HS Chemistry - Chemical Bonding

Ionic Bonding

Overview of The Page

This page will cover:

• How does ionic bonding work?
• What are some properties of ionic compounds?

There are three types of bonding between atoms: ionic bonding, covalent bonding, and metallic bonding. This page looks at ionic bonding.

Ionic bonds are formed between metals and non-metals. The non-metal turns into an anion, leaving it with a negative charge, and the metal turns into a cation, leaving it with a positive charge.

Anions

When an atom $usually a non\-metal$ is turned into an anion, it gains and adds electrons to its valence shell until the valence shell is completed. Cl $Chlorine$, for example, with 7 valence electrons, only needs one more electron to complete its valence shell. Thus, in an ionic reaction, Cl will gain one more electron to get 8 valence electrons, becoming Cl^-. After this, Cl will not take in any more electrons because its outer shell is full $the octet rule$. Thus, the only stable Cl anion is Cl^-.

Cations

When an atom $usually a metal$ is turned into a cation, it loses electrons from its valence shell until the outermost shell is empty, at which point the shell below it, which is full, will become the new outermost shell. Na $Sodium$, for example, has one valence electron in its outer shell. Thus, in an ionic reaction, Na will lose its electron and become Na⁺. After this, Na will not lose any more electrons because it now has a full outer shell $octet rule$. Thus, the only stable Na cation is Na⁺.

But why does it happen this way? Why do metals become cations, while non-metals become anions? Why not the other way?

Metals, with few valence electrons $less than 4$, tend to have lower ionization energies for their valence electrons, while non-metals, with many valence electrons $more than 4$, tend to have higher ionization energies for their valence electrons. The ionization energy is the amount of energy required to remove an electron from an atom. The 1^st ionization energy is the amount required to remove the first electron from the atom $e.g. turning Al into Al^\+^$, the 2^nd ionization energy is the amount of energy required to remove the second electron $e.g. turning Al^\+^ into Al^2\+^$, and so on.

Since in an ionic reaction, the metal has lower ionization energies for its valence electrons than the non-metal, and the non-metal has a much higher electronegativity than the metal, the non-metal can easily pull the valence electron$s$ away from the metal and add them to its valence shell.

The diagram below illustrates this:

Even after bonding, however, the ions are still charged, which means they will also attract other ions with opposite charges.

In this model, the Sodium ions have been outlined in red and the Chlorine ions have been outlined in blue to easily distinguish between the two; these are not their actual colors. The positively charged Sodium ions attract negatively charged Chlorine ions towards them, which attract positively charged Sodium ions towards them, which will attract negatively charged Chlorine ions towards them, and so on.

This continues in three dimensions until no more pairs are nearby, producing a crystal structure. There is multiple Sodium ions surrounding every Chlorine ion, and vice versa. All the Sodium ions surrounding a Chlorine ion are attracted to that Chlorine ion, and all the Chlorine ions surrounding a Sodium ion are attracted to that Sodium ion. There is no distinct NaCl molecule; each ion is surrounded by oppositely charged ions, which it both attracts and is attracted to. This structure is called an ionic crystal.

As there are no distinct NaCl molecules in a NaCl crystal, the formula tells us that there is 1 Na atom for every 1 Cl atom. Thus, in ionic compounds, the formula tells us the ratio of ions, not the total number. This ratio is simplified before being used for the formula - the formula is NaCl, not Na₂Cl₂ or Na₅Cl₅.

After a couple more repetitions of ions bonding to one another, we might get something like this:

This would be in a 3D shape, but it would still have the general shape, where positively and negatively charged ions are alternating in the structure. Each ion is surrounded by oppositely charged ions, and it attracts these surrounding, oppositely charged ions. The Chloride ion surrounded by four Sodium ions isn't just attracted to one of them, it's attracted to all four of them. It has four bonds of attraction, rather than just one like we might see between two atoms in a molecule. Given that ionic compounds have a 3D structure, in reality the Chloride ion would be surrounded by even more Sodium ions, and would thus have more than 4 bonds of attraction. The same goes for the Sodium ions. The ions are held tightly together, and therefore they have high melting and boiling points, as it takes a lot of energy to overcome all these bonds of attraction.

However, the crystal lattice structure also makes ionic compounds very brittle. If you punch a giant salt crystal $not recommended$, you apply a force on some of the ions, but not all of them. We can illustrate this as:

The arrows represent a force $in this case, the punch$ being applied on the ionic compound $in this case, the salt crystal$. The force isn't applied to all the ions. Only some of the ions experience a force. Therefore, only some of the ions are pushed.

The crystal lattice structure has been messed up. Instead of all ions being adjacent to oppositely-charged ions, now some Chloride ions line up with similarly-charged Chloride ions, and some Sodium ions line up with similarly-charged Sodium ions.

The similarly-charged ions will repel, rather than attract, one another, and the ionic compound will split.

The same bonding structure that gives ionic compounds high melting and boiling points makes them brittle as well.

This structure is also responsible for one more property of ionic compounds: electrical conductivity.

Electricity is conducted when an electrical charge flows from one end of a substance to another. On the atomic scale, this means one of two things: either electrons flow from one end of the entire substance to another, or ions flow from one end of the entire substance to another. In the case of an ionic compound, it’s the ions.

When an ionic compound is a solid, the ions are fixed in place by oppositely-charged ions; they can't change their positions relative to one another. Since ions can't move from one end of the substance to another, the substance can't conduct electricity.

But when the ionic compound is a liquid, or dissolved in solution, the ions are free to move about from one end to another. Therefore, when an ionic compound is a liquid, or dissolved in solution, it can conduct electricity.

Practice

What is the ionic formula for an ionic compound made of the elements Potassium (which is a metal) and Oxygen (which is a non-metal)? KO K₂O KO₂ K₃O₂ K₄O₂

Show Answer & Explanation