Explain the reason that the atomic emission spectrum of hydrogen contains only four lines.

Though a hydrogen atom has only one electron, it contains a large number of shells, so when this single electron jumps from one shell to another, a photon is emitted, and the energy difference of the shells causes different wavelengths to be released... hence, mono-electronic hydrogen has many spectral lines.

Explanation:

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a polar bond is formed when electrons are shared unevenly between atoms