THE ATOMIC MODELTHE ATOMIC MODEL

A historical comparison

Early Greek Models

There were two main competing models:

ARISTOTLE’s Four Element Model

•all matter was composed of combinations of air, water, fire and earth.

DEMOCRITUS’s Smallest Particle Model

•matter can be broken down into smaller particles; each type of substance has different types of particles

DALTON

Did various quantitative studies and proposed:

•that all matter is composed of small, solid, indivisible particle spheres called atoms

•all atoms of the same element are identical but different elements had different particles

•atoms can combine in specific whole number ratios

The BILLIARD BALL Model

THOMSON

•Used gas discharge tubes to identify that there is a negative particle in the atom

•Proposed a model that included negative electrons embedded in a positive sphere

•The Raisin Bun or Plum-Pudding Model

Don’t forget Nagaoka

RUTHERFORD

•Was testing Thomson’s model using alpha rays and a gold foil.

•Found a difference in his prediction and the results

Review the Rutherford Model

The atom has a dense, positively charged centrecalled a nucleusAround the atom is a large region of empty spaceNegatively charged electrons can be found orbiting the nucleus in the empty space

Flaw in Rutherford’s Theory•as electrons move about the nucleus, according to classical physics, they will emit electromagnetic radiation and lose energy

•this loss of energy would cause the electrons to slow down and eventually fall into the nucleus (atom collapses!)

•since the above do not occur, a new theory was needed

Bohr discovers that...

Energized gases emit coloured lightDifferent types of gases emit different colours of lightThrough a spectroscope specific bands of colour can be seenEach colour of light is related to a specific amount of energyThe colours of light emitted are related and can be predicted by looking at the number of electrons in an atom

Electrons are found in energy levels

the nucleus is a dense, positive spherea series of shells or energy levels or orbitssurround the nucleuseach level is slightly larger than the previous and is related to more energy

Energy levels...

electrons are found on particular energy levelselectrons may jump from one level to another but they may not travel between levelseach energy level can hold a specific number of electrons (2n2)

energy level 1has 2(1) 2 electrons

energy level 2 can

have up to 2(2) 2=8

electrons

larger energy levels may existbut may be empty

Changing levels...

electrons can jump to higher levels by absorbing energywhen it loses that energy, it will return to its previous levelthe energy released produces the line spectrum Bohr observed

add energylose energy

light is released

The Hydrogen atom

• Bohr observed the following bright line spectrum of hydrogen and was able to make a quantitative connection between each coloured line and a specific electron jump.Electron jumping from

level 2 to level 6 and back

Electron jumping from level 2 to level 5 and back

Electron jumping from level 2 to level 4 and back

Electron

jumpin

g from

level 2

to level

3 and

back

What about the jump from level 1 to level 2?

Released energy produces coloured light related to how much energy is released Line spectra correspond to the energy released

The successes & failures of Bohr’s Theory

•Bohr’s Theory was formulated from a large body of research about the nature of light and energy ==> this large body of evidence resulted in the Quantum Theory

•Bohr’s theory explain the Bright line spectrum of hydrogen very well, but did not work for more complex atoms or ions

•also the scientists (e.g Albert Michelson in 1891) found that the main lines of the bright line spectrum of hydrogen were actually composed of more than one lines

•this led Arnold Sommerfeld (1915) to propose that Bohr’s main energy levels/shells consisted of subshells or sublevels

•by the 1920s Bohr’s planetary model had to be abondoned.

The Quantum (wave) Mechanics Model

Where the Bohr model visualized electrons travelling around the nuclues in orbits of fixed radius, the modern theory views the atom as:

– a positively charged nucleus surrounded by pulsating waves==>proposed by Louis de Broglie==> Clinton Davisson and G.P.Thomson obtained experimental

confirmation==> Erwin Schrodinger theorized that electrons can only

release/absorb certain (quantized) energies because waves can only have whole number of wavelengths; electrons must behave as waves; he developed a wave equation to describe the energy and motion of an electron;

==> in solving the equation, one can deduce the probability of finding the electron in a certain position around the nucleus

The Quantum (wave) Mechanics Model Con’t

it is impossible to simultaneously know the speed and exact location of an electron (Heisenberg uncertainty principle)

Exactly how electrons move in an atom is not known..in terms of location all that is known is the probability of finding an electron in a certain region (electron probability density)

The regions of space where electrons occupy are called orbitals (also called electron clouds), so unlike the Bohr model which uses orbits to pinpoint where electrons are, this theory uses the term orbital..orbitals differ in size, shape, and space orientation

THE ATOMIC MODELEarly Greek ModelsDALTONTHOMSONRUTHERFORDReview the Rutherford ModelBohr discovers that...Electrons are found in energy levelsEnergy levels...Changing levels...The Hydrogen atomThe Quantum (wave) Mechanics ModelThe Quantum (wave) Mechanics Model Con’t