Covalently bonded materials are less dense than metallic or ionically bonded ones because covalent bonds are directional in nature whereas metallic and ionic are not; when bonds are directional, the atoms cannot pack together in a dense manner, yielding a lower mass density than solids with non-directional bonds.
Work Step by Step
A covalent bond forms when electrons are shared between two atoms. An ionic bond forms when an atom donates an electron to another atom. A metallic bond forms between two metals in a lattice structure of positively charged ions in a sea of negatively charged electrons.