In the last lecture you learned about atomic structure of the biological elements
in the first three periods of the periodic table.
In this lecture we're going to move forward
and look at how the electrons in the outermost or valence shell results in bonding arrangements.
So by the end of this lecture, you should be able to explain the difference between covalent,
ionic, and hydrogen bonds, as well as determine the relative strength of each bond type.
Finally towards the end of the lecture,
we’ll take an exploration of the pH scale and learn a little bit about acids, bases, and buffers.
So to begin, let’s look at covalent bonds.
Covalent bonds involve sharing of electrons so each atom has equal interest in the electrons between them.
A single covalent bond involves the sharing of one pair of electrons. For example here, we’ll look at hydrogen gas.
Hydrogens alone have a single electron in the outermost shell which gives them a valence of 1 because ideally,
to fill that innermost shell, they would like two electrons.
So hydrogen will get together with another hydrogen
and they will each share the pair of electrons so that the octet rule is satisfied
and each has a stable outer shell with two electrons.
Next, we’ll look at covalent bonds sharing two pairs of electrons or a double covalent bond.
In this case we’ll use oxygen as an example.
Oxygen has six valence electrons which means that to satisfy the octet rule,
it would like to have another two electrons in its valence shell.
It can satisfy the octet rule simply by sharing two electrons with one other oxygen.
So oxygens now have two pairs of shared electrons in the valence shell -- six of their own,
two pairs of shared that equals 8, satisfies the octet rule, so oxygen and oxygen or oxygen gas is quite stable.
Now, let’s look at a triple covalent bond. For example nitrogen has five valence electrons.
In order to be satisfied, it would like to have three additional electrons so a triple covalent bond
involves the sharing of three pairs of electrons.
It is the strongest of the bonds that we will be looking at in this course.
So nitrogens -- adding the three more electrons sharing with another nitrogen,
finally have 8 electrons in the valence shell.
Three pairs of electrons form a stable molecule of nitrogen gas.
Nitrogen is really hard to break apart, in fact there are only a hundred species of nitrogen fixing bacteria
that are able to separate nitrogen gas, the two nitrogen molecules and break that triple bonding arrangements
to bring all of the nitrogen we have in the plants that grow on earth and thus consume ourselves.
So nitrogen originates from the nitrogen gas in the atmosphere,
the bond is really hard to break because it’s a triple covalent bond, the strongest of the biological bonds.