Neil Bohr discovered the shells that electrons exist in on an atom. They’re sometimes called energy shells, that exist at different distances from the nucleus. The shells are numbered from one onwards. As the shells become larger, they are capable of holding more electrons.
Each shell is labeled from one onwards, increasing in number as you get further from the nucleus, this is known as the principle quantum number. As the shells move further away from the nucleus they can carry more electrons.
Shell 1 can carry 2 electrons
Shell 2 cam carry 8 electrons
Shell 3 can carry 18 electrons
Shell 4 can carry 32 electrons
The relative energy increases as the shell number increases.
As an electron changes shells a certain amount of energy is required. This energy is called a quantum (plural: quanta).
If an electron moves from a higher electron shell to a lower electron energy shell it is said will emit, or the quantum is “emitted” a quantum of energy.
If the electrons go from a lower shell, to an outer higher shell it is said the quantum of energy is “absorbed.”
This is related to quantum mechanics which cannot the exact location of a particle in space, but the probability of finding it at different locations. Quantum mechanices grew out of the fact that electrons orbiting around a nucleus of an atom, could not be fully described as particles, but rather need further explanation.
This has given rise to the wave-particle duality which is a concept in quantum mehcniaces that every particle or quantum can be described as a particle or wave.
This gives rise to the Heisenberg uncertaint principle. This is that the excat speed, and location of a particle or wave is not precise, but rather we focus on where is the greatest probability of finding them.