A point defect in crystals that does not alter the density of the relevant solid is called a "vacancy defect." In a vacancy defect, atoms are missing from their lattice positions in the crystal structure. Despite the absence of these atoms, the overall density of the crystal remains unchanged because the missing atoms create spaces within the lattice that are filled by neighboring atoms. Therefore, the mass per unit volume, and thus the density, remains constant.

In general, point defects such as vacancies, interstitials, and substitutional impurities can affect the density of a solid material. However, among these defects, the presence of interstitials typically increases the density of a solid.

Interstitial defects occur when atoms occupy spaces between the regular lattice sites of the crystal structure. Since these atoms are located in the interstitial spaces, they add to the overall mass and increase the density of the material. This increase in density is because the additional atoms effectively reduce the amount of empty space within the crystal lattice.

Therefore, the presence of interstitial point defects tends to increase the density of a solid material.

Metallic and ionic substances differ significantly in their ability to conduct electricity due to the nature of their bonding and the movement of their constituent particles:

1. Metallic Substances:

   • In metallic substances, such as copper, gold, or aluminum, the atoms are arranged in a closely packed lattice structure.
   • The outermost electrons of the metal atoms are delocalized, meaning they are not associated with any particular atom but are free to move throughout the structure. This is due to the metallic bonding, which involves the attraction between positive metal ions and the sea of delocalized electrons.
   • When a potential difference (voltage) is applied across a metallic substance, the delocalized electrons are free to move in response to the electric field. This movement of electrons constitutes an electric current, allowing metallic substances to conduct electricity efficiently.
   • Metals are generally good conductors of electricity due to the mobility of their delocalized electrons.

2. Ionic Substances:

   • In ionic substances, such as sodium chloride (table salt) or magnesium oxide, the atoms are held together by strong electrostatic forces between positively and negatively charged ions.
   • Ionic compounds form a crystalline lattice structure composed of alternating positive and negative ions.
   • Unlike metallic substances, the electrons in ionic substances are tightly bound to specific ions and are not free to move throughout the crystal lattice.
   • When an electric potential is applied to an ionic substance, the ions are not able to move freely because they are held in fixed positions within the lattice. Thus, ionic substances typically do not conduct electricity as solids.
   • However, when ionic substances are dissolved in water or melted (forming an ionic liquid), the ions become mobile and can carry an electric current. This is because the water molecules surround and separate the ions, allowing them to move and conduct electricity.

In summary, metallic substances conduct electricity due to the free movement of delocalized electrons, whereas ionic substances conduct electricity when dissolved in water or melted, allowing the ions to become mobile.