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