Bohr's theory of the atom was proposed in the early 20th century and was based on the idea that electrons move in fixed orbits around the nucleus. According to this theory, the position and momentum of an electron in an atom can be determined with absolute precision at any given moment, which seems to contradict Heisenberg's uncertainty principle.

Heisenberg's uncertainty principle states that it is impossible to determine the exact position and momentum of a particle simultaneously. This...

The most striking features of atomic spectra (spectrum of only atom but no molecules) is that the emitted radiation has very sharp, discrete wavelengths. These atomic spectra are, therefore, called line spectra.

When electrons are infinite separation from the nucleus then no electrostatic force exists and the energy of electron is zero. When the electrons brought towards the nucleus then the energy is expense to do work and energy is decreased. Therefore, the energy of electrons in shell is negative.

S-orbital cannot accommodate more than two electrons because if three electrons are added in S-orbital then the value of four quantum number for two electrons will be same, which is against the Pauli's exclusion principle.