Explanation:

Na2B4O7

Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is a compound with formula Na 2H 4B 4O 9•nH 2O or, more precisely, [Na•? ?] 2 [B 4O 5(OH)²? 4]. The formula is often improperly written as Na 2B 4O 7•H 2O, reflecting an older incorrect understanding of the anion's molecular structure.