Why is common salt called an ionic compound but sugar is not?

The difference is in how the atoms are held together. In common salt (NaCl), sodium gives away one electron to chlorine, forming Na+ and Cl- ions that attract each other strongly. This is an ionic bond, so NaCl is an ionic compound. In sugar (sucrose), atoms like carbon, hydrogen and oxygen share electrons instead of transferring them, forming covalent bonds, so sugar is a covalent compound. A simple test: dissolve each in water and check if the solution conducts electricity. Salt solution conducts because it splits into free ions, while sugar solution does not conduct because it dissolves as whole neutral molecules with no free ions. Looking alike as crystals does not decide the bond type, the conductivity test does.