In the quantum mechanical model, each allowed has a precisely known energy, but the location of the electron
cannot be determined exactly; rather, the probability of it being at a particular point in space is given by the probability density.
subatomic particle
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probability density
orbital
photon

Orbital

Explanation:

In Bohr's model, the planet model, it was thought that electrons were orbiting a nucleus just as planets are orbiting the sun. Over time, this model was proved to be inconsistent with the further findings, especially with the fact that electrons exhibited wave properties.

This lead to defining orbitals in terms of quantum mechanics. An orbital is defined as a space in which the probability of finding an electron is 95 %. Each allowed orbital has a precisely known energy, this is why we can calculate the energy needed to promote an electron to a higher orbital, as we know the difference in energies between the orbitals.

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