Niels Bohr's model of the atom built upon Ernest Rutherford's nuclear model and addressed some of its key limitations. Rutherford's model proposed that electrons orbit the nucleus in a manner analogous to planets orbiting the Sun. However, this model faced certain problems:
- Electromagnetic Radiation: According to classical electromagnetism, accelerated charges emit radiation. Since electrons in orbit around the nucleus are constantly changing direction, they should lose energy and eventually collapse into the nucleus.
Bohr's Solution: Bohr proposed that electrons can only occupy specific, quantized energy levels or orbits. These orbits are stable and do not emit radiation. Electrons can transition between these orbits by absorbing or emitting energy in discrete packets called quanta or photons. This explanation resolved the issue of electromagnetic radiation and the subsequent collapse of electrons.
- Line Spectra: At the time, it was observed that elements produced distinct line spectra, consisting of discrete lines of different colors. Rutherford's model couldn't explain why only certain frequencies were emitted.
Bohr's Solution: Bohr introduced the concept of quantized energy levels to explain the line spectra. He proposed that electrons exist in fixed energy levels and can move between them by absorbing or emitting energy in the form of photons. When an electron transitions between energy levels, it emits or absorbs a photon of specific frequency. The observed line spectra corresponded to these specific energy transitions.
- Stability of Orbits: Rutherford's model didn't account for the stability of electron orbits. According to classical physics, an electron moving in a circular path would constantly lose energy and spiral into the nucleus.
Bohr's Solution: Bohr postulated that the electron orbits in his model were stationary and stable. He proposed that the electron's angular momentum is quantized, meaning it can only have certain discrete values. This restriction prevents the electron from continuously losing energy and collapsing into the nucleus. The electron's angular momentum is related to its orbital radius and velocity, and it must satisfy specific conditions to remain in a stable orbit.
It's important to note that Bohr's model was a stepping stone in the development of quantum mechanics. While it successfully explained some phenomena, it had its own limitations. Later advancements in quantum theory, such as wave-particle duality and the Schrödinger equation, provided a more comprehensive understanding of atomic structure.