Created for NSTA eBooks+. Original image based on ARSCIMED/SCIENCE PHOTO LIBRARY.

Electron density of a helium atom. This image represents the quantum cloud of electrons surrounding a helium atom. The colours represent the local electric charge density, essentially the probability that one might find one of the atom’s four electrons at a given point. The colours run from blue (highest) to red (lowest). The chaotic appearance of the image is due to the presence of a nearby charged particle; when in equilibrium the atom would appear more symmetrical. This is a more appropriate image than the older ideas of fixed ‘orbitals’ for electrons, sometimes referred to as the Bohr Model, and is due to the principles of modern quantum theory.
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