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the answer is energy.
the bohrs model states that electrons in an atom are arranged in orbits around the nucleus. the atoms in different orbit have different energies. therefore bohr used the term energy levels to describe these orbits of different energies. The energy level an electron normally occupies is called its ground state.
But it can move to a higher-energy, less-stable level, or shell, by
absorbing energy. This higher-energy, less-stable state is called the
electron’s excited state. After it’s done being excited, the electron can return to its original
ground state by releasing the energy it has absorbed.
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