BNG_Formation of Ionic Bonds Lesson

Formation of Ionic Bonds

Recall that in an ionic compound, electrons are transferred. Metals transfer electrons to nonmetals. This results in a positive cation and a negative anion. Because of their opposite charges, these ions are attracted to one another. This attraction is called an ionic bond.

Let's look at this process more closely, focusing on the energy associated with each step. We will use the formation of sodium chloride as our example.

First, sodium loses an electron. You should recall from your study of atomic theory that the energy required to remove an electron is called ionization energy (I.E.). Na(g) → Na* (g) + e I.E. = +495.4 mol Next, chlorine gains that electron. Recall that this energy is called electron affinity (E.A.). Cl(g) + e→ Cl(s) E.A. = =-348.8 k] mol Together these processes have a net energy as shown here: 495.4-348.8 = +146.6 * The + energy means an increase in energy. We know that in order for a bond to form, there must be an overall lowering of energy. So, if this were the only processes involved, these ions would not form. Let's look at the last step, the forming of the bond. The energy associated with this step is called lattice energy. Na (g) + Cl (g) → NaCl(s) L.E. = -787.0 k mol Now we can see that the net energy of the entire process is: Net 495.4+-348.8-787.0 = -640.4 mol mol mol mol This means that the overall energy was decreased; there was a release of energy. When energy is released in a reaction, it is called exothermic.

Lattice Energy

Without lattice energy, the above compound would not exist. Lattice energy is the energy change that would occur if the ions at infinite separation (gaseous) are brought together to form one mole of the solid compound. You can think of this more simply as the energy to bring the ions together. Below is a pictorial representation of lattice energy:

1 mol of gaseous ions (separated ions) with arrow forming 1 mole of solid compound (ions close together)

For any ionic compound, the lattice energy is what makes the compound stable. It must always be large enough to overcome the net energy of the formation of ions (a combination of ionization energy and electron affinity).

We can use this variation of Coulomb's Law to see the factors that affect the magnitude of lattice energy. According to the equation, energy (lattice energy in this case) depends on the charges of the ions and the size of the ions.

COULOMB'S LAW E = Energy E = k (q1q2) / r q1q2 = Charges of the ions forming the compound r = Distance between nuclei k = Proportionality constant

Since we are using this equation to look at lattice energy that means that q1 will be negative and q2 will be positive (because lattice energy refers to an ionic bond). This means that E will be negative. A negative E means and exothermic reaction. Now, let's neglect the positive and negative signs for a moment and just look at the magnitude of the values.  

  • Energy is directly proportional to q1 and q2. If either of these charges (or both) is increased, the magnitude of the lattice energy will increase as well.
  • Energy is inversely proportional to r, the distance between the nuclei. The larger the ions (ionic radius), the greater this distance will be. As r increases, energy decreases.

In conclusion, ions with greater magnitude of charge and smaller size will have greater lattice energy.

Formation of Covalent Bonds

Remember, for ionic bonding to occur, the energy lowering effect of lattice energy must be greater than the net energy raising effects of ionization energy and electron affinity.   For elements with particularly high ionization energies, it is not always possible for ionic reactions to occur.

  • What is the trend for ionization energy? Where are the elements with the highest I.E. found on the periodic table?
  • I.E. increases up and to the right on the periodic table.  Elements with the highest I.E. are located top, right (omitting the noble gases)

This is true when nonmetals combine. They must achieve a lowered energy (required for all bond formation) through a different method, sharing.

The key to understanding how a covalent bond is formed is in the valence electrons. Let's observe how the electrons of two hydrogen atoms behave as they form a covalent bond.

Here is what happens. As the hydrogen atoms approach each other, the electrons of each atom begin to feel the attraction for both nuclei. This causes the electron density to shift to the area between the two nuclei. This electron density attracts both nuclei, pulling them together. By attracting to both nuclei, the electrons can be shared between the atoms, forming a covalent bond. 

At the same time that this attraction is happening, the nuclei repel each other and the electrons also repel each other.   So, there must be a balance between these repulsions and the attraction that the electrons feel for both nuclei. When the attractive forces offset the repulsive forces, the energy of the two atoms decreases and a bond is formed. The bond is the net attractive force. Energy is released by the formation of the bond.

The diagram below shows the relationship between potential energy and internuclear distance of the hydrogen atoms as a covalent bond is formed. Click through the slides to learn the interpretation of this diagram.

Remember to work on the module practice problems as you complete each section of content.

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