Ionic solids are those which form from the ionic bonding of atoms. In this type of bonding, atoms transfer electrons in order to complete their outermost or valence shell. Since electrons are transferred from one atom to another, the atoms involved in the bond become ions. Atoms that give away electrons become cations, or positive ions. This happens because the atoms have lost negatively charged electrons while still retaining the original number of positively charged protons in the nucleus; thus the entire atom now carries a positive charge. Atoms that gain electrons become anions, or negative ions, because they have added negatively charged electrons to their valence shell.
Since an ionic solid consists of cations and anions, the ions tend to cling to one another through electromagnetic attraction. The atoms will become oriented within the solid in a pattern where the ions alternate so that every positive is connected to negatives, and vice versa. This fact creates the shear lines; the solid will break cleanly between the faces of the ions.