text: This is best understood in terms of them both being on the borderline between metals and non-metals - in particular, the fact that their oxides are acidic.\nBoron is quite different to silicon in respect of the fact that its simple covalent compounds are electron-deficient, that is they only have six electrons in the outer level. Thus they are strong Lewis acids.\nSilicon can accept electron pairs, but only by expanding its valency octet.