Particle Nature of Light

• The wave model of light cannot explain why heated objects emit only certain frequencies of light at a given temperature

• Or why some metals emit electrons when colored light of a specific frequency shines on them.

• New Experiments showed that the wave model of light needed to be revised.

Quantum ConceptQuantum Concept Max Planck Max Planck (1900)(1900)

ObservedObserved - emission of - emission of lightlight from hot objects from hot objects

ConcludedConcluded - energy is - energy is emitted in small, specific emitted in small, specific amounts (amounts (quantaquanta))

QuantumQuantum - minimum amount of energy that can be gained or lost by an atom - minimum amount of energy that can be gained or lost by an atom

Quantum ConceptQuantum Concept Planck Planck (1900)(1900)

vs.

Classical Theory Quantum Theory

Because these steps are so small, the energy/temperature seems to rise in a continuous, rather than a stepwise, manner.

The quantum concept

• Planck’s constant has a value of 6.626 x 10–34 J · s, where J is the symbol for the joule, the SI unit of energy.

Remember: Energy is related to frequency (and also wavelength!)

• Looking at the equation, you can see that the energy of radiation increases as the radiation’s frequency, v (of “f”), increases.

• According to Planck’s theory, for a given frequency, , (or f) matter can emit or absorb energy only in whole-number multiples of h; that is, 1h, 2h, 3h, and so on.

The Quantum Concept

• Matter can have only certain amounts of energy—quantities of energy between these values do not exist.

Light as a ParticleLight as a Particle Einstein Einstein (1905)(1905)

ObservedObserved - photoelectric effect - photoelectric effect

The Photoelectric Effect• In the photoelectric effect,

electrons, called photoelectrons, are emitted from a metal’s surface when light of a certain frequency shines on the surface.

Light as a ParticleLight as a Particle Einstein Einstein (1905)(1905)

ConcludedConcluded - light has properties - light has properties of both of both waveswaves and and particlesparticles

““wave-particle duality”wave-particle duality”

PhotonPhoton - - particleparticle of light that of light that carries a quantum of energycarries a quantum of energy

The Photoelectric Effect

• Further, Einstein proposed that the energy of a photon of light must have a certain minimum, or threshold, value to cause the ejection of a photoelectron.

• According to this theory, even small numbers of photons with energy above the threshold value will cause the photoelectric effect.

Bohr’s Model of the Bohr’s Model of the AtomAtom

A combination of Plank’s theory and A combination of Plank’s theory and Einstein’s theories helped Bohr develop Einstein’s theories helped Bohr develop and explain his and explain his EnergyEnergy LevelLevel ModelModel..

•Bohr studied the Atomic Emission Spectrum of the Hydrogen Atom to calculate the energy of each of the energy levels in the atom.

Atomic Emission Spectra of Hydrogen

•The fact that only certain colors appear in hydrogen’s atomic emission spectrum means that only certain specific frequencies of light are emitted.

•Hydrogen’s atomic emission spectrum consists of several individual lines of color, not a continuous range of colors (like a rainbow) as seen in the visible spectrum.

Atomic Emission Spectra

• The atomic emission spectrum of an element is the set of frequencies of the electromagnetic waves emitted by atoms of the element.

•An atomic emission spectrum is characteristic of the element being examined and can be used to identify that element.

Bohr Model Bohr proposed that electrons exist

only in orbits with specific amounts of energy called energy levels

Therefore… e- can only gain or lose certain

amounts of energy only certain photons are produced These photons have different colors

because they have different energy values

Line-Emission Spectrum

ground state

excited state

ENERGY IN PHOTON OUT

•Bohr said the different colors of light emitted were caused by an electron that had been excited away from their ground state near the nucleus, returning to its ground state.

The lowest allowable energy state The lowest allowable energy state of an electron in an atom is called of an electron in an atom is called its its ground stateground state. .

Electrons must absorb a specific Electrons must absorb a specific “quantum of energy” to be excited “quantum of energy” to be excited to a higher energy level. This is to a higher energy level. This is said to be it’s said to be it’s “excited state“excited state””

When the excited electron drops When the excited electron drops back down to its “ground state” it back down to its “ground state” it emits a photonemits a photon of light with the of light with the same energy as the difference same energy as the difference between the energy levels it between the energy levels it drops.drops.

Energy Level Model Bohr measured the wavelengths and

frequencies of the light emitted and looked for patterns.

He came up with an equation to determine the energy of the light that will be emitted or absorbed when an electron changes energy levels.

This is how he calculated the energy of each of the Energy Levels in his model!!!

Bohr Energy Level Model

1

23

456 Energy of photon depends on the difference in energy levels

Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom

•The smaller the electron’s orbit, the lower the atom’s energy state, or energy level.•Conversely, the larger the electron’s orbit, the higher the atom’s energy state, or energy level.

An explanation of hydrogen’s line spectrum

• The four electron transitions that account for visible lines in hydrogen’s atomic emission spectrum are shown.

Other Elements Each element has a unique bright-

line emission spectrum. “Atomic Fingerprint”

Helium

Bohr’s calculations only worked for hydrogen!

Energy states of hydrogen• And although a hydrogen atom contains

only a single electron, it is capable of having many different excited states.