![]() It takes a great deal of energy to change table salt into its constituent elements. Just what caused those electrical forces was not discovered until the atomic nature of matter was elucidated over 100 years later. Davy correctly (as it turned out) deduced that the elements in table salt – what we now know as sodium and chlorine - are held together by “electrical forces”. Not only did it conduct electricity, but when electricity (electrons) was passed through it, it decomposed to produce globules of a shiny, highly reactive metal – sodium (Na), and a pale green gas – chlorine (Cl 2). While solid table salt did not conduct electricity, liquid (molten) salt did. Davy used a Voltaic Pile to study the effects of passing electricity through a range of substances. Based on this difference, we might be tempted to conclude that covalent bonds are not broken when salt melts, but that something stronger that the H-bonds that hold water molecules together are broken - what could that be?Ī hint comes from studies first carried out by the English chemistry Humphrey Davy. On the other hand NaCl melts (solid → liquid) and freezes (liquid → solid), much like water. Under normal circumstances, the carbon atoms react with oxygen (O 2) in the air to form carbon dioxide - a process that requires the addition of lots of energy to reverse (as we will see later). Remember, diamond does not melt it decomposes once enough energy is added to the system to break the C–C bonds. An interesting difference between diamond and sodium chloride occurs on heating. NaCl is a solid at room temperature, with a very high melting point (801 ☌), similar to the melting points of silver (961.78 ☌) and gold (1064.18 ☌), although much lower than the decomposition temperature of diamond (3550 ☌). NaCl is a “continuous compound”, much like diamond (see Chapter 3). The most familiar of these compounds is sodium chloride (NaCl), common table salt. For the sake of simplicity we will confine ourselves (for the moment) to binary compounds - compounds with only two elements in them. Let us take a look at some common ionic compounds and see if we can make some sense of their properties from a consideration of their atomic-molecular structure. This kind of bonding is called ionic bonding (as you are almost certainly already aware). What we have not looked at yet is the extreme case of this kind of distortion, in which the valence electrons are attracted so much by the electronegative atom that they are transferred completely. We have seen that the electron density can be considered to be equally distributed between the bonding atoms, or that it may be distorted by being attracted to the more electronegative atom. ![]() ![]() Our discussion up to now has centered on types of bonds that involve valence electrons being “shared” between different atoms. Chemistry, life, the universe and everything Chapter 4.6: Ionic Bonding ![]()
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