Chemistry is in the electronsChemistry is in the electrons

Electronic structure – how the electrons Electronic structure – how the electrons are arranged inside the atomare arranged inside the atomTwo parameters:Two parameters:– EnergyEnergy– PositionPosition

Learning objectivesLearning objectives

Describe the properties of wavesDescribe the properties of waves

Describe the photon and the relationship Describe the photon and the relationship between light frequency and energybetween light frequency and energy

Setting the scene: Why don’t electrons Setting the scene: Why don’t electrons collapse into the nucleus?collapse into the nucleus?

Like charges attractLike charges attract

The planetary model...doesn’t workThe planetary model...doesn’t work

Unlike the moon, an orbiting charged Unlike the moon, an orbiting charged particle will spiral inwards to the nucleusparticle will spiral inwards to the nucleus

Moving charges lose energy by radiationMoving charges lose energy by radiation

Conventional explanations do not Conventional explanations do not workwork

Yet... atoms exist and electrons are stable Yet... atoms exist and electrons are stable outside the nucleusoutside the nucleus

The iconic symbol of the planetary atom is The iconic symbol of the planetary atom is not correctnot correct

There must be another explanationThere must be another explanation

Light and colourLight and colour

White light contains many coloursWhite light contains many colours

They can be separated by a prism They can be separated by a prism (rainbow)(rainbow)

Absorption determines colourAbsorption determines colour

Objects are the colour of the light Objects are the colour of the light notnot absorbedabsorbed

White absorbs nothingWhite absorbs nothing

Black absorbs everythingBlack absorbs everything

Blue absorbs redBlue absorbs red

Making wavesMaking waves

Light is an lectromagnetic wave:Light is an lectromagnetic wave:– WavelengthWavelength – distance between two peaks – distance between two peaks– FrequencyFrequency - number of wave peaks that pass - number of wave peaks that pass

a fixed point per unit timea fixed point per unit time– AmplitudeAmplitude - height of the peak measured - height of the peak measured

from the center linefrom the center line– VelocityVelocity – speed of the wave – speed of the wave

Wavelength and colourWavelength and colour

Different “colours” of electromagnetic Different “colours” of electromagnetic radiation are waves with different radiation are waves with different wavelengths and frequencies.wavelengths and frequencies.

All electromagnetic radiation has the same All electromagnetic radiation has the same velocity: the speed of light – 3 x 10velocity: the speed of light – 3 x 1088 m/s m/s

Velocity = wavelength x frequencyVelocity = wavelength x frequency

Wavelength proportional to 1/frequency - Wavelength proportional to 1/frequency - as wavelength as wavelength ↑ frequency ↓↑ frequency ↓

The electromagnetic spectrumThe electromagnetic spectrum

Continuous range of wavelengths and Continuous range of wavelengths and frequencies:frequencies:– Radio waves at Radio waves at lowlow-frequency end -frequency end – Gamma rays at Gamma rays at highhigh-frequency end-frequency end– Visible region is a small slice near the middle Visible region is a small slice near the middle

Wavelength of X-ray is same as diameter Wavelength of X-ray is same as diameter of an atom (10of an atom (10–10–10 m) m)Radiation becomes more dangerous as Radiation becomes more dangerous as frequency increasesfrequency increases

Atoms emit and absorb radiation at Atoms emit and absorb radiation at specific wavelengthsspecific wavelengths

Absorption is light removed by the atom from Absorption is light removed by the atom from incident lightincident light

Emission is light given out by an energetically Emission is light given out by an energetically excited atomexcited atom

The absorption and emission lines are at the The absorption and emission lines are at the same wavelengthssame wavelengths

The lines from the H atom form a neat series.The lines from the H atom form a neat series.

Do these spectra have anything to do with the Do these spectra have anything to do with the electronic structure?electronic structure?

Each element has a unique spectrumEach element has a unique spectrum

Electronic structures of each element are Electronic structures of each element are differentdifferent

Spectra can be used to identify elements – Spectra can be used to identify elements – even in very remote locationseven in very remote locations

Light in coherence: LASERSLight in coherence: LASERS

LLight ight AAmplification by mplification by SStimulated timulated EEmission mission of of RRadiationadiation

Laser consists of one wavelength onlyLaser consists of one wavelength only

All the waves are “coherent”All the waves are “coherent”

Three essentials of a laserThree essentials of a laser

Laser medium – solid, Laser medium – solid, liquid, gasliquid, gas

Source of “pump” Source of “pump” energy to excite energy to excite medium –light, medium –light, electrical, chemicalelectrical, chemical

Cavity to “build up” Cavity to “build up” powerpower

All shapes and sizesAll shapes and sizes

Solid-state lasersSolid-state lasers– Crystals, semiconductor chipsCrystals, semiconductor chips

Liquid lasersLiquid lasers– DyesDyes

Gas lasersGas lasers– Helium, Argon, Carbon dioxideHelium, Argon, Carbon dioxide

Other forms of radiation: nuclear Other forms of radiation: nuclear magnetic resonance (NMR)magnetic resonance (NMR)

Magnetic nuclei (like H) in magnetic field Magnetic nuclei (like H) in magnetic field absorb radiationabsorb radiation

Radiation is in the radiowave regionRadiation is in the radiowave region

Measure absorption peak when radiowave Measure absorption peak when radiowave has resonance frequencyhas resonance frequency

Magnetic resonance imaging (MRI)Magnetic resonance imaging (MRI)

The body is full of H atoms (water)The body is full of H atoms (water)

Resonance occurs at different peaks in the Resonance occurs at different peaks in the bodybody

Result is high-resolution map of the inside Result is high-resolution map of the inside of the bodyof the body

Non-invasiveNon-invasive

SafeSafe

ExpensiveExpensive

Problem with light: wave or particle?Problem with light: wave or particle?The photoelectric effectThe photoelectric effect

Light incident on a metal surface causes Light incident on a metal surface causes electrons to be emitted.electrons to be emitted.

Below threshold frequency nothing happensBelow threshold frequency nothing happens

Above threshold, current increases with intensityAbove threshold, current increases with intensity

No current flows

Current flows

Quantization: light as particle Quantization: light as particle andand wavewave

In photoelectric effect, light behaves like a In photoelectric effect, light behaves like a particle.particle.Energy is “quantized” into packetsEnergy is “quantized” into packetsEach packet is a Each packet is a photonphotonPhoton Photon energyenergy depends on frequency: depends on frequency:

EE = h = hνAs frequency increases photon energy increasesAs frequency increases photon energy increases

Photon energy increases from right to leftPhoton energy increases from right to left

““Dangerous” radiation has high photon energy – Dangerous” radiation has high photon energy – UV light, X-raysUV light, X-rays

““Harmless” radiation has low photon energy – Harmless” radiation has low photon energy – IR, radiowavesIR, radiowaves

Blue light has higher energy than red lightBlue light has higher energy than red light