Hobbies And Interests
Covalent
Covalent crystals are held together by covalent bonds; the atoms in the crystal share electrons. The shared electrons are attracted to the nuclei of both the atoms in the bond and that attraction overcomes the repulsion of the two nuclei. The covalent bond is very strong and makes the crystal very stable. Diamonds, graphite and quartz are examples of covalent crystals.
Metallic
Metallic bonds hold metallic crystals together; the individual atoms form a lattice structure and the electrons of the atoms on the outer surfaces of the lattice flow freely around the lattice. This sharing of electrons across the surface can be thought of as a non-localized form of covalent bonding. As you might expect with free moving electrons on the surface, metallic crystals are good conductors of electricity and heat. They are shiny, very hard and have high melting points. Most are malleable and ductile, especially at elevated temperatures. Mercury, copper and iron are examples of metallic crystals.
Ionic
Ionic bonds hold Ionic crystals together; the atoms have lost or gained an electron and so carry a net electrical charge. These charged particles are called ions. The positively charged ions are attracted to the negatively charged ions and this forms the ionic bond. Ionic crystals are hard and brittle with a high melting point. In liquid form or in solution they are electrical conductors. Table salt (NaCl) and potassium hydroxide (KOH) are examples of ionic crystals.
Molecular
Molecular crystals are held together by van der Waals forces or by hydrogen bonding. Van der Waals forces are the attractive forces between molecules and they are much weaker than the attractive forces between atoms. Hydrogen bonding is due to the interaction of a hydrogen atom with a negative ion. As might be expected from this weaker bond, molecular crystals have low melting and boiling points and they are very unstable. Sugar, ice and dry ice, which is just frozen carbon dioxide provide examples of molecular crystals.