Which of the following was explained by the

Bohr atom?
1. atomic spectra
2. the fact that most of the mass of an atom
is in the nucleus
3. nuclear fission
4. the photoelectric effect

The Bohr atom explained atomic spectra.

The correct answer is 1. atomic spectra. The Bohr model of the atom, proposed by Niels Bohr in 1913, helped to explain the behavior of atomic spectra. To understand why atomic spectra were explained by the Bohr atom, let's break it down.

To get the answer, we need to understand what atomic spectra are and how the Bohr model addresses them. Atomic spectra refers to the distinct set of frequencies or wavelengths of light emitted or absorbed by atoms. When an atom absorbs energy, electrons move to higher energy levels, and when they release energy, they fall back to lower energy levels. This results in the emission or absorption of specific frequencies of light, which form the atomic spectra.

The Bohr model proposed that electrons orbit the nucleus of an atom in specific energy levels or shells. These energy levels are quantized, meaning that electrons can only exist in certain fixed orbits with defined energies. According to the Bohr model, when an electron gains or loses energy, it undergoes a transition between energy levels, emitting or absorbing light in the process. Therefore, the Bohr model explained why atoms emit or absorb light at specific frequencies, leading to the understanding of atomic spectra.

On the other hand, the other options were not explained by the Bohr atom. The fact that most of the mass of an atom is in the nucleus (2) was later explained by the discovery of the atomic nucleus and the development of the quantum mechanical model of the atom. Nuclear fission (3) is a process of splitting atomic nuclei and was first explained by Otto Hahn and Fritz Strassmann in 1938. The photoelectric effect (4) was explained by Albert Einstein through his quantum theory of light, which described light as made up of discrete particles called photons.

In summary, the Bohr atom explained the behavior of atomic spectra (option 1) by proposing that electrons occupy specific energy levels and undergo transitions, emitting or absorbing light at specific frequencies. The other options were explained by different scientific advancements and theories.