Glass is manufactured by cooling a melt to produce a rigid structure without crystallization. The binary oxides B₂O₃ and SiO₂ are the primary components of borosilicate glass. Possible structural units include B(μ-O)₄, B(μ-O)₃, B(Ot)₃, Si(μ-O)₄ and Si(Ot)₄ where μ-O and Ot are bridging O atoms and terminal O‾ units, respectively. 

(a) Draw diagrams to show the structures of these units and how they might interconnect where appropriate, paying attention to atomic charges. Comment on the bonding in each unit. 

(b) Suggest other Si- or B-centred structural units that might be present in borosilicate glass. 

(b) Na₂O is added to glass as a modifier. Suggest what role it plays when added to a borosilicate glass containing approximately 70% SiO₂, 14% B₂O₃, 10% Na₂O and 6% Al₂O₃, by weight.