Quantum Chemistry

Flashcards covering key concepts in quantum chemistry, including the photoelectric effect, Compton scattering, and wave-particle duality.

What is quantum mechanics?

Quantum mechanics is the framework for explaining the behavior of matter and energy at the atomic and subatomic levels, where classical mechanics fails.

Why is quantum mechanics important?

Understanding the structure of atoms, properties of solids, interactions with light, nuclear fission and fusion, semiconductors, LEDs, microscopy, and computing.

What did classical physics explain?

Newton’s laws explained motion and matter. Maxwell’s equations explained radiant energy (electromagnetism).

What is the UV Catastrophe?

The UV catastrophe refers to the contradiction between classical physics predictions (continuous emission of energy at shorter wavelengths) and experimental observations of black body radiation.

What was Planck's Quantum Theory about?

Planck proposed that electromagnetic waves are quantized, meaning energy is emitted and absorbed in discrete packets (quanta) rather than continuously.

According to Planck's Quantum Theory, how does matter absorb light?

Matter can only absorb light of frequency in discrete quanta, which we now call photons. E=hv

What is the Photoelectric Effect?

When light of variable frequency is shone on a metal surface, electrons are only emitted at particular frequencies. This is the photoelectric effect.

According to classical theory, what should control light energy in the photoelectric effect?

Classical theory suggested that light intensity should control light energy, meaning electron emission should occur irrespective of frequency.

What was observed in the Photoelectric Effect experiment?

Electron emission depended on the frequency rather than intensity. A certain threshold frequency was required for emission.

How does light behave in the Photoelectric Effect?

Light behaves as a particle (photons), transferring energy to electrons. The work function is the minimum energy required to release an electron.

How is kinetic energy and threshold frequency related in the Photoelectric Effect?

The electron released has a kinetic energy = 1/2mv^2. The threshold frequency (v0), must be exceeded to give photons enough energy to enable electrons to escape from the surface: hv0 = (M), so 0 = (M)/h

How is the Photoelectric Effect experiment performed?

Variable frequency light source, shine light onto a metal surface. Determine the light frequency which causes electrons to be emitted. Measure the energy of the emitted electrons.

What happens during Photoelectron Spectroscopy?

EM radiation (typically X-Ray) is directed onto a molecule/material, and the energy of the electrons emitted is measured: ½ mev2 = h - I (where I is the ionisation energy, instead of the work function).

What is XPS used for?

It can determine composition, and chemical properties of surfaces. Can be used for molecules!

What happens during Compton Scattering?

If light can be described as photons, if they collide with other particles, there should be a change in their momentum (= mass x velocity).

What were Compton's results?

The incident wavelength peak was always present, but also observed a peak at different wavelengths. The shift in 𝝀 varies with the angle

How can the photon model explain Compton Scattering?

The photon model explains the change in energy/momentum of photons scattered by individual electrons. ∆ = (2h/mec) sin2 (½)

What do electrons act like, in addition to particles?

Electrons can also act like waves.

What do diffraction and interference patterns indicate about electrons?

Diffraction and interference patterns indicate wave-like behavior.

What does it mean to say that ligh and electrons exhibit wave-particle duality?

Light behaves as a particle at the atomic scale. The electron can also behave like a wave. This leads to the concept of wave-particle duality