Ionic Bonding and Electron Transfer

Ionic bonds are formed by the transfer of electrons from metals to nonmetals. In comparison to covalent bonds where electrons are shared, in ionic bonds metals tend to lose and nonmetals tend to accept electrons — but why? 

The most stable electron configuration of an atom is with a full octet. Achieving this state in conjunction with lowering the potential energies is a major driving force for bond formation. 

But how to predict when electrons will be transferred and not shared? For this, examine the ionization energies and electron affinities of atoms.

On the periodic table, s-block metals have low ionization energies. This facilitates easier loss of electrons to reach an octet while concurrently forming cations. 

On the contrary, nonmetals from the p-block, except for noble gases, have high electron affinities and readily accept electrons to form anions. The oppositely charged cations and anions experience strong electrostatic interactions, attracting each other to form ionic bonds.

Ionic compounds often form hard and well-defined crystal structures with high melting points, due to strong electrostatic attraction between ions.

Consider lithium fluoride, an ionic compound made of lithium and fluorine. When lithium loses an electron to become a lithium cation with a duet, the electron is being taken up by fluorine to form the anion with an octet.

According to Coulomb's law, these two charged particles are attracted to each other to form lithium fluoride, a neutral compound. 

The Lewis model can be used to describe and predict the chemical formula of ionic compounds. The Lewis symbol shows charges in the upper-right hand corner and depicts anions and cations with the symbol written within brackets. 

Consider strontium chloride, a commonly used salt to get a bright red color in fireworks. The Lewis symbol for strontium has two, and chlorine has one unpaired electron.

Strontium must lose two electrons, while chlorine must gain one electron to reach an octet. Therefore, one strontium cation binds to two chloride anions, in the ratio of 1:2.