Question

From your knowledge about the distribution of electrons in their shells and from the atomic number (in parentheses), indicate the most likely charge on the ion when this atom forms an ion. (Remember the 2-8-18 shell distribution.) Oxygen (8) +1 +2 -2 -1

Answer 1

If the atom gains electron then the ion will be electronegative but when it looses then it is electropositive.

Answer 2

Actually any atom tries to reach its nearest noble gas electron distribution, which is 8 electrons in its last shell (2 in case of helium). To achieve the total of 8 electrons in its last shell an atom takes up or gives up electrons.

Like in case of oxygen, it has 6 electrons in its last shell, thus generally takes up 2 more electrons to achieve the electron distribution of noble gas neon(10) and it can be written as O⁻².

Now when an atom takes up electron it becomes electronegative(because of more negative charged entities in it i.e, electrons)  and when one gives up electron it becomes electropositive(because of extra positive charged entities in it i.e, proton).

But there can be some extra valencies other than the most stable one(which has the noble gas configuration). For example, oxygen has valencies of +1, +2, -1 other than -2(most stable).