Atoms: Not to Be Cut. Dalton’s Theory He deduced that all elements are composed of atoms. He deduced that all elements are composed of atoms. Atoms are.

Atoms: Not to Be CutAtoms: Not to Be Cut

Dalton’s TheoryDalton’s Theory

He deduced that all He deduced that all elementselements are composed of are composed of atoms. atoms.

Atoms are indivisible Atoms are indivisible particles.particles.

Atoms of the Atoms of the samesame element element are identical.are identical.

Atoms of Atoms of differentdifferent elements elements are are differentdifferent..

CompoundsCompounds are formed by are formed by the joining of atoms of two the joining of atoms of two or more elements.or more elements.

Thomson ModelThomson Model In In 18971897, J.J. Thomson , J.J. Thomson

proposed that an atom is proposed that an atom is made of even made of even smallersmaller particles with the “particles with the “PlumPlum PuddingPudding” model. ” model.

Atoms were made from a Atoms were made from a positively positively chargedcharged substancesubstance with negatively charged with negatively charged electrons electrons scatteredscattered about, about, like raisins in a pudding.like raisins in a pudding.

In 1908, Ernest Rutherford’s In 1908, Ernest Rutherford’s experiment Involved firing a stream of experiment Involved firing a stream of tiny tiny positively chargedpositively charged particles at a particles at a thin sheet of thin sheet of gold foilgold foil

RutherfordRutherford

Rutherford reasoned Rutherford reasoned that all of an atom’s that all of an atom’s positively charged positively charged particles were particles were containedcontained in the in the nucleus. The nucleus. The negatively charged negatively charged particles were particles were scatteredscattered outside the outside the nucleus around the nucleus around the atom’s atom’s edgeedge..

Bohr ModelBohr ModelIn 1913, the In 1913, the

Danish scientist Danish scientist Niels Bohr Niels Bohr proposed that the proposed that the electrons in an electrons in an atom existed at atom existed at specificspecific energy energy levels.levels.

Bohr ModelBohr Model According to Bohr’s According to Bohr’s

atomic model, atomic model, electrons move in electrons move in definite definite orbitsorbits around around the nucleus, much the nucleus, much like planets circle the like planets circle the sun. These orbits, or sun. These orbits, or energy energy levelslevels, are , are located at certainlocated at certain distances from the distances from the nucleus.nucleus.

Bohr Atom ModelBohr Atom Model

In this model, the In this model, the nucleus is orbited by nucleus is orbited by electrons, which are electrons, which are in different energy in different energy levels. levels.

The Bohr Model of the Atom The Bohr Model of the Atom • Bohr’s model of the atom

– Quantized energy levels

– Electron moves in a circular orbit

– Electron jumps between levels by absorbing or emitting photon of a particular wavelength

The Energy Levels of Hydrogen The Energy Levels of Hydrogen Atomic states Atomic states

Excited stateExcited state – – atom with excess atom with excess energy energy

Ground stateGround state – – atom in the lowest atom in the lowest possible state. possible state.

When an atom When an atom absorbs energy from absorbs energy from an outside source it an outside source it enters an excited enters an excited state. state.

A. The Energy Levels of A. The Energy Levels of Hydrogen Hydrogen Energy level diagramEnergy level diagram

• Energy in the photon corresponds to the energy used by the atom to get to the excited state.

A. The Energy Levels of A. The Energy Levels of Hydrogen Hydrogen Only certain types of photons are produced when atoms Only certain types of photons are produced when atoms

release energy. Why? release energy. Why?

A. The Energy Levels of A. The Energy Levels of Hydrogen Hydrogen Quantized Energy LevelsQuantized Energy Levels

– Since only certain energy changes occur the H atom must contain discrete energy levels.

B. B. The Bohr Model of the Atom The Bohr Model of the Atom • Bohr’s model of the atom

– Quantized energy levels

– Electron moves in a circular orbit

– Electron jumps between levels by absorbing or emitting photon of a particular wavelength

Electrons circle the nucleus due to the Electric force

Bohr’s Picture of the AtomBohr’s Picture of the Atom

Allowed Orbits

1

2

3

4

5

n =Electronin lowest“allowed”

energy level(n=1)

Electronin excited

state(n=5)

Before

1

2

3

4

5

Electron falls to the lowest

energy level

After

Radiatedphoton

Note: There are many more energy levels beyond n=5, they are omitted for simplicity

Atomic RadiationAtomic Radiation

The difference in energy, E, is given by:

E = E5 – E1 = hphoton

h = Planck’s constant = 6.6x10-34 [J s] = frequency of light [hz]

The energy of the light is DIRECTLY PROPORTIONAL to the frequency, .

Recall that the frequency, , is related tothe wavelength by:

c = c

So, higher frequency higher energy lower wavelength

This is why UV radiation browns your skinbut visible light does not !

It is now “known” that when an electron is in an “excited state”,it spontaneously decays to a lower-energy stable state.

