Exitstion Flashcards
Excitation by collision
Using gas-filled tubes with a metal grid between the filament and the anode, we can show that gas atoms can absorb energy from colliding electrons without being ionised. This process, known as excitation, happens at certain energies,. If a colliding electron loses all its kinetic energy when it causes excitation, the current due to the flow of electrons through the gas is reduced. If the colliding electron does not have enough kinetic energy to cause excitation, it is deflected by the atom, with no overall loss of kinetic energy.
Excitation every
The energy values at which an atom absorbs energy are known as its excitation energies. We can determine the excitation energies of the atoms in the gas-filled tube by increasing the potential difference between the filament and the anode and measuring the pd when the anode current falls.
What happens when excitation occurs
When excitation occurs, the colliding electron makes an electron inside the atom move from an inner shell to an outer shell. Energy is needed for this process, because the atomic electron moves away from the nucleus of the atom. The excitation energy is always less than the ionisation energy of the atom, because the atomic electron is not removed completely from the atom when excitation occurs.
Every level diagram
The lowest energy state of an atom is called its ground state. When an atom in the ground state absorbs energy, one of its electrons moves to a shell at higher energy this shows the allowed energy values of the atom. Each allowed energy corresponds to a certain electron configuration in the atom. The energy levels below the ionisation level would then need to be shown as negative values,
De excitation
The electron configuration in an excited atom is unstable because an electron that moves to an outer shell leaves a vacancy in the shell it moves from. Sooner or later, the vacancy is filled by an electron from an outer shell transferring to it. When this happens, the electron emits a photon. The atom therefore moves to a lower energy level (the process of de-excitation).
The energy of the photon is equal to the energy lost by the electron and therefore by the atom.
Excitation using photons
An electron in an atom can absorb a photon and move to an outer shell where a vacancy exists - but only if the energy of the photon is exactly equal to the gain in the electron’s energy In other words, the photon energy must be exactly equal to the difference between the final and initial energy levels of the atom. If the photon’s energy is smaller or larger than the difference between the two energy levels, it will not be absorbed by the electron.
Fluorescence
The fluorescent tube is a glass tube with a fluorescent coating on its inner surface. The tube contains mercury vapour at low pressure.
When the tube is on, it emits visible light because:
• ionisation and excitation of the mercury atoms occur as they collide with each other and with electrons in the tube
• the mercury atoms emit ultraviolet photons, as well as visible photons and photons of much less energy, when they de-excite
• the ultraviolet photons are absorbed by the atoms of the fluorescent coating, causing excitation of the atoms
• the coating atoms de-excite in steps and emit visible photons.
Coulorful spectra
The wavelengths of the lines of a line spectrum of an element are characteristic of the atoms of that element. By measuring the wavelengths of a line spectrum, we can therefore identify the element that produced the light. No other element produces the same pattern of light wavelengths. This is because the energy levels of each type of atom are unique to that atom. So the photons emitted are characteristic of the atom.
• Each line in a line spectrum is due to light of a certain colour and therefore a certain wavelength.
• The photons that produce each line all have the same energy, which is different from the energy of the photons that produce any other line.
• Each photon is emitted when an atom de-excites due to one of its electrons moving to an inner shell.
• If the electron moves from energy level E, to a lower energy level E,
Wave particle duality- the wave like nature
The wave-like nature is observed when diffraction of light takes place. This happens, for example, when light passes through a narrow slit. The light emerging from the slit spreads out in the same way as water waves spread out after passing through a gap.
The narrower the gap or the longer the wavelength, the greater the amount of diffraction.
Wave particle duality - particle like nature
• The particle-like nature is observed, for example, in the photoelectric effect. When light is directed at a metal surface and an electron at the surface absorbs a photon of frequency f, the kinetic energy of the electron is increased from a negligible value by hf. The electron can escape if the energy it gains from a photon exceeds the work function of the metal.
