Structure and Bonding [3]: Ionic Bonding - Part 1 (A Level Chemistry)


Uploaded by freeeschool on 01.09.2012

Transcript:
A Level Biology: Structure and Bonding 3 – Ionic Bonding (Part 1)
Hi! Welcome to my third video on the series about Structures and Bonding. Today, we are
going to be looking at Ionic Bonding.
What we have previously gone through and looked at the structure of different types of atoms.
What we are going to do today is to understand why certain atoms join together. You have
all probably heard of the chemical called sodium chloride, which is normal table salt.
Today, we are going to look at why it is that the sodium and the chlorine join together.
An ionic compound is one that is produced from a metal and a non-metal. Now a metal
is always found on the left hand side of the Periodic Table and a non-metal is found on
the right. You will be able to see on the Periodic Table that it has this squiggly line
on the right hand side, and that divides up the metals from the non-metals.
What we need to establish is what an ion is. An ion is produced when an atom either losses
or gains electrons. I’m going to explain this with the example of sodium chloride,
specifically sodium at the start here. Sodium (Na), if you look at that in your Periodic
Table, is on the left hand side. It is in Group 1 and so you should realize that it
has one electron in its outer shell.
So in order for the sodium atom to become an ion, what it needs to do is it needs to
lose this single electron in its outer shell and that’s what all atoms are aiming to
do or trying to do. They are trying to reach the state in which they got a full outer shell.
Now a sodium atom could either gather more electrons to it which is seven and the quickest
way to do that is to lose one. When it does that, it loses this electron and it becomes
an ion. It now has a charge. The charge it now has is single positive and the reason
for that, if you look back up at the previous videos where I have discussed proton number
and mass number, you’ll know that sodium has a proton number of 11. That means that
it has eleven protons, so eleven positive charges. Now, because this has become an ion,
what happened is it lost a single electron. Now, instead of having 11 electrons, it now
has 10. So it has 11 positive charges and 10 negative ones and hence why it has the
charge of single positive.
Now, chorine is different. Chlorine is on the right hand side and it is non-metal. It
has seven electrons in its outer shell which tells you that it is in Group 7. Now, in order
for chlorine to have a full outer shell which is trying to get to, it can either gain one
electron to get to eight or lose seven and so the quickest method is to gain one.
So this shows our chlorine atom with 2, 8, and 7. So chlorine, that has an atomic number
of 17 and so you can see these 17 crosses here and in order for it to become an ion,
what it must do is it must gain an electron. So they are using an example of gaining it
from sodium. It then reaches a full outer shell and now, it has 2, 8 and 8 and now,
it has 18 electrons.
The reason why chlorine has a negative charge is the fact that it got 17 protons and now,
it got 18 electrons and hence, Cl-.
This diagram here takes us through the entire process. These both represent a sodium atom
and a chlorine atom. What is going to happen is the electron from the sodium is going to
exchange with the electron from the chlorine and now we have a sodium ion, which is a positive
charge and we have a chlorine ion which has a negative charge. What you will find is because
one has a positive and the other has a negative, then they would be attracted to one another
and join up and hence, you have sodium chloride, NaCl.
Now, there is a simpler way of doing this. Instead of drawing all the electron shells,
sometimes, you might be asked in an exam question or it might show you an example of whether
to draw the outer shell and this is what it is illustrating. Sodium is from Group 1; a
single electron in its outer shell. Chlorine is from Group 7; seven electrons in its outer
shell. This is shown that the sodium losing the electron and the chlorine gaining it and
hence, the positive charge for the sodium and negative for the chlorine.
In summary, ionic bonding is where ions of opposite charges join. In the case of sodium
and chlorine, sodium has a positive charge, chlorine has a negative and hence why they
joined together.
Ions are formed when electrons are lost or gained. In the case of sodium, it produces
an ion when it loses its electron. Chlorine forms an ion when it gains an electron. All
ions from Group 1 elements have a positive charge and all ions formed from Group 7 have
a negative charge.
[end of audio – 04:54] A Level Biology: Structure and Bonding 3 – Ionic
Bonding (Part 1) Page…1