energy levels and photon emission Flashcards
how do electrons in atoms exist
in discrete energy levels. electrons in an atom can only exist in certain well defined energy levels
what is each level given
a number, with n = 1 representing ground state
how can electrons move down energy levels
by emitting a photon
what can the value of each photon be
these transitions are between definite energy levels so the value can only take a certain allowed value
why is the joule not used
the values are so tiny it makes sense to use eV
what is an electron volt defined as
the kinetic energy carried by an electron after it has been accelerated through a potential difference of 1 volt
what does 1eV equal
1.60 x 10^-19 J
what is the energy carried by each photon equal to
the difference in energies between the two levels
show an equation representing transition from a higher energy to a lower energy
E(delta) = E2 - E1 = hf = hc/lambda
how can electrons move up energy levels
absorbing a photon with the exact energy difference between the two levels
what is the movement of an electron to a higher energy level called
excitation
what if an electron is removed from the atom
we say the atom is ionised
what is the ionisation energy of an atom
the amount of energy needed to completely remove an electron from the atom from the ground state, n = 1
how do fluorescent tubes produce light
they use excited electrons
what do fluorescent tubes contain
mercury vapor, across which an initial voltage is applied
what does the high voltage do
accelerates fast moving free electrons that ionise some of the mercury atoms, producing more free electrons
what happens when the flow of mercury atoms collides with electrons in other mercury atoms
the electrons in the mercury atoms are excited to higher energy levels
what happens next
when these electrons return to their ground state, they emit photons in the UV range
what absorbs these photons
a phosphor coating on the inside of the tube absorbs these photons, exciting its electrons to much higher orbits.
what happens next
these electrons cascade down to the energy levels emitting many lower energy photons in the form of visible light
what do fluorescent tubes also produce
line emission spectra
how do you get a line spectrum
if you split light from a fluorescent tube with a prism or a diffraction grating you get a line spectrum
what is a line spectrum seen as
a series of bright lines against a black background
what does each line correspond to
a particular wavelength of light emitted by the source
what wavelengths can you see
since only certain photon energies are allowed you can only see wavelengths corresponding to these energies
what does shining a white light through a cool gas give
an absorption spectrum
what does a continuous spectra contain
all possible wavelengths
what is the spectrum of white light
continuous
what happens if you split the light up with a prism
the colours will merge into each other with no gaps in the spectrum
what emits a continuous spectrum
hot things emit a continuous spectrum in the visible and infrared
why are all wavelengths allowed
because electrons are not confined to to energy levels in the object. the electrons are not bound by atoms and are free
what do cool gases do
remove certain wavelengths from a continuous spectrum
what do you get when light with a continuous spectrum of energy passes through a cool gas
line absorption spectrum
where will most of the electrons in a gas atom be in low temperatures
ground states
what can these electrons absorb
only photons with energy equal to the difference between two energy levels
what happens to the photons
photons of the corresponding wavelengths are absorbed by the electrons to excite them to higher energy levels
