In class last Tuesday, we discussed covalent bonds, where two atoms form a bond by sharing their electrons. As we noted in class, the electrons are shared because the two atoms have similar electronegativities: the pull that each atom has on the electron is about equal. If one atom has a higher electronegativity than the other, like the O-H bond, then the more electronegative oxygen atom tends to hoard the electrons more. However, the electrons are still being partially shared. Given all of this, one might ask what would happen if one atom were much more electronegative than the other. If one atom stole the electrons away from the other completely, could a bond still be formed?
The answer to this question is yes. Ionic bonding is when electrons are transferred from one atom to another rather than being shared. As we learned in class, atoms need to have certain numbers of valence electrons to be stable; specifically, they like to have the same number of valence electrons as the noble gases (found in the last column of the periodic table). Let’s use sodium and chlorine as examples of this concept.
Sodium (Na) has eleven electrons. Two are used in the first shell of electrons, and 8 are used in the next, leaving one lonely electron in a third shell. If this electron were lost, then sodium would have two full shells of electrons, the same as noble gas neon (Ne).
Chlorine (Cl), on the other hand, has 17 electrons. Two fill the atom’s first shell, and the next 8 fill up the second shell, leaving 7 valence electrons. If chlorine can grab one more electron, it will have 8 electrons in its third shell, giving it three full shells of electrons. This would be the same as the noble gas argon (Ar). Because having another electron would make chlorine very stable, the atom is very electronegative, just like oxygen.
So sodium wants to ditch an electron, and electronegative chlorine wants that electron. If the two come together, sodium’s valence electron will be transferred over to chlorine, making both atoms much more stable. The sodium, having lost an electron, will now have more protons than electrons and therefore have a positive charge. The chlorine, being in the opposite situation, has a negative charge. Because opposite charges attract, the two ions (charged atoms) will stick together. This is called an ionic bond (ions = ionic). This particular bond between sodium and chlorine forms the molecule NaCl, better known as table salt.
One interesting property of ionic compounds is that they can form crystals. A crystal is a solid material where all of the atoms or molecules form a highly structured repeating pattern. For instance, a snowflake is a crystal made of water molecules, and a diamond is a crystal made of carbon atoms. One can describe a crystal by the size, shape, and contents of its smallest repeating part, called the unit cell. This can be pictured using the analogy of a tile floor. One individual tile is the smallest individual piece (the unit cell), and when it is repeated it forms a larger, more complex floor pattern (the crystal).
NaCl in particular forms a unit cell that is shaped like a cube. There are 8 corners to the cube, which are occupied by four Na atoms and four Cl atoms in a pattern called the “simple unit cell.”
As you can see, the Na and Cl ions are arranged such that they are always closest to the oppositely charged ion, which is what makes the structure favorable.
There are many kinds of unit cell structures, which are explained very well by the video below. I thought their 3D animations of the unit cell and crystals structures were particularly effective at communicating a scientific concept through an artistic medium.
www.youtube.com/watch?v=CsnNbuqxGTk