Before

n = 1

n = 2

n = 3

n = 4

n = 5

Energy Electronin excited

state(higher PE)

E5

E4

E2

E3

E1

E5 > E4 > E3 > E2 > E1

After

n = 1

n = 2

n = 3

n = 4

n = 5

Energy Electronin lowest

state(lower PE)

E5

E4

E2

E3

E1

One example could be:

Light has a Light has a dualdual nature nature______________________________________________________

WaveWave (electromagnetic) - Interference (electromagnetic) - Interference - Diffraction- Diffraction ParticleParticle (photons) - Photoelectric effect (photons) - Photoelectric effect - Compton effect- Compton effect

Wave - Particle Duality for lightWave - Particle Duality for light

What about Matter?What about Matter?______________________________________________________________

If light, which was traditionally understood If light, which was traditionally understood as a wave also turns out to have a particle as a wave also turns out to have a particle nature, might matter, which is traditionally nature, might matter, which is traditionally understood as particles, also have a wave understood as particles, also have a wave nature?nature?

Yes!Yes!

Louis de Broglie’s hypothesisLouis de Broglie’s hypothesis________________________________________________________

The The dual dual nature of matternature of matter In 1924, Louis de Broglie proposed that In 1924, Louis de Broglie proposed that

matter particles have wave nature.matter particles have wave nature. A particle with momentum A particle with momentum pp has a has a matter matter

wavewave associated with it, whose associated with it, whose wavelength is given by wavelength is given by

λλ = h/p = h/mv = h/p = h/mv

p

h

p

h

The connecting link – Planck’s constantThe connecting link – Planck’s constant______________________________________________________________

DualDual Nature Nature

Radiation E = hfRadiation E = hf

Matter Matter λλ = h/p = h/p

Matter Waves ?Matter Waves ?

Particles, like photons, also have a wavelength given by:Particles, like photons, also have a wavelength given by:

= h/p = h / mv

That is, the wavelength of a particle depends on its momentum, just like a photon!

The main difference is that matter particles have mass, and photons don’t !

That is, the wavelength of a particle depends on its momentum, just like a photon!

The main difference is that matter particles have mass, and photons don’t !

Why isn’t the wave nature of matter more Why isn’t the wave nature of matter more apparent to us…?apparent to us…?______________________________________________________________________

Planck’s constant is Planck’s constant is so smallso small that we don’t that we don’t observe the wave behaviour of ordinary observe the wave behaviour of ordinary objects – their de Broglie wavelengths objects – their de Broglie wavelengths could be many orders of magnitude smaller could be many orders of magnitude smaller than the size of a nucleus! than the size of a nucleus!

Real photographs of an Real photographs of an electronelectron

interference pattern…interference pattern…

70 [s]70,000

electronsNotice the clear interferencefringes. Clear indication ofwave phenomenon.

Are matter waves for real?!Are matter waves for real?! ____________________________________________________________________

In 1927 In 1927 Davisson and GermerDavisson and Germer showed that electrons can showed that electrons can diffract – they act like waves diffract – they act like waves

Big application – Big application – Electron MicroscopesElectron Microscopes

Matter Waves (cont)Matter Waves (cont)

Compute the wavelength of a 1 [kg] block moving at 1000 [m/s].

= h/mv = 6.6x10-34 [J s] / (1 [kg])(1000 [m/s])

= 6.6x10-37 [m].

This is immeasureably small

For ordinary “everyday objects”, we don’t experience that matter can behave as a wave.

But, what about small But, what about small particles ?particles ?Compute the wavelength of an electron

(m = 9.1x10-31 [kg]) moving at 1x107 [m/s].

= h/mv = 6.6x10-34 [J s]/(9.1x10-31 [kg])(1x107 [m/s]) = 7.3x10-11 [m].

= 0.073 [nm]

These electronshave a wavelength in the region

of X-rays

How do we see ?How do we see ?Light reflects (scatters) from

a surface and reaches our eye.

Our eye forms an image of the object.

Wavelength versus SizeWavelength versus SizeEven with a visible light microscope, we are limited to beingable to resolve objects which are at least about 10-6 [m] = 1 [m] = 1000 [nm] in size.

This is because visible light, with a wavelength of ~500 [nm] cannotresolve objects whose size is smaller than it’s wavelength.

Bacteria, as viewed using visible light

Bacteria, as viewed using electrons !

Electron MicroscopeElectron Microscope

This image was taken with a Scanning Electron Microscope (SEM).

These devices can resolve features downto about 1 [nm]. This is about 100 times better than can be done with visible light microscopes!

This image was taken with a Scanning Electron Microscope (SEM).

These devices can resolve features downto about 1 [nm]. This is about 100 times better than can be done with visible light microscopes!

The electron microscope is a device which uses the wave behavior of electrons to make images which are otherwise too small for visible light!

IMPORTANT POINT HERE:High energy particles can be used to reveal the structure of matter !