Day 25

Context

We now deviate from temporary dipoles for a short time to consider how atoms stay together.

Explanation

The periodic table is a great tool for predicting why atoms stay together. We will start with ionic compounds that stay together because they transfer electrons and create oppositely charged ions.

After this class you should be able to:

• Use the periodic table to predict the charge on common monatomic ions.
• Give formulas for ionic compounds.
• Say why atoms stay together in salts.

The Periodic Table Revisited

We previously learned that the elements in a column of the periodic table have similar properties. The elements in the last column of the table are called the noble or inert gases. All of them are gases and they are not very reactive.

What is responsible for reactivity? Well, if anything moves it will be the electrons, so it is the electrons that determine reactivity. The easier it is for the electrons to move around the more reactive it is. Chemical properties are determined by the electrons in the atom.

So, what does that mean about the Noble gases? It must mean that the electrons are held tightly in those atoms and are not easily transferred or shared. We could say that they are in a low energy state. Everything likes to be in a low energy state.

The bottom line for us is that the number of electrons in a noble gas must be a preferred number of electrons to have. That turns out to be true and it explains a lot about how salts, or ionic compounds, are formed.

Today's Main Idea!

Application of the Main Idea

Lets review a couple of things about the periodic table before we begin. Specifically remember that the sequencial numbers are the atomic numbers and that they tell us how many protons are in the nucleus of an atom of that element.

Look at flourine, F, number 9. F has 9 protons, so how many electrons does a neutral flourine atom have? If it is neutral it must have the same number of electrons as it does protons, so a neutral flourine atom would have 9 electrons. But flourine is right next to a noble gas, neon, Ne, which is number 10. A neutral neon atom has 10 electrons.

If flourine just had one more electron it would be like a noble gas!

That is exactly what it does. The flourine atom gets another electron to have the same number of electrons as the noble gas neon. Does it become neon? No, the element is determined by the number of protons and that didn't change, so it is still flourine. It does have an extra electon now, so we say it is an ion that has a negative one, -1, charge. It could be written F^-.

What would chlorine, Cl, do? It, like flourine, just needs one more electron to be like the noble gas argon, Ar. It would also become an ion with a -1 charge. What about bromine or any of the other elements in that column. All of them would create ions with a -1 charge. What could oxygen, #8, do? It would have to gain two electrons and would be a negative ion with a -2 charge.

Now look at sodium, #11, Na. Na has 11 protons and, when neutral, 11 electrons. If we apply our main idea we might consider being either like neon, with 10 electrons, or argon, with 18 electrons. It is clearly easier to lose one electron to be like neon than to gain seven to be like argon. Sodium would lose one electron to be like neon. It would then have one more proton than it does electrons. Since protons have positive charges, Na would become positive and have a +1 charge. It would be written Na⁺.

What about Li, K, or any of the elements in that first column? All of them would give up one electron to become like a noble gas and have a +1 charge. What about the second column? All of those elements would give up two electrons and have a +2 charge.

Another Thing to Remember!

For this class we can assume that all of the elements in the first column will go to a +1 charge, all of the elements in the second column will go to a +2 charge, and all of the elements in the next to last column will go to a -1 charge. These statements are generally true even outside of this class. Another thing we will assume for this class, that isn't as generally true, is that oxygen will go to a -2 charge. You can't do this with any other elements! The other elements also tend toward having the same number of electrons as a noble gas, but they may do it in a different way. Or they may undergo other transformations that we aren't ready to discuss.

Here is a chart summarizing some of this.

Symbol	#Protons	#Electrons
F	9	9
F-	9	10
Cl	17	17
Cl-	17	18
O	8	8
O2-	8	10
Na	11	11
Na+	11	10
Mg	12	12
Mg2+	12	10

Creating Ions

You can't just get electrons from nowhere or just make them disappear, so how are these ions formed?

How are all charges created? By transferring electrons! To create these ions we put an atom that wants to gain an electron next to an atom that wants to give up an electron and together they transfer electrons to form ions.

So, combining Na with Cl results in NaCl where Na is a positive ion and Cl is a negative ion, both having a charge of one (+1 for Na and -1 for Cl). These kind of substances are called ionic compounds or salts.

A Condition

When you create ionic compounds the overall charge of the resulting compound must be zero. The positive charges must balance out the negative charges. In NaCl the Na is +1 and the Cl is -1. +1-1=0, so it balances. This will work with any combination of elements from column one with an element from the next-to-last column, eg. KF. By the way, the elements of the next-to-last column are called halogens, a term I will now use to refer to those elements.

But what about other combinations? What happens when you combine Mg with Cl? Mg will get a +2 charge and Cl will get a -1 charge, so they won't cancel out. One Mg with one Cl doesn't balance out the charges. To balance the charges we need to combine two Cl with each Mg. That way the two Cl would provide a total of -2 charge and balance the +2 charge of the Mg. The formula for the compound would be MgCl₂.

Can you predict the correct formulas for the following? Hover to see answers.

• Li and Br LiBr
• Na and O Na₂O
• Ba and Br BaBr₂
• Sr and O SrO
• or my favorite, Cs and I? CsI

Why Do the Atoms Stay Together in Ionic Compounds?

In ionic compounds there are oppositely charged ions. Opposite charges attract each other. The atoms in ionic compounds stay together because opposite charges attract. Do they have molecules?

Remember that a molecule is formed when two or more atoms are hooked together, stay together, and act like one unit. An example would be H₂O. The two hydrogens are attached to one oxygen and they float around together in liquid water. When water evaporates those same atoms stay together and go into the air.

Does NaCl do the same thing? Ionic compounds including NaCl dissolve in water. But what is floating around in the water? We can test this using electricity.

In order for electricity to flow you need a power source, a closed circuit or loop, and mobile charged particles. In metals the mobile charged particles are electrons. It turns out that normal water is not very conductive. Water doesn't have enough mobile charged particles to turn a light bulb on. But when you dissolve NaCl in water the light goes on bright. Here is a video showing this point.

Since the light goes on after dissolving NaCl, when the NaCl dissolves it must provide mobile charged particle. They would be the Na⁺ and Cl^- ions. The ions become free to move around in the water. The conductivity after dissolving in water tells us that no two atoms are stuck together. The Na⁺ and Cl^- ions are independent of each other. Salts don't have molecules!

Salts and Poyatomic Ions

So far we have only talked about salts formed form single element ions. Polyatomic ions are charged particles that have two or more atoms that are connected and act as one unit. Some common polyatomic ions are NH₄⁺, NO₃^-, SO₄^2-, and CO₃^2-.

Each polyatomic ion has a charge associated with it that doesn't change. The charges on the examples above don't change. These polyatomic ions can be combined with the elements we have identified as forming salts or with other polyatomic ions to form ionic compounds. It is always the case, however, that the charges of the ions should balance out: all electrons must be accounted for. Here are some examples: NH₄Cl, (NH₄)₂SO₄, NaNO₃.

The hydroxide ion (OH^-) is an important ion because an aqueous (water) solution is determined to be basic (as opposed to acidic) if there are extra hydroxide ions in the solution. Many bases are ionic compounds that have OH^- as the negative ion. NaOH, sodium hydroxide, is a good example of a base. When NaOH is placed in water it separates into Na⁺ and OH^- ions that are free to float around in the solution. Any of the elements in the first two columns (excluding hydrogen) combined with the hydroxide ion will form an ionic compound that will be a base. Some examples: LiOH, Mg(OH)₂, etc. Any substance that increases the hydroxide ion in an aqueous solution is a base.

Homework

The homework associated with Day 25 is on Canvas.