Covalent bonds can also be formed in such a way as to form a giant molecule, such as happens in diamond. Here, each of the four valency electrons of a carbon atom is shared with one of the valency electrons of another carbon atom, so that every carbon atom in the structure has four different carbon atoms bonded to it.
In simple molecules, the molecules are held together by strong covalent bonds, but the bonds between different molecules are relatively weak, and therefore easily broken. Many compounds containing covalent bonds, therefore, are liquids or gases at room temperature, although at lower temperatures they form crystalline solids; those that are solids at room temperature have relatively low melting and boiling points.
These compounds are frequently insoluble in water, as the mechanism for dissolving described for ionic compounds cannot occur. However, water is also capable of dissolving covalent compounds which form hydrogen bonds, because instead of the ionic attachment, the water molecules can attach themselves to the molecules of the covalent compounds, with a similar result.
Ionic Bonds
Ionic bonds are formed through the electrostatic attraction between two oppositely charged ions. This type of bond normally occurs between metallic and non-metallic elements. Metals typically have few valency electrons, and occur in groups I, II, and III of the periodic table; reactive non-metals typically have more electrons in their outermost shells, and occur in groups IV, V, VI, and VII. An ionic bond occurs when a metal loses all its valency electrons, leaving a positively charged ion with a noble gas structure, and a non-metal gains the number of electrons it requires to give it a noble gas structure, and becomes a negative ion.
The ionic bond occurs by means of the attraction between these charged particles, but because there are, of course, millions and millions of them in any given sample, they do not just pair off to make simple “molecules”. Instead, the positive ions cluster as close as they can round the negative ions, and similarly the negative ions cluster round the positive ions. The net result of this process is that a regular lattice of ions is formed, whose structure, and hence the shape of the resulting ionic crystal, depends on the sizes of the ions and their relative charges. There must, in the end, be an equal number of ions of each charge to ensure that the final structure is neutral. However, it must be noted that there is no such thing as a single molecule of an ionic compound, in the way that there can be a single molecule of a covalent compound. A useful way to represent a compound formed by an ionic bond is to show the charges on the ions e.g. Na+Cl-.
Typically, compounds containing ionic bonds are crystalline solids. This is because the arrangement of the ions is dictated by their sizes and charges. Every ion is strongly bonded to its neighbours, and the whole is thus held in a rigid lattice with a specific shape.
They have high melting and boiling points. This is because the very strong bonding associated with ionic compounds means that much energy is required to break these bonds.
They are soluble in water. This is because water is a so-called ionizing solvent, because of its polar nature. The water molecule is capable of attaching itself to both positive and negative ions, water is therefore capable of carrying away the ions into solution (in the form of "hydrated" ions), so that the solid eventually dissolves. Non-polar solvents do not dissolve ionic solids.
Compounds containing ionic bonds also conduct electricity when in solution (water) or when molten. This is because both the solution and the molten compound contain ions that can move about when an electric current is passed through. Solid ionic compound cannot conduct electricity as the ions are held in their rigid lattice, and are not free to move.
Shayon King