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The elements present in group 1A, 2A, or 3A in the periodic table, lose electrons, whereas the elements present in groups 5A, 6A, or 7A in the periodic table, gains electrons to get the same number of electrons present in the nearest noble gas and form positive and negative ions respectively.

The atomic number of barium (Ba) is 56${}^{\mathbf{56}}\mathbf{Ba}$. is metal and belongs to group 2A in the periodic table.

${}^{\mathbf{56}}\mathbf{Ba}$is an alkaline earth metal, so like other group 2A elements, it loses two electrons for attaining the same number of electrons present in the nearest noble gas${}^{\mathbf{56}}\mathbf{Xe}$ Hence, the monatomic ion of Ba is${\mathbf{Ba}}^{\mathbf{+}\mathbf{2}}$

The atomic number of selenium (Se) is 34.${}^{\mathbf{34}}\mathbf{Se}$ is non-metal and belongs to group 6A in the periodic table.

${}^{\mathbf{34}}\mathbf{Se}$like other groups, 6A elements gain two electrons for getting the same number of electrons present in the nearest noble gas${}^{\mathbf{36}}\mathbf{Kr}$Hence, the monatomic ion of Se is${\mathbf{Se}}^{\mathbf{-}\mathbf{2}}$