# Covalent and Ionic bonds with Semi-metals

why are some semimetals considered as a metal, and some, a nonmetal when paired with some atoms. Like in BF, it is an ionic bond, but in SiCl, it is a covalent bond. Do their electronegativities matter?

• I would never call a $\ce{B-F}$ bond ionic. – Ivan Neretin Feb 18 '16 at 14:24

Yes, in fact difference in electronegativity is the defining property. Ionic character of a bond is given by: $$\mathrm{\% IC = 1-exp[-\frac{1}{4}(\chi_A - \chi_B)^2]}$$ Where $\chi_A$ and $\chi_B$ are the electronegativities of the anion and cation species. When $|\chi_A - \chi_B|$ exceeds 1.665 the ionic character transitions from less than $50\%$ ionic to more than $50\%$ ionic, making the compound ionic.