As 1s electrons are closer to the nucleus than 2s electrons,they experience higher nuclear charge(i.e.,they are highly bound to atom than 2s electrons).so,2s electrons can easily escape from nucleus and can form Be2+(since it is stable configuration,He) but 1s electrons cannot escape from nucleus. Solution As 1s electrons are closer to the nucleus than 2s electrons,they experience higher nuclear charge(i.e.,they are highly bound to atom than 2s electrons).so,2s electrons can easily escape from nucleus and can form Be2+(since it is stable configuration,He) but 1s electrons cannot escape from nucleus..