Electrons are added to the same principal energy level.Explanation;Shielding effect refers to the electrons in an atom shielding each other from the pull of the nucleus. It describes the attraction between the electrons and the nucleus of an atom. It is the effect where the inner electrons help "shield" the outer electrons and the nucleus from each other.When moving from left to the right of a period, the number of electrons increases and the strength of shielding increases. As you move across period the number of shells remain same, the shielding effect will also remain constant.
This is because the number of orbital shells does not increase. The non-valence orbitals remain the same with maximum electron configuration; with only the valence orbital changing. Therefore the valence orbital is shielded the same, by the non-changing lower orbital levels, from the attraction force of the atomic nucleus.
i would say it is
condensation. good luck!