Electromagnetic radiation Flashcards
Electronvolt
The energy gained by an electron travelling through a potential difference of one volt
If an electron, with a charge of 1.6 × 10-19 C, travels through a potential difference of 1 V, the energy transferred is equal to:
E = Q V = (1.6 x 10^-19) C x 1V = 1.6 x 10^-19
therefore 1 eV = 1.6 x 10^-19 J
Electronvolt Relation to Kinetic Energy
When a charged particle is accelerated through a potential difference, it gains kinetic energy
If an electron accelerates from rest, an electronvolt is equal to the kinetic energy gained:
Electrons & energy levels
Electrons in an atom occupy certain energy states called energy levels
Electrons will occupy the lowest possible energy level as this is the most stable configuration for the atom
When an electron absorbs or emits a photon, it can move between these energy levels, or be removed from the atom completely
Excitation
When an electron absorbs enough energy to move up to a higher energy level
When an electron moves to a higher energy level, the atom is said to be in an excited state
To excite an electron to a higher energy level, it must absorb a photon
Electrons can also move back down to a lower energy level by de-excitation
To de-excite an electron to a lower energy level, it must emit a photon
Ionisation
When an atom gains or loses an orbital electron and becomes charged
When an electron is removed from an atom, the atom becomes ionised
An electron can be removed from any energy level it occupies
However, the ionisation energy of an atom is the minimum energy required to remove an electron from the ground state of an atom
Fluorescent Tube
Fluorescent tubes are partially evacuated glass tubes filled with low-pressure mercury vapour with a phosphor coating on the glass
Fluorescence process
- When a high voltage is applied across the tube, electrons flow from the cathode to the anode producing an electron beam
- These beam electrons collide with the electrons in the mercury atoms transferring kinetic energy in the collision
- The atomic electrons in the mercury atoms are excited and move to a higher energy level
- This high energy level state is unstable and so the electrons de-excite i.e. move back to their original ground state
- As they de-excite, the electrons release that energy by emitting photons in the UV range of wavelengths
- The UV photons then collide with electrons in the atoms of the phosphor coating and excite them into a higher energy level
- As these phosphor electrons de-excite, they do so in stages emitting photons in the visible light range of wavelengths
Line Spectra & Energy Levels
Energy levels can be represented as a series of horizontal lines
The line at the bottom with the greatest negative energy represents the ground state
The lines above the ground state with decreasing energies represent excited states
The line at the top, usually 0 eV, represents the ionisation energy
Line Spectra
Line spectra occur when excited atoms emit light of certain wavelengths which correspond to different colours
The emitted light can be observed as a series of lines with spaces in between
These series of lines are called line or atomic spectra
Each element produces a unique set of spectral lines
No two elements emit the same set of spectral lines, therefore, elements can be identified by their line spectrum
There are two types of line spectra: emission spectra and absorption spectra
Emission Spectra
When an electron transitions from a higher energy level to a lower energy level, this results in the emission of a photon
Each transition corresponds to a different wavelength of light and this corresponds to a line in the spectrum
The resulting emission spectrum contains a set of discrete wavelengths, represented by coloured lines on a black background
Each emitted photon has a wavelength which is associated with a discrete change in energy, according to the equation:
Difference in Discrete Energy Levels
