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The concept of quantized energy levels, first proposed by Neils Bohr, states that electrons can only exist in certain possible energy levels, which he pictured as orbits around a nucleus since the energy of an electron is proportional to its distance from the nucleus.
nuclear shielding
Energy levels or Energy
Niels Bohr first suggested that electrons orbited the nucleus in fixed energy levels.
Electrons are arranged around the atomic nucleus forming the electron clouds.
Ernest did an experiment where he shot alpha particles into a gold foil that proved the concept of a nuclear atom, that is an atom with a nucleus made of protons and neutrons and have electrons moving around the nucleus in some way. His experiment did not show the position of electrons or energy levels or even how they moved.
Electrons
Electrons -- in energy levels outside the nucleus. Protons in the nucleus. Neutrons in the nucleus.
Electrons -- in energy levels outside the nucleus. Protons in the nucleus. Neutrons in the nucleus.
Who? Niels Bohr started the concept of electrons orbiting the nucleus, but I'm not sure he called them shells, but more along the lines of energy levels.
those closest to the nucleus
The answer which I found turns out to be energy level