An ionic bond is an electrical attraction between two oppositely charged atoms or groups of atoms. Normally, atoms are neutral and have no charge. However, in order to gain stability they will sacrifice their neutrality by either losing one or more of its outermost electrons thus becoming a positive ion (cation) or they will gain one or more electrons thus becoming a negative ion (anion). Elements that are described as "metallic" tend to lose electrons and elements that are described as "non-metallic" tend to gain electrons. Once this has happened, the resulting charged atoms will attract each other. That electrical attraction between two oppositely charged ions is referred to as an ionic bond. All salts are ionic.
Polarity of the Ionic Bond
Because the bonding electrons are under the domain of the non-metal in an ionic bond, the bond is said to be polar. Polar bonds generate a dipole moment, an electrical force which is generated because of the unequal distribution of the bonding electrons between the two bonded atoms. In the case of an ionic bond that unequal distribution is extreme. The dipole moment that is generated is quite large compared to polar bonds of the covalent bond.
A dipole moment is a vector measurement. A vector is a measurement that has a magnitude and a directional component. Any force (mechanical, electrical, magnetic, etc.) will be vector measurement. Examples would be velocity and forces. The other type of measurement is scalar measurement. These are measurements that have only a magnitude component but no directional one. Examples would be temperature, mass, speed, etc. Scalar measurements are handled differently than vector measurements when it comes to math operations. For example when you add or subtract scalar measurements you pay no attention to direction. With vector forces, you must pay attention to the direction and the angle between the vectors. Often the use of trigonometry is called for in order to resolve vectors.
Characteristics of Ionic Compounds
- Crystalline solids at room temperature
- Have higher melting points and boiling points compared to covalent compounds
- Conduct electrical current in molten or dissolved state
- Are extremely polar bonds
- Most are soluble in water but not soluble in non-polar solvents
What determines what the charge is on an ion?
Elements combine to make the compound which is as stable as possible - the one in which the greatest amount of energy is evolved in its making. The more charges a positive ion has, the greater the attraction towards its accompanying negative ion. The greater the attraction, the more energy is released when the ions come together.
That means that elements forming positive ions will tend to give away as many electrons as possible.
Energy is needed to remove electrons from atoms. This is called ionization energy. The more electrons removed, the greater the total ionization energy becomes. Eventually the total ionisation energy needed becomes so great that the energy released when the attractions are set up between positive and negative ions isn't large enough to cover it.
The element forms the ion which makes the compound most stable - the one in which most energy is released over-all.
Why is Calcium Chloride CaCl2 Rather than CaCl or CaCl3?
If one mole of CaCl (containing Ca+ ions) is made from its elements, it is possible to estimate that about 171 kJ of heat is evolved.
However, making CaCl2 (containing Ca2+ ions) releases more heat, 795 kJ. That extra amount of heat evolved makes the compound more stable, which is why CaCl2 rather than CaCl is formed.
To make one mole of CaCl3, one can estimate that 1341 kJ of heat would be needed. This makes this compound completely non-viable. Why is so much heat needed to make CaCl3? It is because the third ionization energy (the energy needed to remove the third electron) is extremely high (4940 kJ mol-1) because the electron is being removed from the 3-level rather than the 4-level. Because it is much closer to the nucleus than the first two electrons removed, it is going to be held much more strongly.
A similar sort of argument applies to the negative ion. For example, oxygen forms an O2- ion rather than an O- ion or an O3- ion, because compounds containing the O2- ion turn out to be the most energetically stable.