## Chemistry: The Central Science (13th Edition)

The order in increasing size is $$Se\lt Se^{2-}\lt Te^{2-}$$
$$Se^{2-}, Te^{2-}, Se$$ Ions are larger than their corresponding neutral atoms, since the addition of new electrons increases the electron-electron repulsions, screening the attraction from the nucleus. So each electron in ions experiences less attraction than each electron in corresponding neutral atoms. Therefore, $Se^{2-}\gt Se$. For ions carrying the same charge, ionic radii increase as we move down a column in the periodic table. In the periodic table, $Se^{2-}$ and $Te^{2-}$ are in the same column. But $Te^{2-}$ is below $Se^{2-}$, so $Te^{2-}\gt Se^{2-}$. Overall, the order in increasing size is $$Se\lt Se^{2-}\lt Te^{2-}$$