Atomic models from
Dalton to Bohr

 is not a real thing, but is used to explain, mimic or simulate reality,
 is used as a tool,
 is used to predict what happens in the real world,
 is changed or modified until it best fits new information,
 may have some limitations or be valid only under certain conditions.
Examples: globes, computer simulations, product prototypes

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All matter consists of atoms
Each element has its own atom type
Atoms of different elements have different properties
Atoms of two elements can combine to form compounds
Atoms are never created, destroyed or subdivided

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First to include sub-atomic particles
(electrons) that had been seen in cathode ray tube experiments
Model is of a positively charged sphere with negatively charged electrons embedded in it
Positive ‘dough’ and negative ‘raisins’ make up an atom that is neutral over all

Ernest Rutherford
Nuclear (‘beehive’) model (1911)
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Tested Thomson’s theory with the famous
“gold foil experiment”
His results suggested that the atom was mostly empty space with a very dense positivelycharged ‘nucleus;’ he later discovered that protons were the positivelycharged part
In this model, negatively-charged electrons existed within the empty space
**Neutrons were discovered much later by
James Chadwick (in 1932); why so late?

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Bohr knew that a new model was needed, primarily because of a major problem with the Rutherford model
:
Bohr and his contemporaries knew that if a charged particle accelerates, it must give off energy, likely in the form of light.
The electrons, which are definitely charged (-) and accelerating
(changing direction constantly) should therefore give off energy and eventually spiral in towards the nucleus and cause the atom to collapse. Problem? Yes: Atoms don’t collapse!
Bohr was very interested in the newly-developed quantum theory of light proposed by Einstein and
Planck, and thought it could be applied to the problem with Rutherford’s model…so… more about quantum theory next, then back to Bohr…

Bohr’s Inspiration for a Better
Model of the Atom…but still not the best…

 Light travels through space as an electromagnetic wave.
Waves are characterized by their wavelength, λ, and frequency, f, and amplitude, A.
A
• Light colour is related to the wavelength (and frequency) of a wave. Red light has a longer wavelength and lower frequency than blue light.

A spectrum containing all colours of visible light is called a continuous spectrum. This is what we see if we pass white light through a prism.

Quantum – a specific allowable value.
Quanta – a set of specific allowable values.
Example – A staircase is like a set of quanta.
Each stair is an allowable position. Other than when travelling between steps, an individual step is the only place you may exist. Compare to a ramp.

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Max Planck first hypothesized that the energy of an oscillating atom was not continuous (or wavelike) when he studied blackbody radiation
Albert Einstein stated that if the energy of the vibrating atoms was quantized, the light they emit must also be quantized
Einstein earned a Nobel Prize when he used this new
‘Quantum Theory of Light’ to explain the photoelectric effect; one quantum of light (called a ‘photon’) could release one electron from a metal surface; higher-energy photons were more likely to liberate electrons than lowenergy photons

 Light is also thought to propagate through space as individual particles called photons.
 Each photon has a specific amount of energy that is related to the wave characteristics and to the colour of light, that is, blue photons have more energy than red photons

 An element in a gaseous state produces light when it is heated to a certain temperature
 By passing this light through a prism, we can see its ‘bright-line’ or ‘emission’ spectrum

 Bohr wanted his atomic model to explain the bright-line spectra of the elements
 Since only certain distinct colours of light could be absorbed or emitted by atoms,
Bohr reasoned that this related to distinct
‘energies’ of the electrons inside the atom
Conclusion: Electrons have distinct energies, and are therefore ‘quantized’

And now we can finish the story…
Niels Bohr
Refined nuclear model (1913) a.k.a.“Bohr-Rutherford Model”
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Nucleus containing protons (+) ((and neutrons)),
Electrons (-) are organized into specific energy levels orbiting the nucleus, and are thereby ‘quantized.’
Bohr’s model allows the electrons to exist in specific allowable energy levels that are identified by the principle quantum number, n. The allowable values of n are 1,2,3, …
Electrons follow “occupancy rules”
Although technically ‘historical,’ this model is very useful.
It is still used daily by students and scientists alike.

Electron occupancy rules:
2n 2 n = energy level
* Electrons will always occupy the lowest energy level available. *

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Electrons are arranged in fixed energy states.
When an element is heated, electrons are promoted to higher energy states (excited states). When electrons return to a lower energy state, energy is given off in the form of light (a photon is emitted).
The movement of an electron between energy levels is referred to as a transition.
The type (colour) of light emitted is related to the size of the transition. Many transitions produce photons that are not in the visible region.
It is important to note that Bohr primarily studied hydrogen.

Usefulness of the Bohr-Rutherford Model
1.
Periodic Trends
 Valence electrons = group # (A groups)
 Common ion charges (A groups)
 Ionization energy
 Stability of Noble Gases and trends with successive ionization energies.
 Elements in group 1 have an unusually high 2 nd group 2 have an unusually high 3 rd
I.E ; those in
I.E. etc.. The B-R model explains this by suggesting that once an element has achieved an octet, it is in a stable arrangement that matches a noble gas.
 Atomic radii
 Reactivity

1 679
2 085
492
734
579
781
1 062
1 004
1 255
1 525
1 313
2 374
521
898
801
1 091
1 400
1 313
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Si
P
S
Cl
Ar
F
Ne
Na
Mg
Al t
Elemen
H
He
Li
Be
B
C
N
O
Usefulness of the Bohr-Rutherford Model
Stability of Noble Gases; trends with successive ionization energies
1st 2nd 3rd 4th
Ionization Energy (kJ/mol)
5th 6th 7th 8th 9th 10 th
5 251
7 297 11 814
1 757 14 855 21 013
2 423 3 658 25 028 32 827
2 355 4 623 6 226 37 836 47 276
2 857
3 388
4 575
5 299
7 480 9 449 53 270 64 360
7 470 10 994 13 328 71 339 83 600
3 378
3 967
4 565
1 448
1 815
6 042
6 177
6 920
8 416
9 382
9 546
11 023
12 200
13 378
15 163
15 241
16 640
17 866
20 115
92 042
7 731 10 550 13 629 18 040 21 747 25 675
2 741 11 583 14 845 18 378 23 348 27 518
105 754
1 573
1 902
2 259
2 297
2 664
3 223
2 915
3 378
3 851
3 948
4 353 16 090 19 796 23 783 29 333
4 961 6 274 21 273 25 414 29 854
4 565 6 998 8 494 27 122 31 736
5 164
5 772
6 544
7 391
9 334 11 032 33 618
8 812 11 969 13 811
130 834

Usefulness of the Bohr-Rutherford Model
2. Predictions For Compounds
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Bonding ratios (MgF
2
, CaF
2
, SrF
2
, etc.)
Bond polarity (electronegativity)
Bonding types – covalent vs. ionic
3. Physical Properties
 Solubility
 Melting and boiling points
 Viscosity