chpt. 2: the atom history of the atom 1.greek philosophers (400bc): - first proposed that matter was...
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Chpt. 2: The Atom
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History of the Atom1. Greek Philosophers (400BC):
- first proposed that matter was composed of minute particles- believed that the tiny particles of which all matter was composed were so small that nothing smaller was possible
‘Atomos’ Greek word for indivisible - ATOM
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2. John Dalton (1808):Dalton’s Atomic Theory
- All matter is made up of very small particles called atoms
- All atoms are indivisible. They cannot be broken down into simpler particles
- Atoms cannot be created or destroyed
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What is inside the atom???
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3. William Crookes – cathode ray tube (1875):Discovery of the Electron
,- passed electric current through gases at low pressure
- invisible radiation that caused the glass to glow came from *cathode (-)
- called cathode rays
- showed existence of this radiation by placing Maltese Cross inside the tube
*Note: Cathode = plate connected to negative end of batteryAnode = plate connected to positive end of battery
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4. J.J Thomson – cathode ray tube experiments (1897):
- devised experiment to investigate if cathode rays consisted of charged particles
- cathode rays attracted up towards positive plate (anode) => consisted of negatively charged particles
- hence cathodes are streams of negatively charged particles called electrons
Definition: cathode rays are streams of negatively charged particles called electrons
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Thomson’s Experiment
Voltage source
+-------
Passing an electric current makes a beam appear to move from the negative to the positive end
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Voltage source
+
-
By adding an electric field he found that the moving pieces were negative
Thomson’s Experiment
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Further experiment:
- he found electrons were also deflected in magnetic field
- found ratio of charge to mass of the electron (e/m):
(electrical charge of electron)(mass of electron) = 1.76 x 108 coulombs = 1 gram of electrons
*Note: In 1891 George Stoney proposed that the smallest amount of electric charge be called an electron.
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Thomson's ‘Plum Pudding Model’ of the Atom (1898):
Proposed that since atoms are neutral each one consists of:- - a sphere of positive
charge- - electrons embedded
randomlyDough = positive charge
Raisins = electrons
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5. Robert Millikan (1909):
- Experiment to measure size of charge on electron – Oil Drop Experiment
- Charge of one electron = 1.6 x 10-19 coulomb
THUS…. Mass of e- = 9.11 x 10-31g
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Discovery of radiation led to the use of alpha particles in experiments
Alpha particles are positively charged particles produced by certain radioactive substances
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6. Ernest Rutherford (1909):
Rutherford discovered the nucleus and the proton
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Rutherford's Gold Foil Experiment
- bombarded tin foils of gold with alpha (α) particles - If plum pudding model was correct he expected:
The alpha particles to pass through without changing direction very much
Lead block
Uranium
Gold Foil
Fluorescent Screen
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What He Got!!!!!
*Note: Detector flashes - of light produced when α particles strike zinc/sulphite screen
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Results• Most alpha particles passed straight through undeflected
• Some were deflected at wide angles
• Few deflected back along own path
Explanation/Conclusion• Atom mainly empty space occupied by electrons (negative)
• Both the mass and positive charge were concentrated in a small dense core which he called the nucleus
Rutherford’s Gold Foil Experiment
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Rutherford – discovery of protons (1924):
• Light atoms (oxygen, nitrogen) were bombarded with alpha particles - small POSITIVE charged particles were given off
• This did not occur with heavier metals e.g. gold
• Explanation – alpha particles were breaking up the nuclei of the lighter atoms to release positively charged particles
• referred to these small positive particles as protons
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7. James Chadwick (1932):
• Search for a neutral particle to cement the nucleus
• Bombarded beryllium with alpha particles
• Produced neutral particles – neutron
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Properties of Sub-Atomic Particles
Name Relative Charge
Relative Mass
Location
Proton +1 1 nucleus
Electron -1 1/1836 (no
mass)
outside nucleus
Neutron 0 1 nucleus
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• Small, indivisible spheres
Dalton Model of the Atom
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J.J. Thompson’s Model of Atom
• Plum Pudding Model, 1896
•Thought an atom was like plum pudding
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Rutherford’s Model of the Atom
• Rutherford Model, 1911
• Thought atom was mostly empty space:
- Nucleus
- Electrons (negatively charged) revolving around nucleus
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Bohr’s Model of the Atom
Neils Bohr, 1913
• Similar to Rutherford’s model
• Thought atom was mostly empty space:
- Nucleus in center is dense, positively charge- Electrons in orbits around nucleus
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(Modern) Quantum Mechanical Model of the Atom
Heisenberg, Schrodinger, many others, ~1926
• Think atom is mostly empty space:- Nucleus in center is dense,
positively charge
- Electrons cannot locate
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Evidence for the existence of small particles!!!
Why is it possible to smell the perfume that someone is wearing from several metres away?
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Diffusion• The process by which molecules of a substance spread through a solid, liquid or gas.
• Some examples which can be demonstrated in the lab:-
Gas Jar full of air
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Demonstration• Diffusion of ink in water
• Diffusion of NH3 and HCl
• Diffusion of smoke in air
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Diffusion of NH3 and HCl
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Diffusion of NH3 and HClWord Equation:Ammonia + Hydrogen chloride = Ammonium
chloride (Gas) (Gas) (White powdered ring)
Chemical Equation:NH3 + HCl = NH4Cl