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Chpt. 2: The Atom1History of the AtomGreek 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 possibleAtomos Greek word for indivisible - ATOM22. John Dalton (1808):Daltons 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 destroyed3What is inside the atom???43. 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 battery5

64. 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 electrons7Thomsons ExperimentVoltage source+-------Passing an electric current makes a beam appear to move from the negative to the positive end8Voltage source+-By adding an electric field he found that the moving pieces were negative Thomsons Experiment9 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.10

11Thomson'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 randomly

Dough = positive chargeRaisins = electrons125. 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 10-31g13Discovery of radiation led to the use of alpha particles in experiments

Alpha particles are positively charged particles produced by certain radioactive substances146. Ernest Rutherford (1909):

Rutherford discovered the nucleus and the proton15Rutherford's Gold Foil Experiment

- bombarded tin foils of gold with alpha () particles - If plum pudding model was correct he expected:

The alpha particles to pass through without changing direction very much

Lead blockUraniumGold FoilFluorescent Screen16

What He Got!!!!!*Note: Detector flashes - of light produced when particles strike zinc/sulphite screen17Results Most alpha particles passed straight through undeflected

Some were deflected at wide angles

Few deflected back along own pathExplanation/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 nucleusRutherfords Gold Foil Experiment18Rutherford 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 protons197. James Chadwick (1932): Search for a neutral particle to cement the nucleus

Bombarded beryllium with alpha particles

Produced neutral particles neutron

20Properties of Sub-Atomic ParticlesNameRelative ChargeRelative MassLocation

Proton+11nucleusElectron-11/1836 (no mass)outside nucleusNeutron01

nucleus21 Small, indivisible spheres

Dalton Model of the Atom22J.J. Thompsons Model of Atom

Plum Pudding Model, 1896

Thought an atom was like plum pudding23Rutherfords Model of the Atom

Rutherford Model, 1911

Thought atom was mostly empty space:

- Nucleus

- Electrons (negatively charged) revolving around nucleus24Bohrs Model of the Atom

Neils Bohr, 1913

Similar to Rutherfords model

Thought atom was mostly empty space:

- Nucleus in center is dense, positively charge- Electrons in orbits around nucleus

25(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

26Evidence for the existence of small particles!!!

Why is it possible to smell the perfume that someone is wearing from several metres away?

27Diffusion 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 air28Demonstration

Diffusion of ink in water

Diffusion of NH3 and HCl

Diffusion of smoke in air29

Diffusion of NH3 and HCl30Diffusion of NH3 and HClWord Equation:Ammonia + Hydrogen chloride = Ammonium chloride (Gas) (Gas) (White powdered ring)

Chemical Equation:NH3+HCl= NH4Cl31