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Ionic compounds (more commonly known as salts) are formed when metals react with nonmetals.
A common salt, table salt, or sodium chloride (NaCl) is formed from a sodium ion, Na+ and a chloride ion, Cl-.
A sodium atom, loses an electron and gives it to a chlorine atom.
The sodium atom, is now an ion and has a positive charge, while the chlorine atom changes to an ion and has a negative charge.
Once they are formed, the positive ion and negative ion attract each other in an ionic bond.
The reason why electrons are transferred between these atoms is because for any stable compound to form from its elements, there must be a net lowering of potential energy (exothermic).
The reason why ionic compounds are stable is because of lattice energy, the amount that the potential energy of the system is lowered when ions in one mole of the compound are brought from a gaseous state to the positions the ions occupy in a crystal of the compound.
The potential energy is lowered because the ions have a net attraction for each other.
Since this change is exothermic, the lattic energy appears with a negative sign.
Because of IE and EA (see previous section), relatively little energy is needed to remove electons from the left side of the periodic table (for example, Na) and add electrons to the right side of the periodic table (for example, Cl).
The Octet Rule
Sodium has an electron configuration of 1s22s22p63s1.
The electron that is lost is the one least tightly held, which is the single 3s electron.
The electron configuration of the Na+ ion is then 1s22s22p6.
The removal of the first electron does not require much energy because the first IE is small.
The reason why sodium does not form Na2+ ions is because the second IE (the energy to break a second electron away) is much stronger, which involves breaking into the 2s22p6 core.
Calcium, for example has an electron configuration of 1s22s22p63s23p64s2.
It forms Ca2+ ions because it is easy to lose the 2 4s electrons, which are the outermost ones.
Now the electron lost from the sodium is gained by the chlorine atom.
The chlorine atom has an electron configuration of 1s22s22p63s23p5.
When it gains an electron, the chloride ion (Cl-) has a configuration of 1s22s22p63s23p6.
It will not gain another electron because that would mean entering an orbital of the next higher shell, 4s.
Basically, when ions are formed, they like to form a noble gas configuration.
This is the octet rule, which states that atoms of most representative elements tend to gain or lose electrons until they have obtained a configuration that is the same as that of the nearest noble gas.
All of the noble gases except helium have valence shells with eight electrons, which is why it is called the octet rule.
So, the rule can be restated to say that atoms tend to gain or lose electrons until they have achieved an outer shell that contains an octet of electrons (eight electrons).
Exceptions to the Octet Rule
The octet rule works well for the representative metals (Group IA, IIA) and the nonmetals, but not for the transition elements and post-transiton elements.
This is because they have d and f subshell orbitals.
For example, tin (a post-transition metal) forms two ions, Sn2+ and Sn4+.
The electron configurations are:
Sn [Kr] 4d105s25p2
Sn2+ [Kr] 4d105s2
Sn4+ [Kr] 4d10
Neither of these configurations are noble gas configurations though.
For iron ([Ar] 3d64s2), the 4s2 can be lost to make Fe2+.
But Fe3+ can be made because a 3d5 (half-filled) subshell is more stable.