2.5 - Energy Levels & Photo Emission

Question 1

What is ionisation?

Answer 1

The removal, or addition, of an electron from, or to, an atom when given sufficient energy.

Question 2

What is excitation?

Answer 2

When an electron is given enough energy to move up an energy level, but not enough to leave the atom.

Question 3

When does florescence occur?

Answer 3

When an electron (in atomic orbit of a mercury atom) is excited and then de-excites, releasing a UV photon.
This UV light excites the electrons in the phosphor coating which, as a result creates a fluorescent glow.

Question 4

What are photons?

Answer 4

Massless ‘packets’ of electromagnetic energy.

Question 5

What is the equation to find the energy of a photon?

Answer 5

E = hf
E = hc/λ

h = Plank’s constant
c = speed of light
λ = wavelength
f = frequency

Question 6

How do you calculate the number of photons incident on a surface per second?

Answer 6

Power of light source / Energy of one photon

Question 7

What are electron energy levels?

Answer 7

Electrons in an atom can only have a specific amounts of energy. (energy levels are the shells around the atom)

Question 8

What is the lowest energy level called?

Answer 8

Ground state.

Question 9

What are 3 ways that electrons gain energy and move up energy levels?

Answer 9

• Collisions with other atoms or electrons
• Absorbing a photon
• A physical source, like heat

Question 10

What is line spectra?

Answer 10

The wavelengths of light emitted from an element when its electrons de-excite.

Question 11

What are the 2 types of line spectra?

Answer 11

Emission spectra and absorption spectra.

Question 12

How do you get a line spectrum?

Answer 12

Passing light from a fluorescent tube through a diffraction grating or prism, you get a line spectrum. Each line in the spectrum represents a different wavelength of light.

Question 13

What is the emission spectra?

Answer 13

• When a photon is emitted after de-excitation, each transition lets out a different wavelength of light.
• The resulting emission spectrum contains a set of discrete wavelengths, represented by coloured lines on a black background.

Question 14

What is the absorption spectra?

Answer 14

• When electrons excite after the absorption of a photon. When white light passes through a cool, low pressure gas it is found that light of certain wavelengths are missing, this shows on the spectrum.
• Colour background and black lines (black lines - energy levels)

Question 15

What can be used as evidence for the discrete energy levels in atoms?

Answer 15

Emission and absorption spectra.

Question 16

What is the definition of discrete energy levels?

Answer 16

The specific energy levels of emitted photons when electrons in orbit get excited.

Question 17

What is the equation for energy of the emitted photons in atoms?

Answer 17

ΔE = hf = E₁ - E₂
E₁ = Energy of higher level
E₂ = Energy of the lower level
h = Planck’s constant
f = Frequency of photon (Hz)

Question 18

Using the wave equation, how do you find the wavelength of the emitted / absorbed radiation?

Answer 18

λ = hc / E₂ - E₁

Question 19

What is wave-particle duality?

Answer 19

The fact that light behaves as both a particle and a wave.

Question 20

What is the evidence for wave-particle duality?

Answer 20

• Proof that light behaves like a particle - The photoelectric effect (as light here interacts with matter)
• Proof that light behaves like a wave - The Young’s Double Slit experiment (as light diffracts).

Question 21

What is the equation to find the De Broglie wavelength?

Answer 21

λ = h /mv
h = Plank’s constant
mv = momentum

Question 22

What is proof that electrons have wave-like properties?

Answer 22

Electron diffraction as only waves can diffract.

Question 23

What is the interference pattern of an electron?

Answer 23

Concentric rings.

Question 24

When are diffraction patters on accelerated electrons observed?

Answer 24

When accelerated electrons in a vacuum tube interact with the space in a graphite crystal.

Question 25

When does diffraction occur with particles?

Answer 25

When a particle interacts with an object of about the same size as its de Broglie wavelength.

Question 26

In electron diffraction experiments, what does a smaller accelerating voltage achieve?

Answer 26

Slower electrons, giving more widely-spaced rings.

Question 27

How can momentum and KE be related? (equations)

Answer 27

E = p²/2m
or
p = √2mE
(E = KE)

Question 28

How can momentum and KE be used in the de Broglie equation?

Answer 28

λ = h/√2mE
(E = KE)

Question 29

How is the radius of the diffraction pattern and the wavelength related?

Answer 29

The longer the wavelength, the more the light spreads out, therefore there is a larger radius produced.
(directly proportional)