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Mr. Shields Regents Chemistry U06 L03

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Bohr ModelBohr Modele- transitions from a higher energy levels to lowerenergy levels release energy in the form of photons.

Bohr’s model correctly predicted hydrogen’s 4 visibleLines in the emission spectra but incorrectly predicted the emission spectra for all other atoms.

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Bohr ModelBohr Model

Niels Bohr1885-1962

Obviously the Bohr model didn’tAccurately describe the atom.

In the 1920’s a new model of the Atom began to emerge.

It was known as the Quantum Mechanical model.

Let’s look at some of the key conceptsThat lead to this model

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The electron as a WaveThe electron as a Wave

Louis Victor de Broglie1892-1987

1924 – As a graduate studentde Broglie began consideringBohr’s quantized atom.

If light could behave as a particleThen why, de Broglie wondered,couldn’t particles (like electrons)behave as waves?

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The wave nature of an The wave nature of an electronelectron

De Broglie developed his idea and established a Mathematical relationship that related wavelengthTo mass & velocity of a moving object.

h = 6.626 x 10-34 j sec

In other words,Anything that movesbehaves as a wave!

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HeisenbergHeisenberg

In 1913 Bohr showed how electrons werequantized

In 1924 De Broglie showed how an electroncould behave as a wave.

In 1925 Werner Heisenberg establishedwhat was known as matrix mechanics toexplain atomic behavior

Werner Heisenberg1901- 1976

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SchrodingerSchrodinger

In 1926 Erwin Schrodinger proposed aWave Mechanics model of the atom.

The matrix and wave quantum model of the atom divided scientists into two camps.

Erwin Schrodinger1887 - 1961

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The debateThe debate

In 1926 Werner Heisenberg began his job as Assistant toNiels Bohr in Copenhagen. Later that year Schrodinger cameTo debate the two alternative theories with Bohr.

Neither model was satisfactory but Schrodinger showed theequivalence of the matrix and wave versions of QuantumMechanics.

After Bohr presented a statistical interpretation of the wave function, these theories formed thebasis of what is now regarded asquantum mechanics.

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Uncertainty PrincipleUncertainty Principle

1927 - Heisenberg proposed what he is most well Known for … The Uncertainty PrincipleUncertainty Principle

“It is impossible to simultaneously know both thePosition and Velocity (a vector) of a particle at theSame time.”

A photon has about the same energy as an electron. A photon striking an electron causes the electronTo change both position and velocity. So an electronsPosition can not be specified with precision.

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Uncertainty principleUncertainty principle

The consequence is that we can only give aProbability of finding an electron in a given location.

Is the electron

*here

Ans: yes to all but each has a different probability

here

*

Or here?

*

Some location probabilitiesare higher than others.

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The Quantum Mechanical modelThe Quantum Mechanical model

Electrons behave as a Electrons behave as a wavewave or a or a particleparticle with specific with specific allowable energy values – they are quantized.allowable energy values – they are quantized.

Electrons are located in “probabilityElectrons are located in “probability regions” of spaceregions” of space

These are known as atomic These are known as atomic orbitalsorbitals. They are . They are notnot the same as Bohr’s circular the same as Bohr’s circular orbitsorbits!!

AnAn OrbitalOrbital is defined as a region in space in which is defined as a region in space in which there is a there is a 90% probability90% probability of finding the electron of finding the electron

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Hydrogen’s 1st orbital; electron probability plot

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The Quantum Mechanical modelThe Quantum Mechanical model

Just like the Bohr Model there are Just like the Bohr Model there are Principal Principal quantum numbers (n)quantum numbers (n)

n indicates n indicates distancedistance from the nucleus from the nucleus As n increase distance from the nucleus increasesAs n increase distance from the nucleus increases

n Specifes major energy levels called the n Specifes major energy levels called the Principal Energy LevelsPrincipal Energy Levels

There are currently 7 principal energy levelsThere are currently 7 principal energy levels

Principal energy levels containPrincipal energy levels contain Energy SublevelsEnergy Sublevels

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Energy SublevelsEnergy SublevelsEach Principal Energy level contains 1 more sublevels than thelast principal energy level, beginning with 1 sublevel in the1st Principal Energy Level

Sublevels are labeled s, p, d, and f (there are no others)

So… number ofn sublevels Sublevel1 1 1s2 2 2s 2p3 3 3s 3p 3d 4 4 4s 4p 4d 4f5 4 5s 5p 5d 5f6 4 6s 6p 6d 6f7 4 7s 7p 7d 7f

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Energy SublevelsEnergy SublevelsFurthermore, within each energy sublevel there is anywherefrom 1 to 7 orbitals

Sublevel # of Orbitals

s 1 s orbitalp 3 p orbitalsd 5 d orbitalsf 7 f orbitals

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Energy SublevelsEnergy SublevelsElectrons occupy the energy sublevels.

These energy sublevels refer to the 3 dimensional regions in space called atomic Orbitals.

Remember: orbitals are defined as the regionIn space in which there is a 90% probabilityof finding the electron.

So what do these orbitals look like?

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The “s” orbital

90% probabilityInside circle

All “s” orbitals are spherical

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The s orbitals of the 1st three Energy Levels

n=1 n=2

n=3

2s

Note the growth in size as n increases

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The 3 “p” orbitals of the 2nd Principal Energy Level

The “Dumbbell” shaped p Orbitals

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So what do these 1s 2s and 2p atomicorbitals look like as they surround thenucleus ?

The number of orbitals in a given principle energy level (n) is equal to n2

Remember “2n2 “ for calculating the # of e- in each Bohr orbit ?

Well 2 is the max # of e- allowed in each orbital hence 2n2 = max # of e- per principle quantum number

Calculating # of Orbitals

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Neon with fully occupied 1s, 2s, 2p orbitals

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All the orbitals of the s, p, d, and f sublevels

Only need to know these

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Orbital EnergyOrbital EnergyAs the principal quantum number (n) increases, in Other words as we move further from the nucleus,The energy of the electrons in those principal Energy levels increase;

Energy: 1 < 2 < 3 < 4 < 5 < 6 < 7

Also, within any principal energy level the energy ofThe sublevels increase from s to f;

Energy: s < p < d < f

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Electron Orbital fillingElectron Orbital filling

This increasing energy sequence defines into which Orbitals Electrons go as theyare added to the atom .

For example lets look at Rubidium …

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3d > 4s

4d > 5s

4f > 5s,5p,6s

SublevelEnergy increasesFrom s to f.

Some sublevelsWith a lower n mayActually be at aHigher energyThan some e-

in a higher n !

We’ll look at this“SWITCHING”Again later