masses. .
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NUCLEAR REACTIONS --
Top numbers are the mass numbers (protons + neutrons).
Bottom numbers are the charge numbers (atomic numbers).
The sum of the numbers on the right must equal the sum of the numbers on the left.
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He + Be ---> C + n 24496120
1.
n + U ---> Np + e 019223893239-1
0.
NUCLEAR FISSION & THE CHAIN REACTION --
Transmutation-- Changing an element into a different element. Let’s make a new element!n + U ---> Np + e 0192238 93239-1
0Oooooh… a new element!Nuclear Fission-- The breaking up of an isotope into smaller elements.
U + n ---> Ba + Kr + 3 n + ENERGY!!92235015614136920
1Oh, oh… U fissions! And the three new neutrons are available to continue a chain
reaction!
92235NUCLEAR BOMB--Needs a Critical Mass to sustain a chain reaction.
The Critical Mass (about 50 kilograms) is the minimum amount of U needed to sustain a chain reaction. Remember that the neutrons must hit the nucleus of the atom (the flea in
Yankee Stadium) to cause another fission. So to make the bomb one merely needs to
accumulate the critical mass and it’s
92235
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