Quantum Cloud Model Why did Bohr propose the orbit or planetary model? Why did Bohr propose the orbit or planetary model? He based it on experiments with hydrogen and the line emission spectra that it produces He based it on experiments with hydrogen and the line emission spectra that it produces

Another View

Just the Line Spectrum

Originally only the Balmer Series were noted as they were within the visible portion of the electromagnetic spectrum Originally only the Balmer Series were noted as they were within the visible portion of the electromagnetic spectrum Using other detection instruments the following two series were found: Using other detection instruments the following two series were found: Lyman Series in the ultraviolet range Lyman Series in the ultraviolet range Paschen Series in the infrared range Paschen Series in the infrared range

Interpretation………….. Adding energy to an atom will excite electrons causing them to jump to a higher energy level Adding energy to an atom will excite electrons causing them to jump to a higher energy level The original location of the electron is referred to as the Ground State The original location of the electron is referred to as the Ground State The higher level is called the Excited State The higher level is called the Excited State When the electron returns to the ground state it gives off energy equal to the difference between the two levels (excited and ground state) as electromagnetic radiation with its specific wavelength and frequency When the electron returns to the ground state it gives off energy equal to the difference between the two levels (excited and ground state) as electromagnetic radiation with its specific wavelength and frequency

Bohr was able to calculate the radius from each wavelength detected on the line emission spectra Bohr was able to calculate the radius from each wavelength detected on the line emission spectra Bohr correlated these radii to the angular momentum of electrons using a formula that included an “n” term which was a whole number integer Bohr correlated these radii to the angular momentum of electrons using a formula that included an “n” term which was a whole number integer Assigned the letter K to the first orbit; L to the second; M to the third, etc. Assigned the letter K to the first orbit; L to the second; M to the third, etc. Today we call them Energy Levels Today we call them Energy Levels

Everything Isn’t OK in Bohr Land The math didn’t work for all elements The math didn’t work for all elements Other scientists take up the quest: Other scientists take up the quest: De Broglie, French, 1924 De Broglie, French, 1924 Heisenberg, German, 1927 Heisenberg, German, 1927 Schrödinger, Austrian, 1926 Schrödinger, Austrian, 1926 Led to the Quantum Theory being applied to electron location Led to the Quantum Theory being applied to electron location

Important contributions- Important contributions- deBroglie – wavelike properties of electrons deBroglie – wavelike properties of electrons Heisenberg Uncertainty Principle: Heisenberg Uncertainty Principle: “it is impossible to determine simultaneously both the position and velocity of an electron or any other particle” “it is impossible to determine simultaneously both the position and velocity of an electron or any other particle” Think of a spinning fan, ou know that there are blades but its position at any moment isn’t known Think of a spinning fan, ou know that there are blades but its position at any moment isn’t known

Schrödinger- formulated the equations for quantum mechanics Schrödinger- formulated the equations for quantum mechanics Also known as Wave Mechanics Also known as Wave Mechanics Employs upper level math beyond the scope of this course and introductory college course Employs upper level math beyond the scope of this course and introductory college course

Quantum Mechanics “highly mathematical theories of atomic structure based on the belief that energy is absorbed and radiated in definite units (quanta)” “highly mathematical theories of atomic structure based on the belief that energy is absorbed and radiated in definite units (quanta)” Concludes that the energy of an electron is restricted to certain definite values Concludes that the energy of an electron is restricted to certain definite values

Quantum Numbers Values used to express the energy of an electron which describe its position, shape, magnetic moment, and spin Values used to express the energy of an electron which describe its position, shape, magnetic moment, and spin Each electron has a unique set of four quantum numbers Each electron has a unique set of four quantum numbers Also tells you the properties of the atomic orbitals Also tells you the properties of the atomic orbitals

Principal Quantum Number The energy level in which the electron resides The energy level in which the electron resides Symbol is n Symbol is n Allowed values are 1, 2, 3…∞ Allowed values are 1, 2, 3…∞ The number of electrons that can reside in an energy level is 2n 2 The number of electrons that can reside in an energy level is 2n 2 Also tells you how many sublevels or orbitals found in an energy level- n 2 Also tells you how many sublevels or orbitals found in an energy level- n 2

Angular Momentum Quantum Number Refers to the shape of the orbital Refers 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-1 Values 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 Number Indicates the orientation of an orbital around the nucleus Indicates the orientation of an orbital around the nucleus Symbolized by m Symbolized by m Allowed values –l l Allowed values –l l For example, p orbitals have an l value of 1; therefore, For example, p orbitals have an l value of 1; therefore, = p x ; 0 = p y ; +1 = p z -1 = p x ; 0 = p y ; +1 = p z

Spin Quantum Number Originally not included but was added to account for the magnetic properties of the electron Originally not included but was added to account for the magnetic properties of the electron Electrons spin like a top or the Earth Electrons spin like a top or the Earth Symbol is s Symbol is s Only two possible spin states Only two possible spin states +1/2 or -1/2 +1/2 or -1/2

So what do we do with them? Given the sequence of four numbers you can tell where an electron is located Given the sequence of four numbers you can tell where an electron is located Example: 2,1,-1,+1/2 Example: 2,1,-1,+1/2 Tells you the electron is located in the second energy level in a p orbital; the orbital is on the x-axis (p x ), and has a +1/2 spin Tells you the electron is located in the second energy level in a p orbital; the orbital is on the x-axis (p x ), and has a +1/2 spin

And more… Can determine the sequence of electrons in an atom by following three rules: Can determine the sequence of electrons in an atom by following three rules: AUFBAU Principle AUFBAU Principle PAULI EXCLUSION Principle PAULI EXCLUSION Principle HUND’S RULE HUND’S RULE

AUFBAU Principle – an electron occupies the lowest energy orbital that can receive it AUFBAU Principle – an electron occupies the lowest energy orbital that can receive it PAULI EXCLUSION Principle – no two electrons in the same atom can have the same set of four quantum numbers (for electrons to occupy the same orbital they must have opposite spin) PAULI EXCLUSION Principle – no two electrons in the same atom can have the same set of four quantum numbers (for electrons to occupy the same orbital they must have opposite spin)

HUND’S RULE – orbitals of equal energy are occupied by one electron before any orbital is occupied by a second electron; all electrons in singly occupied orbitals must have the same spin state HUND’S RULE – orbitals of equal energy are occupied by one electron before any orbital is occupied by a second electron; all electrons in singly occupied orbitals must have the same spin state We will use two notations for electron sequencing: We will use two notations for electron sequencing: Electron Configuration Electron Configuration Orbital Notation Orbital Notation