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.
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