Question

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

Answer 1

Beryllium (Be) is the element belongs to second  group in the periodic table. It is amphoteric and reacts with acid and bases and is invariably divalent. Its oxide i.e., Beryllium Oxide dissolves in the excess of Sodium Hydroxide (NaOH) and gives sodium berrylate $NaBeO_{ 2 }$ and water.

      $BeO\quad +\quad 2NaOH\quad \longrightarrow \quad NaBeO_{ 2 }\quad +\quad H_{ 2 }O$

The dipositive ion of Beryllium Oxide has a soluble Noble gas core and also $Be^{ 2+ }= 1s^{ 2 }$

Answer 2

Beryllium  is an element with atomic number 4

and  it is is a bivalent  element.

Beryllium oxide is soluble in excess of NaOH  and gives SODIUM BERYLLATE  

chemical equation:

BeO+ 2NaOH +H2O --->Na₂[Be(OH)₄]  

BERYLLIUM DIPOSITIVE  IS Be²⁺ and  HAS NOBLE GAS CONFIGURATION OF 1S²