Electronic Electronic Structure of AtomsStructure of Atoms

Chapter 6Chemistry 100

What is light?What is light? Light is obviously real - it is part of our

world. Darkness is the absence of light Light is NOT a solid, a liquid, or even a gas So what is it? It is a form of energy We call it a form of radiant energy

because it carries energy through space

Electromagnetic radiationElectromagnetic radiation Visible light is a type of electromagnetic

radiation Other types include: infra-red, ultra-violet,

X-rays, gamma rays, radar waves, microwaves, radio and TV waves

Electromagnetic radiation has wave like properties

WavesWaves

Wavelength (lamda) Frequency (nu) Speed c (see?) = c c = 3.00 108m/s for all

types of electromagnetic radiation.

So how is IR different from UV, for example?

Electromagnetic SpectrumElectromagnetic Spectrum Different types of electromagnetic

radiation have different wave lengths () and different frequencies ()

Frequency: number of cycles (vibrations) per second. Unit is second-1 or s-1. The Hertz is the SI unit for frequency.

82,000 s-1 is the same as 82 kHz (kiloHertz)

Units for wavelengthUnits for wavelengthUnit Symbol Length (m) Type of RadiationAngstrom Å 10-10 X-rayNanometre nm 10-9 UV & visibleMicrometre m 10-6 IRMillimetre mm 10-3 IRCentimetre cm 10-2 microwaveMetre m 1 TV, Radio

Max Planck and his Max Planck and his constant constant hh Suggested that

energy is quantized - comes in small chunks

E = h where n = 1, 2, 3

Compare the potential energy of a brick on a staircase to one on a slope

Can this be true?Can this be true? We do not find that energy is quantized in

everyday life - h is very small. Cannot see the difference between 200,000,000h and

200,000,001 h Einstein used Planck’s idea to explain the

photoelectric effect For electromagnetic radiation, E = h where is

the frequency of the radiation. High frequency more energy

What is light?What is light? When we look how light behaves in

experiments with lens, mirrors, etc., we are led to believe that light has wave properties

In the photoelectric effect, light appears to consist of particles - which we call photons

Dual nature of electromagnetic radiation

Bohr’s AtomBohr’s Atom Bohr said: if energy is quantized then the

energy of an electron in an atom is quantized

So the radius of its orbit cannot be any arbitrary value but must obey the quantum theory. Only certain orbits are allowed

Allowed Orbitals in Bohr’s Allowed Orbitals in Bohr’s AtomAtom

2

2

220

4

n

22

20

n

nZ

h8meE

...3 ,2 ,1n where nZmehr

The quantity n is a quantum number

Bohr’s Atom 1913Bohr’s Atom 1913 Electrons move in orbitals with specified

radii Each orbital is associated with a specific

energy This explains why atoms emit (or absorb)

light of well-defined frequency. Examples: the yellow sodium street light and the neon tube.

Wave BehaviourWave Behaviour Louis de Broglie (1892-1987): If light can

have both wave and particle behaviour, why not wave behaviour for all particles?

= h/m He talked about matter waves

Matter wavesMatter wavesFind for electron moving at 5.97 106 m/s

rays- Xof wavelength the toSimilar nm122.0m1022.1kg1

g10J1

sm.kg1)s/m1097.5)(g1011.9(

Js1063.6mvh

10

322

629

34

Find for baseball moving at 100 km/h

meaningfulor - measurable be to small tooFar m1065.1hr

s3600km1

m1000kg1

g10J1

sm.kg1)hr/km100)(g145(

Js1063.6mvh

34

32234

HeisenbergHeisenberg Postulated that there is a limit to how

precisely we can measure both position and momentum

The measurement effects the object being measured

Heisenberg’s Uncertainty Principle

Schrödinger’s wave Schrödinger’s wave equationequation In 1926, Schrödinger put de Broglie’s

and Heisenberg’s ideas together and came up with the wave equation

The quantity 2 provides information about the electron's position when it has

energy E!

!!equation!ugly truly A EVdxd

m8h

2

2

2

2

Quantum NumbersQuantum Numbers Schrödinger's wave equation has three

quantum numbers. Principal quantum number n. Has integer values 1,

2, 3 Azimuthal quantum number, l. Allowed values

values of 0, 1... up to n - 1 Magnetic quantum number, ml. Allowed values -l …

0 … +l There is also the Spin quantum number, ms. It

can have a value of -½ or +½

Atomic orbitalsAtomic orbitals The first shell n = 1 The shell nearest the nucleus l = 0 We call this the s subshell (l = 0) m = 0 There is one orbital in the subshell

s = -½ The orbital can hold two electronss = + ½ one with spin “up”, one “down”

No two electrons in an atom can have the same value for the four quantum numbers: Pauli’s Exclusion Principle

The second shellThe second shelln = 2 l = 0 or 1 There are two subshells

l = 1 The p subshellm = -1, 0, +1 Three orbitals in the subshells = -½ or + ½ Each orbital can hold 2 electrons.

p subshell can hold 6 electrons

l = 0 The s subshellm = 0 One orbital in the subshells = -½ or + ½ Subshell can hold two electrons

The second shell can hold 8 electrons:

2 in s orbitals and 6 in p orbitals

If the principal quantum number is n, the shell can hold up to 2n2 electrons

s Orbitals are Sphericals Orbitals are Spherical

p Orbitals are Dumbbell p Orbitals are Dumbbell ShapedShaped

d Orbitals are Complexd Orbitals are Complex

Aufbau Aufbau PrinciplePrinciple

1s 2s2p 3s 3p 4s3d 4p 5s 4d 5p 6s4f 5d 6p 7s 5f 6d6f

Let’s do Sodium, Z = 11Let’s do Sodium, Z = 11 Aufbau Principle 1s 2s 2p 3s …. First 2 electrons 1s2 that’s 2 Next 2 electrons 2s2 that’s 4 Six this time 2p6 that’s 10 1 more to go 3s1 that’s all, folks

Electronic configuration of Na is 1s22s22p63s1

Hund’s RuleHund’s Rule

The configuration with the maximum spin is more stable.

Shall we use1s 2s 2p() () ()()

Or, shall we use1s 2s 2p() () ()()()

Shorthand Shorthand configurationsconfigurations The configuration of Neon is: 1s22s22p6

Na is 1s22s22p63s1, or in short form: [Ne]3s1 The configuration of Argon:1s22s22p63s23p6

K is: 1s22s22p63s23p64s1, which in short form becomes [Ar]4s1

Note the similarity of the two elements from the same group in the periodic table.The incomplete orbitals are 3s1 and 4s1.

Same group, similar Same group, similar configurationconfiguration

Fluorine: [He]2s22p5

Chlorine: [Ne]3s23p5

Bromine: [Ar]3d104s24p5

Iodine: [Kr]4d105s25p5

The outer-shell configuration in each case is s2p5

We need not be concerned with the d electrons here because d10 is a filled subshell.

Electronic Configuration Electronic Configuration & Periodic Table& Periodic Table

I’m in a spin!!!I’m in a spin!!! Nitrogen has Atomic Number 7 Electronic Configuration: 1s222s22p3

Let’s draw an orbital diagram: