Lesson 1 - Ions and Ionic Bonding

Reminder:

An atom is a neutral particle that consists of a central nucleus made up of protons and neutrons, surrounded by electrons which are found in shells.

e.g. a sodium atom

If we find sodium's symbol on the Periodic Table, we can see that the mass number is 23 and the atomic number is 11. This means a sodium atom has;

11 protons

11 electrons

12 neutrons (remember, to work out the number of neutrons we take away the atomic number from the mass number).

In an atom, the number of protons = the number of electrons. This is why atoms are neutrally charged!

If we remember back to how electrons orbit an atom's nucleus, we always fill from the inner shell first, then outwards. The first shell can hold a maximum of 2 electrons and the second shell a maximum of 8 electrons. Because a sodium atom has 11 electrons, 2 go in the first shell, then 8 in the second shell, then 1 in the outer (third) shell. This is why sodium is found it group 1 of the Periodic Table - because it has one electron in its outer shell.

Question..... which group of elements on the Periodic Table are completely unreactive?

Think.....

Think..............

Think........................

Think ........................................

The answer is Group 0 (The Noble Gases). These elements, such as neon, helium, argon, etc, are completely unreactive! But why??

The answer is because they have a FULL OUTER SHELL OF ELECTRONS.

Electron configurations:-

Helium: 2

Neon: 2,8

Argon: 2,8,8

This is the only group of elements on the Periodic Table where the atoms of their elements have a full outer shell of electrons. This means every other element on the Periodic Table is reactive and takes part in chemical reactions.

FACT: Elements on the Periodic Table react so that their atoms can obtain a full outer shell of electrons.

Let's look at a really simple example... let's go back to our example of a sodium atom.... why is sodium reactive?

We saw that a sodium atom has the electron configuration 2,8,1. This means it only has 1 electron in its outer shell! To be stable, an atom of sodium wants to try and get a full outer shell of electrons. There are two ways a sodium atom could do this.....

1. It could either gain 7 electrons (this would give it 8 in the outer shell - full!)

2. It could lose its 1 outer shell electron (this would mean the next inner shell would be full, as it has 8 in it already).

Which of these two are easier do you think.... lose 1 electron, or gain 7? It's basic maths.... it's going to be easier to just lose 1 electron!

When the sodium atom loses its 1 outer shell electron, it now has an imbalance of protons and electrons. It still has 11 protons, but now only has 10 electrons. This means that the species we are left behind with is no longer neutrally charged! We have turned a sodium atom into a sodium ion (which has a 1+ charge). We would call this an Na⁺ ion.

Let's consider a second case.... looking at a chlorine atom. Chlorine is in group 7 of the Periodic Table and has the electron configuration 2,8,7. The easiest way for it to obtain a full outer shell would be to GAIN 1 electron.

Because the chlorine atom has now gained 1 electron, it now ends up with 17 protons and 18 electrons. This means it has a negative charge overall. We have turned it into a chloride ion, Cl^-. Notice that the negative ion name is not a chlorine ion... but a chlorIDE ion. This is a naming convention you must learn.

In general...

• Metal atoms find it easier to lose electrons and they become positively charged ions (which we call cations)

• Non-metal atoms find it easier to gain electrons and they become negatively charged ions (which we call anions).

We can show the overall dot and cross diagram, which explains where the electrons originally came from and how they were transferred from one atom to the next.

Ionic bonding ONLY occurs when your compound contains a metal AND a non-metal element, because the metal needs to lose electrons and the non-metal needs to gain electrons.

This means that if you have a substance only made up of metals or only made up of non-metals, it cannot be ionically bonded and must be something else.... next lesson!