Ionic Bonds are a type of chemical bond that typically forms between metal and non-metal atoms. I say typical because it is also possible between certain non-metal molecular ions such as Ammonia (NH3). Unlike Covalent bonds, the electrons in Ionic bonds are not shared equally. They are instead transferred between species. Ionic bonds are formed by the electrostatic attraction between oppositely charged ions.
The images above show beautifully, that when a metallic species gives up its electron(s) it becomes more positively charged (what we call a cation), and the atom receiving the extra electron(s) becomes negative (what we call an anion). It is these positive and negative charges on these ions that form ionic bonds. For the most part these compounds follow the octet rule of filling electron shells to became stable, but this is a general rule, not absolute. There are many exceptions (particularly with metals).
Properties of Ionic bonded compounds are that they conduct electricity while molten or in a solution, they generally have a high melting point and tend to be soluble in water. In the real world, ionic compounds exist not as pairs of bonded atoms, but in arrangements of ions set in very specific shapes and forms. What we call a crystal lattice. This specific form repeated millions of times gives ionic compounds their 3 dimensional shape (salt crystals for example). Ionic bonds are the strongest of all bonds (Ionic > Covalent > Hydrogen).
Pure ionic bonds do not exist in nature. They are all covalent to some degree. The greater the electronegativity difference between atoms, the more ionic the bond. For reference again:
>1.7 EN difference = Ionic bond
0.4-1.7 EN difference = Polar Covalent
< 0.4 EN difference = Covalent
The example of Sodium Chloride (NaCl), or table salt is a good one to explain ionic bonds. When Na and Cl are combined, the sodium atoms lose an electron becoming cations (Na+), while the Chlorine atoms each gain an electron to form anions (Cl-). These oppositely charged ions are then attracted to each other in a 1:1 ratio to form Sodium Chloride (NaCl). Depending on the charges on the atoms, this ratio can vary. The compound Potassium Oxide (K2O) for example, the potassium exists in a 2:1 relationship with Oxygen because Oxygen’s charge is -2; so two K atoms (charge of +1) are needed per O atom to balance the charges. The greater the charge, the greater the attraction, and the stronger the ionic bond.
To remove electrons from atoms in this process requires the input of energy or heat, and is therefore endothermic. However the formation of ionic compounds is exothermic and creates heat or energy as a byproduct. Ionic bonding however will only occur if the reaction is what we call favourable. Essentially the bonded atoms must have a lower energy requirement than the free atoms. The larger the energy change, the stronger the ionic bond. Because metals have low electronegativity, and non-metals have high electronegativity, the resulting energy change of the reaction is more favourable when metals lose electrons and non-metals gain them.
This can be explained easier by reviewing the 3 steps in ionic bonding, in this case again with NaCl:
Please note that this is a simplification that does not take into effect a solid product forming into a crystal lattice (as occurs in reality) or beginning with gaseous Cl (Cl2). See here for a more detailed picture.
Ionic bonding can be broken down into 3 steps: ex. sodium chloride
Formation of the sodium ions (ionization)
Na(g) + energy ——-> Na+(g) (DH = +496 kJ/mol)
Requires 496kj/mol input to remove electrons.
Formation of the chloride ions (electron affinity)
Cl(g) + e- ——-> Cl-(g) + energy (DH = -348 kJ/mol)
Releases 348kJ/mol of heat/energy in this reaction.
Formation of the ion pair (bond energy)
Na+(g) + Cl-(g) ——-> NaCl(g) + energy (DH = -504 kJ/mol)
Releases 504kJ/mol of heat/energy in the final formation of NaCl.
DH = ΔH = Change in Enthalpy (heat energy).
We use Hess’s Law, which simply put means that the total energy change in a reaction is equal to the sum of the energy changes that occur.
So adding up each reaction we get:
DHrxn = DH1+ DH2 + DH3 = -356 kJ/mol
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