Ionic and Molecular Compounds

Structures and Properties of Ionic Compounds

When a bond forms between a cation and an anion, we call it an ionic bond. Ionic bonds occur when the cation donates electrons to the anion so that way they can both have an entire outer shell.

For example, when sodium (Na) bonds with chlorine (Cl) to make the compound NaCl, the sodium ion (Na⁺) donates one electron to the chlorine ion (Cl^-). Sodium has one valence electron, while chlorine has seven valence electrons. They both want to have an entire outer shell and become more stable. So, sodium gets rid of its single electron in the outer shell and gives it to chlorine since chlorine needs one electron to fill its outermost shell. Even atoms like to help others by giving away what they don't need to those who do!

Fig. 2: The ionic bond between Sodium and Chlorine, Isadora Santos - StudySmarter

What keeps the ions in an ionic bond together? Electrostatic forces between the metal and the non-metal hold the atoms together in an ionic bond!

When a compound comprises a negative and a positive ion, they are considered an ionic compound. A positive ion is called a cation, whereas a negative ion is called an anion.

• Metal ions lose electrons to form cations, while non-metals gain electrons to form anions.

Ionic compounds have the following properties:

• They have strong electrostatic attractions.

• They are hard and brittle.

• Ionic compounds have a crystal lattice structure.

• Ionic compounds have high melting and boiling points.

• Ionic compounds can conduct electricity only when in liquids or if dissolved.

Electronegativity

Electronegativity is an atom's ability to attract a shared pair of electrons. To determine whether a compound is ionic or not, we can take a look at the difference in electronegativity between the two atoms. We can use the periodic table to compare the electronegativity between two atoms, and if the difference between them is greater than 1.2, they will form an ionic compound! Notice that in the periodic table below, electronegativity increases across a period (from left to right) and decreases down a group.

Naming Ionic And Molecular Compounds

When naming ionic compounds, there are specific rules that we need to follow:

1. We always write ionic compounds in the following form: cation + anion.

2. If the cation has more than one charge, we need to write the positive charge using roman numbers. We always need to state the oxidation number, except for groups 1, 2, and Al³⁺, Zn²⁺, Ag⁺, and Cd²⁺. For example, if we have Fe⁺³, then we would write its name as Iron (III), but if we have Zn²⁺, we would write its name as Zinc.

3. The anion will keep the beginning to its name, but -ide needs to be added to the end.

To make things easier, let's look at an example!

Well, that was pretty easy! Unfortunately, not all compounds are that easy to name. When we come across polyatomic ions, the naming is slightly different. Most common polyatomic ions are negatively charged (anions), except for the ammonium ion (NH₄⁺) and the mercury (I) ions (Hg₂⁺²). When polyatomic ions are present, they will always keep their name! So, the easiest way to name compounds involving polyatomic ions is to memorize their names!

Here is a list of the most common polyatomic ions you might encounter:

Let's look at some problems involving polyatomic ions.

1) Name the following ionic compound: CoCO₃

First, notice that CO₃ is a polyatomic anion: CO₃^-2. Cobalt (Co) is a transition metal, so it can have many charges. Since there is a -2 charge on CO₃^-2, we can assume that the charge in Co is +2. In other words, Co⁺² will give away two valence electrons, and CO₃^-2 will accept two valence electrons.

Since a polyatomic anion is present, we have to maintain its name. By looking at the list of polyatomic ions, we know that the name for CO₃^-2 is carbonate. So, the name of this compound will be Co⁺² metal + polyatomic anion: Cobalt (II) carbonate.

2) Write the formula for the following ionic compound: Magnesium sulfate

We know that magnesium (Mg) cation has a charge of +2 and that sulfate is a type of polyatomic anion with the formula SO₄^2- . Since the charge of both the cation and the anion is the same, they cancel each other, so we don't need to write it. So, the formula for magnesium sulfate would be MgSO_4.

Now, let's look at the molecular compound nomenclature. Naming molecular compounds is easier than ionic compounds' nomenclature when it comes to naming them.

1. First, look at the first nonmetal and write its numerical prefix. However, if the first nonmetal has a prefix of 1, do not add the "mono" prefix.

2. Write the name of the first nonmetal.

3. Write the numerical prefix of the second nonmetal.

4. Write the base name of the second nonmetal and change the end to -ide.

The numerical prefixes that you need to learn if you haven't yet are the following:

Feeling confused? Let's look at some examples!

Difference Between Ionic and Molecular Compounds

Now that we learned about the structure and properties of ionic compounds, let's look at what molecular compounds to learn how they differ from ionic compounds. When nonmetals are joined together by covalent bonds, they form molecular compounds. Instead of a cation giving away its electrons to an anion as it happens in ionic bonding, covalent bonding consists of sharing valence electrons between two atoms.

To better understand how nonmetals form covalent bonds, let's look at the figure below. Here, one carbon atom bonds to two oxygen atoms to form carbon dioxide CO₂. Carbon has four valence electrons, and oxygen has six valence electrons.

They both want to have full outer shells (8 electrons), so they share electrons between them! Each oxygen atom will share two electrons with carbon, and carbon will share two electrons with each oxygen atom.

Properties of Simple Covalent Molecules

Simple covalent molecules have low melting and boiling points. They are also insoluble in water and are considered poor conductors of electricity since they cannot carry a charge (they are neutral). Common examples of simple covalent molecules include CO₂, O₂, and NH₄.

Properties of Covalent Macromolecules

Macromolecules are also called giant covalent structures. These compounds are also molecular compounds, but they have different properties. Macromolecules have high melting and boiling points, and they are hard and strong. They are also insoluble in water and are unable to conduct electricity. Some examples of macromolecules include silicon and diamond.

Conductivity of Molecular and Ionic Compounds

Let's talk a bit more about the conductivity of molecular and ionic compounds. Ionic compounds are capable of electrical conductivity only when molten or dissolved. When the ionic solid gets dissolved in water or when in its molten state, the ions separate and become free to move around and conduct electricity.

Covalent compounds, on the other hand, are incapable of conducting electricity because they have no charged particles that can freely move. The only exception is graphite. Graphite has loosely held electrons that can move through the solid structure, conducting electricity.

Ionic and Molecular Compounds Examples

Now, let's take a look at examples involving ionic and molecular compounds. Some examples of ionic compounds include CuCl, and CuSO_4.

Cuprous chloride (CuCl) is an ionic solid that has a melting point of 430 °C. In organic chemistry, CuCl can be used in a reaction with aromatic diazonium salts to form aryl chlorides. It can also be used as a catalyst in other organic reactions. Copper (II) sulfate is also an ionic solid, and it has a melting point of 200 °C. CuSO4 has many uses, such as a soil additive in agriculture and as a wood preservative.

Examples of molecular compounds include N₂O₄, and CO. Dinitrogen tetroxide (N₂O₄) is a gas at STP. It was a boiling point of 21.2 °C. N₂O₄ can be used as a fuel additive, for example, as a rocket propellant! Carbon monoxide (CO) is also a gas at STP, and it has a boiling point of -191.5 °C. Carbon monoxide can be very dangerous. For example, when a person gets CO poisoning, these carbon monoxide molecules bind to hemoglobin instead of oxygen molecules.

I hope you are more comfortable with ionic and molecular compounds now; maybe you can tell them apart by their specific properties!