Name an element which is invariably bivalent and whose oxide is soluble in excess of `NaOH` and its dipositive ion has a noble gas core.

Explanation

The elements which satisfies the following requirements:
(a). Invariably bivalent.
(b). Oxide is soluble in `NaOH`.
(c ) Dipostive ion has noble gas core is beryllium.
(i) The bivalent ion of beryllium is `Be^(2+)`.
(ii). Since `BeO` is amphoteric. It reacts both with an acid and an alkali, whereas other oxides of group `2` elements are basic and do not react with `NaOH`.
`BeO+2NaOHrarrNa_(2)BeO_(2)+H_(2)O`
(iii) `Be^(2+)` ion has noble gas core
`Be^(2+)`ion `1s^(2)2s^(0)` or `[He]2s^(0)`