2.5 Energy levels & photon emission Flashcards
What is ionisation of an atom
the removal or addition of an electron from or to an atom when given sufficient energy
What is excitation?
when an electron is given enough energy to move up an energy level but not enough to leave the atom
how does a fluorescent tube work?
- 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
What is a photon
a massless packet or a quantum of electromagnetic energy
What are electron energy levels?
the certain specific energy of an electron in an atom
How are electron energy levels represented?
as a series of stacked horizontal lines increasing in energy
What energy level do electrons normally occupy?
the lowest energy level available, known as ground state
how can electrons move up in energy?
- 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
what is the process of electrons moving through an excited state either to, ionisation, or returns back?
- 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
When does line spectra occur?
- occurs when excited atoms emit light of certain wavelengths corresponding to different colours
What is line spectra?
the emitted light observed of coloured lines with dark spaces between
What are the 2 types of line spectra?
emission spectra
absorption spectra
What is the main feature of line spectra?
it ia purely unique
- every element produces its own set of spectral lines, therefore elements can be identified using this
how does emission spectra occur?
- 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
What is absorption spectra?
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
What are the properties of an absorption spectrum?
- 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
What is the difference in discrete energy levels?
difference between two energy levels is equal to a specific photon energy
What is wave-particle duality?
- light behaving as a particle ie photons and a wave
how is light a particle?
- it interacts with matter such as electrons as a particle
- proves b the photoelectric effect
How is light a wave?
- it propagates through space as a wave
- proven by diffraction and interference of light in Young’s double slit
What are photons?
quanta of energy that behave as a particle
What is the photon model of light?
- 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
What is the wave theory of light?
- 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
What occurs if the frequency of the incident light is above the threshold and the intensity of the light is increased?
more photoelectrons are emitted per second
