Final answer:
The valence shell of a monatomic ion of the formula Sn²+ contains 50 electrons, as two electrons are lost from the neutral atom's valence shell, resembling the noble gas krypton (Kr).
Step-by-step explanation:
The valence shell of a monatomic ion of the formula Sn²+ contains 50 electrons. Tin (Sn) normally has 50 electrons as a neutral atom, with the electron configuration as:
1s²2s²2p¶3s²3p¶4s²3d±04p¶5s²4d±05p². When it forms a 2+ ion, it loses two electrons from its valence shell. This leaves the valence shell configuration resembling that of the noble gas krypton (Kr), which is 1s²2s²2p¶3s²3p¶4s²3d±04p¶.