2.5 Energy levels & photon emission

Question 1

What is ionisation of an atom

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

how does a fluorescent tube work?

Answer 3

• high voltage applied across mercury vapour accelerates fast moving free electrons which collide with the mercury atoms
• mercury electrons are excited and the return to the ground state, releasing a UV photon
• the tubes phosphorus coating absorbs the UV photons and its electrons are excited, they cascade down energy levels emitting visible light photons

Question 4

What is a photon

Answer 4

a massless packet or a quantum of electromagnetic energy

Question 5

What are electron energy levels?

Answer 5

the certain specific energy of an electron in an atom

Question 6

How are electron energy levels represented?

Answer 6

as a series of stacked horizontal lines increasing in energy

Question 7

What energy level do electrons normally occupy?

Answer 7

the lowest energy level available, known as ground state

Question 8

how can electrons move up in energy?

Answer 8

• they must gain energy to move up
• this can happen when the electrons absorb energy
• this can be by
• collisions with other atoms or lectrons
• absorbing a photon
• a physical source, such as heat

Question 9

what is the process of electrons moving through an excited state either to, ionisation, or returns back?

Answer 9

• as the electron gains energy it moves up through the energy levels becoming in an excited state
• if the electrons gain enough energy to be removed from the atom entirely it is known as ionisation
• when the electron returns to lower energy state from its excited state it returns in the form of a photon

Question 10

When does line spectra occur?

Answer 10

• occurs when excited atoms emit light of certain wavelengths corresponding to different colours

Question 11

What is line spectra?

Answer 11

the emitted light observed of coloured lines with dark spaces between

Question 12

What are the 2 types of line spectra?

Answer 12

emission spectra
absorption spectra

Question 13

What is the main feature of line spectra?

Answer 13

it ia purely unique
- every element produces its own set of spectral lines, therefore elements can be identified using this

Question 14

how does emission spectra occur?

Answer 14

• when an electron transitions from a higher energy level to a lower energy level a photon is emitted
• each transition corresponds to a different wavelength of light corresponding to a line in the spectrum
• resulting in emission spectrum containing a set of discrete wavelengths

Question 15

What is absorption spectra?

Answer 15

when white light passes through a cool, low pressure gas it is found that the light of certain wavelengths are missing
- this spectrum is called an absorption spectrum

Question 16

What are the properties of an absorption spectrum?

Answer 16

• consists of a continuous spectrum containing al the colours with dark lines at certain wavelengths
• these dark lines correspond exactly to the differences in energy levels in an atom
• when returning to lower levels, the photons are emitted in all directions rather than in the original direction of the white light

Question 17

What is the difference in discrete energy levels?

Answer 17

difference between two energy levels is equal to a specific photon energy

Question 18

What is wave-particle duality?

Answer 18

• light behaving as a particle ie photons and a wave

Question 19

how is light a particle?

Answer 19

• it interacts with matter such as electrons as a particle
• proves b the photoelectric effect

Question 20

How is light a wave?

Answer 20

• it propagates through space as a wave
• proven by diffraction and interference of light in Young’s double slit

Question 21

What are photons?

Answer 21

quanta of energy that behave as a particle

Question 22

What is the photon model of light?

Answer 22

• electromagnetic waves carry energy in discrete packets called photons
• the energy of the photons are quantised according to the E=hf eq
• in the photoelectric effect each electron can absorb only a single photon
• only the frequencies of light above the threshold frequency will emit a photoelectron

Question 23

What is the wave theory of light?

Answer 23

• frequency of light can give rise to photoelectric emission if th exposure time is long enough
• this is because the energy absorbed by each electron will increase gradually with each wave
• the KE of the emitted electrons should increase with radiation intesity
• in the photoelectric effect, this is not what is observed

Question 24

What occurs if the frequency of the incident light is above the threshold and the intensity of the light is increased?

Answer 24

more photoelectrons are emitted per second

Question 25

The wave theory suggest, frequency of light can give rise to photoelectric emission if the exposure time is long enough, what is wrong with this?

Answer 25

photoelectrons will be released immediately if the frequency is above the threshold for that metal

Question 26

The wave theory suggest, the energy observed by each electron will increase gradually with each wave, what is wrong with this?

Answer 26

• energy is absorbed instantly
• photoelectrons are either emitted or not emitted after exposure to light

Question 27

The wave theory suggest, the KE of emitted electrons should increase with radiation intensity , what is wrong with this?

Answer 27

• if the intensity of the light is increased
• more photoelectrons are emitted per second