but as we add electrons, repulsion can push some of them out into the higher energy 4s level.
if we add 1 e- to Sn+ (previous example): let's name that e- A.
A experiences a repulsive force from the e- in 3d1, making it difficult for it to move to 3d orbital. however, A needs more energy if it wants to move up to the 4s orbital (higher than 3d).
the 'repelling' energy exerted by the existing e- (3d1) is greater than the energy required for A to move up to 4s orbital.
'energy of repulsion w 3d1' > difference in energy level btwn 3d and 4s
therefore A moves up to 4s. forming a complete subshell: 4s2. now Sn+ has become a neutral-charged Sn atom.