STRUCTURE AND BONDING
Ionic (Electrovalent) Bonding
Noble gases like neon or argon have eight electrons in their outer shells (or two in the case of helium). These noble gas structures are thought of as being in some way a “desirable” thing for an atom to have. When other atoms react, they try to organize electrons such that their outer shells are either completely full or completely empty.
Chemical reactions occur so that atoms attain inert gas configuration by either losing valency electrons as in the case of metals, or gaining electrons as in the case of non-metals.
Ionic bonding in sodium chloride
Sodium (2, 8, 1) has 1 electron more than a stable noble gas structure (2,8). If it gave away that electron it would become more stable. Chlorine (2, 8, 7) has 1 electron short of a stable noble gas structure (2,8,8).
If it could gain an electron from somewhere it too would become more stable. If a sodium atom gives an electron to a chlorine atom, both become more stable. The sodium has lost an electron, so it no longer has equal numbers of electrons and protons. Because it has one more proton than electron, it has a charge of 1+.
If electrons are lost from an atom, positive ions are formed. Positive ions are sometimes called cations because they move to the cathode during electrolysis. The chlorine has gained an electron, so it now has one more electron than proton.
It therefore has a charge of 1-. If electrons are gained by an atom, negative ions are formed. A negative ion is sometimes called an anion since it drifts to the anode during electrolysis.
The nature of ionic bond
The sodium ions and chloride ions are held together by the strong electrostatic attractions between the positive and negative charges. You need one sodium atom to provide the extra electron for one chlorine atom, so they combine together 1:1. The formula is therefore NaCl.
Again, noble gas structures are formed, and the magnesium oxide is held together by very strong attractions between the ions.
The ionic bonding is stronger than in sodium chloride because this time you have 2+ ions attracting 2- ions.
The greater the charge, the greater the attractive force. The formula of magnesium oxide is MgO.
- Electrons are transferred from one atom to another resulting in the formation of positive and negative ions.
- The electrostatic attractions between the positive and negative ions hold the compound together.
Properties of ionic compounds
- All compounds with ionic bonding produce giant ionic structures.
- Consist of oppositely charged ions arranged in an ionic lattice, the ions are held together by strong ionic bonds. e.g. NaCl is composed of Na+ ions and Cl- ions.
- These bonds are hard to break, therefore ionic substances have very high melting and boiling points.
- All exist as solids.
- They conduct electricity when molten, because the ions are free to move, but do not conduct when solid.
- They conduct electricity in the aqueous state because the ions are free to move.
- Most ionic substances are soluble in water because the polar water molecules can accommodate the charged ions.
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