Quantum Cloud ModelQuantum Cloud Model

Why did Bohr propose the orbit or Why did Bohr propose the orbit or planetary model?planetary model?

He based it on experiments with He based it on experiments with hydrogen and the line emission hydrogen and the line emission spectra that it producesspectra that it produces

Another ViewAnother View

Just the Line SpectrumJust the Line Spectrum

Originally only the Balmer Series Originally only the Balmer Series were noted as they were within were noted as they were within the visible portion of the the visible portion of the electromagnetic spectrumelectromagnetic spectrum

Using other detection instruments Using other detection instruments the following two series were the following two series were found:found: Lyman Series in the ultraviolet rangeLyman Series in the ultraviolet range Paschen Series in the infrared rangePaschen Series in the infrared range

Interpretation…………..Interpretation………….. Adding energy to an atom will excite Adding energy to an atom will excite

electrons causing them to jump to a electrons causing them to jump to a higher energy levelhigher energy level

The original location of the electron is The original location of the electron is referred to as the referred to as the Ground StateGround State

The higher level is called the The higher level is called the Excited Excited StateState

When the electron returns to the When the electron returns to the ground state it gives off energy equal ground state it gives off energy equal to the difference between the two to the difference between the two levels (excited and ground state) as levels (excited and ground state) as electromagnetic radiation with its electromagnetic radiation with its specific wavelength and frequencyspecific wavelength and frequency

Bohr was able to calculate the radius Bohr was able to calculate the radius from each wavelength detected on from each wavelength detected on the line emission spectra the line emission spectra

Bohr correlated these radii to the Bohr correlated these radii to the angular momentum of electrons angular momentum of electrons using a formula that included an “using a formula that included an “n”n” term which was a whole number term which was a whole number integerinteger

Assigned the letter K to the first Assigned the letter K to the first orbit; L to the second; M to the orbit; L to the second; M to the third, etc.third, etc.

Today we call them Today we call them Energy LevelsEnergy Levels

Everything Isn’t OK in Bohr Everything Isn’t OK in Bohr LandLand

The math didn’t work for all The math didn’t work for all elementselements

Other scientists take up the quest:Other scientists take up the quest: De Broglie, French, 1924De Broglie, French, 1924 Heisenberg, German, 1927Heisenberg, German, 1927 Schrödinger, Austrian, 1926Schrödinger, Austrian, 1926 Led to the Quantum Theory being Led to the Quantum Theory being

applied to electron locationapplied to electron location

Important contributions-Important contributions- deBroglie – wavelike properties of deBroglie – wavelike properties of

electronselectrons Heisenberg Uncertainty Principle:Heisenberg Uncertainty Principle: ““it is impossible to determine it is impossible to determine

simultaneously both the position simultaneously both the position and velocity of an electron or any and velocity of an electron or any other particle”other particle”

Think of a spinning fan, ou know Think of a spinning fan, ou know that there are blades but its that there are blades but its position at any moment isn’t knownposition at any moment isn’t known

Schrödinger- formulated Schrödinger- formulated the equations for quantum the equations for quantum mechanicsmechanics

Also known as Also known as Wave Wave MechanicsMechanics

Employs upper level math Employs upper level math beyond the scope of this beyond the scope of this course and introductory course and introductory college coursecollege course

Quantum MechanicsQuantum Mechanics ““highly mathematical highly mathematical

theories of atomic structure theories of atomic structure based on the belief that based on the belief that energy is absorbed and energy is absorbed and radiated in definite units radiated in definite units (quanta)”(quanta)”

Concludes that the energy of an Concludes that the energy of an electron is restricted to certain electron is restricted to certain definite valuesdefinite values

Quantum NumbersQuantum Numbers

Values used to express the Values used to express the energy of an electron which energy of an electron which describe its position, shape, describe its position, shape, magnetic moment, and spinmagnetic moment, and spin

Each electron has a unique set of Each electron has a unique set of four quantum numbersfour quantum numbers

Also tells you the properties of Also tells you the properties of the atomic orbitalsthe atomic orbitals

Principal Quantum NumberPrincipal Quantum Number

The energy level in which the The energy level in which the electron resideselectron resides

Symbol is Symbol is nn Allowed values are 1, 2, 3…∞Allowed values are 1, 2, 3…∞ The number of electrons that can The number of electrons that can

reside in an energy level is 2nreside in an energy level is 2n22

Also tells you how many sublevels Also tells you how many sublevels or orbitals found in an energy or orbitals found in an energy level- nlevel- n22

Angular Momentum Angular Momentum Quantum NumberQuantum Number

Refers to the shape of the orbitalRefers to the shape of the orbital Symbol is l (like “L” but lower case)Symbol is l (like “L” but lower case) Values it can assume are 0 to n-1Values it can assume are 0 to n-1 If l = 0 it is a “s orbital”If l = 0 it is a “s orbital” If l = 1 it is a “p orbital”If l = 1 it is a “p orbital” If l = 2 it is a “d orbital”If l = 2 it is a “d orbital” If l = 3 it is a “f orbital”If l = 3 it is a “f orbital”

Magnetic Quantum NumberMagnetic Quantum Number

IIndicates the orientation of an ndicates the orientation of an orbital around the nucleusorbital around the nucleus

Symbolized by Symbolized by mm Allowed values –l...0...+lAllowed values –l...0...+l For example, p orbitals have an l For example, p orbitals have an l

value of 1; therefore, -1...0...+1value of 1; therefore, -1...0...+1 -1 = p-1 = pxx ; 0 = p ; 0 = pyy ; +1 = p ; +1 = pzz

Spin Quantum NumberSpin Quantum Number

Originally not included but was Originally not included but was added to account for the magnetic added to account for the magnetic properties of the electronproperties of the electron

Electrons spin like a top or the EarthElectrons spin like a top or the Earth Symbol is Symbol is ss Only two possible spin statesOnly two possible spin states +1/2 or -1/2+1/2 or -1/2

So what do we do with them?So what do we do with them?

Given the sequence of four Given the sequence of four numbers you can tell where an numbers you can tell where an electron is locatedelectron is located

Example: 2,1,-1,+1/2Example: 2,1,-1,+1/2 Tells you the electron is located Tells you the electron is located

in the second energy level in a in the second energy level in a p p orbitalorbital; the orbital is on the x-; the orbital is on the x-axis (axis (ppxx), and has a +1/2 spin), and has a +1/2 spin

And more…And more… Can determine the sequence of Can determine the sequence of

electrons in an atom by following electrons in an atom by following three rules:three rules:

AUFBAU PrincipleAUFBAU Principle PAULI EXCLUSION PrinciplePAULI EXCLUSION Principle HUND’S RULEHUND’S RULE

AUFBAU Principle – an electron AUFBAU Principle – an electron occupies the lowest energy occupies the lowest energy orbital that can receive itorbital that can receive it

PAULI EXCLUSION Principle – no PAULI EXCLUSION Principle – no two electrons in the same atom two electrons in the same atom can have the same set of four can have the same set of four quantum numbers (for electrons quantum numbers (for electrons to occupy the same orbital they to occupy the same orbital they must have opposite spin)must have opposite spin)

HUND’S RULE – orbitals of equal HUND’S RULE – orbitals of equal energy are occupied by one electron energy are occupied by one electron before any orbital is occupied by a before any orbital is occupied by a second electron; all electrons in singly second electron; all electrons in singly occupied orbitals must have the same occupied orbitals must have the same spin statespin state

We will use two notations for electron We will use two notations for electron sequencing:sequencing:

Electron Configuration Electron Configuration Orbital NotationOrbital Notation