>>13554110triangles in architecture etc do not posses sides that have a prefered alignment to a given point aka an optimal angle regarding each other side. Unless you apply pressure through gravity in a point no angle is preferred. In chemistry on the other hand you see that bonds are made of electrons inhabiting certain molecular orbitals and these electrons repell each other and the orbitals themselves follow a certain orientation to (roughly) maximize space between them. In your oxirane example the atoms are covalently bond but there is a preferred angle that is wider than 60° because, again, electrons repell each other. Just read up on Coulomb forces and the Pauli principle.
