Atomic Spectra

A spectroscope is an instrument that disperses the light emitted by an excited gas into the different frequencies the light contains.

Light with multiple wavelengthsWavelengths separated

refraction

Atomic Spectra

An emission spectrum consists of the various frequencies of light given off by an excited substance. Below is the part of the emission spectra of sodium.

Atomic Spectra

An absorption spectrum consists of the various frequencies absorbed by a substance when white light is passed through it. The frequencies in the spectrum of an element fall into sets called spectral series. Seen here is the spectral series of hydrogen.

Origin of Absorption Spectra

Spectrum of hydrogen

• When energy is supplied to the H gas sample, individual atoms absorb different amount of energy and electron shift to different energy levels, I,e., 3rd, 4th, 5th…….

• The electrons are in higher energy levels are relatively unstable and drop back to lower energy levels.

• This process of dropping back, energy is emitted in the form of line spectrum containing various lines of particular frequency and wavelength.

Spectrum of Hydrogen

Electron Waves and Orbits

An electron can circle a nucleus only in orbits that contain a whole number of de Broglie Wavelengths. The quantum number n of an orbit is the number of electron waves that fit into the orbit.

Electron Waves and Orbits

Electrons seemed to be locked into these wave patterns around the nucleus.

Quantum Mechanics

The theory of quantum mechanics was developed by Erwin Schrödinger, Werner. According to quantum mechanics, the position and momentum of a particle cannot both be accurately known at the same time. Only its most probable position or momentum can be determined.The most probable distance between the proton and electron for a hydrogen atom turns out to be about 0.89Å, the same as Niels Bohr.

Erwin Schrödinger (1887-1961)

Quantum Numbers and Atomic Orbitals

n = principal quantum number (shell)describes orbital sizespecifies primary energy level

greater n, higher E (like Bohr’s E)l = angular momentum quantum number (subshell)

(azimuthal quantum number)describes orbital shape l = 0 = s orbital

l = 1 = p orbital inc E l = 2 = d orbital within l = 3 = f orbital n

n 1

0n -1

ml = magnetic quantum numberdescribes orbital orientation

the direction is spacealso tells the number of orbitals of same

energy (degenerate orbitals)

So, the combination of n, l, and ml, completely describes a specific orbital, its size , shape and orientation.

To make a complete picture, a 4th quantum number added, ms.

ms = spin quantum number - describes the electron spin in the orbital

m

ms 1/2 or

Quantum Numbers and Atomic Orbitals

Quantum Numbers

principal quantum number n 1,2,3,…..

orbital quantum number l 0,1,2,….n-1

magnetic quantum number m -l to +l for n=2 -2,-1,0,1,2spin magnetic quantum number +½ or –½ spin

Its like your address. To find where you are you need to know 4 things: state, city, street, house #. To know where or what state the electron is in you need to know the four quantum numbers.

Quantum #’s are like an Address.

What do you need to know to find out where you live?

State City Street House

Principle Quantum # (n)

Angular Quantum # (l)

Magnetic Quantum # (ml)

Spin Quantum # (ms)

Shapes of Atomic Orbitals

s orbital P orbital

1s 3s2s2px

3pz

3d2py

2pz

3py

3px

Quantum Numbers

s and p orbital

Movie

d orbital

Movie

Aufbau Principle

-For an atom in its ground state ( the lowest energy

configuration) fill the lowest energy orbital first

then go up in energy until all the electrons are used.

1s2s2p3s3p4s3d4p5s4d…..

Predicting Orbital Filling

Diagonal Rule

1s

2s 2p

3s 3p 3d

4s 4p 4d 4f

5s 5p 5d 5f

6s 6p 6d 6f

7s 7p 7d 7f

Energy Levels

For H atom All other atoms4s_4p_ _ _ 4d_ _ _ _ _4f _ _ _ _ _ _ _ 4s _

3d _ _ _ _ _

3p _ _ _

3s _ 3p _ _ _ 3d _ _ _ _ _ 3s _

2p _ _ _ (degenerate orbitals)

2s _ 2p _ _ _(degenerate orbitals) 2s _

1s _ 1s _

Inc

EIn

c E

Pauli Exclusion Principle• No two electrons in the same atom can have

the same four quantum numbers

• Each electron has a unique address

• Designated by a set of quantum numbers (n, l ,ml , ms)

• So: (1,0,0, +1/2) indicates an electron in a 1s orbital

• While (3,1, -1, +1/2) would be a 3p electron

A. (3,2,1,+1/2)B. (3,2,-1,+1/2)C. (3,1,1,+1/2)D. (3,2,2,-1/2)E. (3,2,0,+1/2)

Which of the following does not represent a 3d electrons?

What are the atomic numbers of elements

whose outer most electrons are represented by

2p3 and 3p6?

N= 1s22s22p3

Ar=1s22s22p63s23p6

Next class Chemical bonding