History of Atomic TheoryHistory of Atomic Theory

Atomic models from Atomic models from

Dalton to BohrDalton to Bohr

A ‘Model’… A ‘Model’… 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

Historical Models of the AtomHistorical Models of the Atom

John DaltonJohn Dalton‘‘Billiard ball’ model (1803)Billiard ball’ model (1803)

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

Historical Models of the AtomHistorical Models of the Atom

J. J. ThomsonJ. J. Thomson‘‘Raisin bun’ model (1897)Raisin bun’ model (1897)

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

Historical Models of the AtomHistorical Models of the Atom

Ernest RutherfordErnest RutherfordNuclear (‘beehive’) model (1911)Nuclear (‘beehive’) model (1911)

Tested Thomson’s theory with the famous “gold foil experiment”

His results suggested that the atom was mostly empty space with a very dense positively-charged ‘nucleus;’ he later discovered that protons were the positively-charged 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?

Historical Models of the AtomHistorical Models of the AtomNiels BohrNiels BohrRefined nuclear model (1913)Refined nuclear model (1913)

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…

?

The Quantum ModelThe Quantum Model

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

the best…the best…

Light defined as a WaveLight defined as a Wave Light travels through space as an electromagnetic wave. Light travels through space as an electromagnetic wave.

Waves are characterized by their wavelength, Waves are characterized by their wavelength, λλ, and , and frequency, f, and amplitude, A.frequency, f, and amplitude, A.

• Light colour is related to the wavelength (and frequency)Light colour is related to the wavelength (and frequency) of a wave. Red light has a longer wavelength and lower of a wave. Red light has a longer wavelength and lower frequency than blue light. frequency than blue light.

A

Relationship Between Wavelength Relationship Between Wavelength and Colour of Lightand Colour of 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.

The Quantum ModelThe Quantum Model

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.

Origins of Quantum TheoryOrigins of Quantum Theory 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 low-energy photons

Light Defined as a ParticleLight Defined as a Particle(quantum model)(quantum model)

Light is also thought to propagate through space Light is also thought to propagate through space as individual particles called photons. as individual particles called photons.

Each photon has a specific amount of energy that Each photon has a specific amount of energy that is related to the wave characteristics and to the is related to the wave characteristics and to the colour of light, that is, blue photons have more colour of light, that is, blue photons have more energy than red photonsenergy than red photons

Atoms and LightAtoms and Light

An element in a gaseous state produces light An element in a gaseous state produces light when it is heated to a certain temperaturewhen it is heated to a certain temperature

By passing this light through a prism, we can By passing this light through a prism, we can see its ‘bright-line’ or ‘emission’ spectrumsee 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…And now we can finish the story…

Niels BohrNiels BohrRefined nuclear model (1913)Refined nuclear model (1913)a.k.a.“Bohr-Rutherford Model”a.k.a.“Bohr-Rutherford Model”

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.

Bohr-Rutherford ModelBohr-Rutherford ModelElectron occupancy rules:Electron occupancy rules:

2n2

n = energy level

* Electrons will always occupy the lowest energy level available. *

Electrons are arranged in fixed energy states.Electrons are arranged in fixed energy states. When an element is heated, electrons are promoted to When an element is heated, electrons are promoted to

higher energy states (excited states). When electrons higher energy states (excited states). When electrons return to a lower energy state, energy is given off in the return to a lower energy state, energy is given off in the form of light (a photon is emitted).form of light (a photon is emitted).

The movement of an electron between energy levels is The movement of an electron between energy levels is referred to as a transition.referred to as a transition.

The type (colour) of light emitted is related to the size of The type (colour) of light emitted is related to the size of the transition. Many transitions produce photons that are the transition. Many transitions produce photons that are not in the visible region.not in the visible region.

It is important to note that Bohr primarily studied It is important to note that Bohr primarily studied hydrogen.hydrogen.

What Bohr proposed…What Bohr proposed…

Electron transitions of HydrogenElectron transitions of Hydrogen

Usefulness of the Bohr-Rutherford ModelUsefulness 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 2nd I.E ; those in

group 2 have an unusually high 3rd 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

Usefulness of the Bohr-Rutherford Model Stability of Noble Gases; trends with successive ionization energies

Element

Ionization Energy (kJ/mol)

1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th

H 1 313

He 2 374 5 251

Li 521 7 297 11 814

Be 898 1 757 14 855 21 013

B 801 2 423 3 658 25 028 32 827

C 1 091 2 355 4 623 6 226 37 836 47 276

N 1 400 2 857 4 575 7 480 9 449 53 270 64 360

O 1 313 3 388 5 299 7 470 10 994 13 328 71 339 83 600

F 1 679 3 378 6 042 8 416 11 023 15 163 17 866 92 042 105 754

Ne 2 085 3 967 6 177 9 382 12 200 15 241 130 834

Na 492 4 565 6 920 9 546 13 378 16 640 20 115

Mg 734 1 448 7 731 10 550 13 629 18 040 21 747 25 675

Al 579 1 815 2 741 11 583 14 845 18 378 23 348 27 518

Si 781 1 573 3 223 4 353 16 090 19 796 23 783 29 333

P 1 062 1 902 2 915 4 961 6 274 21 273 25 414 29 854

S 1 004 2 259 3 378 4 565 6 998 8 494 27 122 31 736

Cl 1 255 2 297 3 851 5 164 6 544 9 334 11 032 33 618

Ar 1 525 2 664 3 948 5 772 7 391 8 812 11 969 13 811

Usefulness of the Bohr-Rutherford Model

2. Predictions For Compounds Bonding ratios (MgF2, CaF2, SrF2, etc.) Bond polarity (electronegativity) Bonding types – covalent vs. ionic

3. Physical Properties Solubility Melting and boiling points Viscosity