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
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
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
What is inside the atom???
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
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
Thomson’s Experiment
Voltage source
+-------
Passing an electric current makes a beam appear to move from the negative to the positive end
Voltage source
+
-
By adding an electric field he found that the moving pieces were negative
Thomson’s Experiment
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.
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
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
Discovery of radiation led to the use of alpha particles in experiments
Alpha particles are positively charged particles produced by certain radioactive substances
6. Ernest Rutherford (1909):
Rutherford discovered the nucleus and the proton
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
What He Got!!!!!
*Note: Detector flashes - of light produced when α particles strike zinc/sulphite screen
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
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
7. James Chadwick (1932):
• Search for a neutral particle to cement the nucleus
• Bombarded beryllium with alpha particles
• Produced neutral particles – neutron
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
• Small, indivisible spheres
Dalton Model of the Atom
J.J. Thompson’s Model of Atom
• Plum Pudding Model, 1896
•Thought an atom was like plum pudding
Rutherford’s Model of the Atom
• Rutherford Model, 1911
• Thought atom was mostly empty space:
- Nucleus
- Electrons (negatively charged) revolving around nucleus
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
(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
Evidence for the existence of small particles!!!
Why is it possible to smell the perfume that someone is wearing from several metres away?
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
Demonstration• Diffusion of ink in water
• Diffusion of NH3 and HCl
• Diffusion of smoke in air
Diffusion of NH3 and HCl
Diffusion of NH3 and HClWord Equation:Ammonia + Hydrogen chloride = Ammonium
chloride (Gas) (Gas) (White powdered ring)
Chemical Equation:NH3 + HCl = NH4Cl