MCAT Content / Electronic Structure / Photoelectric Effect

Photoelectric effect

Topic: Electronic Structure

When light strikes materials, it can eject electrons from them. This is called the Photoelectric effect.

In the photoelectric effect, electrons are emitted from matter (metals and non-metallic solids, liquids, or gases) as a consequence of their absorption of energy from electromagnetic radiation of high frequency (short wavelength), such as ultraviolet radiation.

All atoms have their electrons in orbitals with well-defined energy levels. When electromagnetic radiation interacts with an atom, it can excite the electron to a higher energy level, which can then fall back down, returning to the ground state. However, if the energy of the light is such that the electron is excited above energy levels associated with the atom, the electron can break free from the atom leading to ionization of the atom. These escaping electrons are called photons. The energy of a photon is equal to the product of Planck’s constant and frequency of EM Wave.

E = hf

With h is Planck constant 6.62607004 × 10-34 m2 kg / s

The energy required to knock off an electron is called the ionization energy (Φ). When the absorbed energy is less than an atom’s ionization energy (threshold), its electron will jump to a higher energy level, but will not leave the atom. If the photon’s energy is higher than ionization energy, the electron will leave the atom with kinetic energy:

KE = hf – Φ

 


Practice Questions

 


MCAT Official Prep (AAMC)

Physics Online Flashcards Question 14



Key Points

• Photoelectric effect: When light strikes materials, it can eject electrons from them.

• The energy in an EM wave is composed of energy quanta called photons. The energy of a photon is equal to the product of Planck’s constant and frequency of EM Wave.

• Each photon must individually have enough energy that meets the threshold or higher for an electron to come off the atom.


Key Terms

Photon: the smallest discrete amount of electromagnetic radiation. It is the basic unit of all light, carrying the energy E= hf.

Ionization energy: The energy required to knock off an electron.

Electron: each electron has a negative charge (-1) with weight so small it’s normally negligible as compared to proton or neutron.

Ionization: any process that leads to the dissociation of a neutral atom or molecule into charged particles (ions).



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