Final answer:
Bohr proposed that an electron is only found in specific circular paths, or orbits, around the nucleus. These orbits represent fixed energy levels, and the electrons can only occupy these discrete levels.
Step-by-step explanation:
Bohr proposed that an electron is found only in specific circular paths, or orbits, around the nucleus. According to Bohr's Model, electrons move around the nucleus in circular paths that have certain allowed radii. These paths are known as energy levels (described by the principal quantum number n) and the electrons can only occupy these discrete levels, similar to the way a person can only stand on the rungs of a ladder.
In Bohr's theoretical model, particularly for the hydrogen atom, an electron in a specific orbit has a fixed energy level. Orbits closer to the nucleus have lower energy, and those farther away have higher energy. Transitions between these orbits involve the absorption or emission of energy in quantized amounts.
Unlike the orbits suggested by Bohr, orbitals are a quantum mechanical concept introduced later by Erwin Schrödinger, which corresponds to the regions in space where there is a probability of finding an electron.