Video Transcript
In this video, we will learn
how the outer electrons of atoms can be shared to form covalent bonds. We’ll look at the forces
between electrons and nuclei that explain why these bonds form and look at ways
of representing them.
A proton is a subatomic
particle. Protons have a specific type of
charge that we call positive charge. And we say that a single proton
has a charge of one plus. An electron is a different type
of subatomic particle. Each electron has a charge that
is the same size but of the opposite type to the charge of a proton. So electrons are said to be
negatively charged with a charge of one minus.
Particles with opposite charges
attract each other, while particles with the same charge repel. Positively charged protons
attract negatively charged electrons. So when a proton and an
electron meet, they can form an atom. The electron doesn’t just stick
to the proton for reasons that are far too complicated to go into in this
video. Instead, we have the nucleus of
the atom deep in the center, and the electron occupies a much larger electron
cloud.
When electrons and a nucleus
come together, they can lose energy to their surroundings and become more
stable. But that doesn’t mean that
atoms cannot get any more stable. Atoms can gain or lose
electrons forming ions or they can come together to form what we call covalent
bonds. Electrons around the nucleus
can’t all be in the same space at the same time. When you add electrons to a
nucleus, the electrons fill up the space. And like tiers at a stadium,
the further from the nucleus you go, the more space there is.
Electrons are said to occupy
shells around the nucleus. The first shell can only
contain two electrons. The second electron shell can
contain a maximum of eight electrons. The bigger the shell, the more
electrons that can fit. An atom of hydrogen has one
electron and one proton. The electron is quite tightly
bound to the hydrogen nucleus with its one plus charge.
An atom of helium has just
enough electrons to fill the first shell. These two electrons are held
much more tightly to the nucleus, which has a two plus charge than in
hydrogen. An atom of lithium has three
electrons, so the first two are really tightly bound to the nucleus, which has a
three plus charge. And the third electron, forced
to be much further from the nucleus by the other two electrons, is much less
tightly bound.
An atom of neon has 10
electrons, two in the first shell and eight in the second shell. So atoms of helium and atoms of
neon both have full outer shells. But atoms of fluorine have only
nine electrons, two in the first shell and seven in the second. This means in an atom of
fluorine, there is space in the second shell for one more electron.
When we bring a lithium and
fluorine atom together, the outer electron of lithium is in a competition
attracted to the lithium nucleus and to the fluorine nucleus. The electron can hop over and
the force of attraction between the electron and the nine-plus-charged fluorine
nucleus is much much greater than the force of attraction it felt to the lithium
nucleus.
While there are many other
factors in the reaction of lithium and fluorine, this is a contributing
factor. The ion arrangement is more
stable than the atom arrangement, so we have a lithium ion, Li+, and a fluorine
ion, F-. You’ll generally see this
called a fluoride ion. These differently charged ions
attract each other in an ionic bond, forming lithium fluoride. However, we’re not specifically
looking at ionic bonds in this video. It’s just helpful to see what
happens when you take things to extremes. In less extreme cases, rather
than atoms becoming ions, atoms can share electrons.
What we have here are two atoms
of fluorine. There’s no reason for electrons
to jump from one atom to the other because they’re identical. But that doesn’t mean we
couldn’t get a configuration that’s even more stable. If two fluorine atoms get close
to each other, the outer electrons of each atom will be attracted to the nucleus
of the other atom. As the forces balance out, the
most stable arrangement emerges where two electrons are in between the atoms
being equally shared. By sharing electrons, these two
atoms have managed to fill their outer shells without gaining or losing
electrons and becoming ions.
This arrangement is much more
stable than when the atoms were separated. Chemists have discovered
there’s only electrons in the outer shell of atoms that are shared in covalent
bonds. We call electrons in the outer
shell of an atom or ion the valence electrons. So we sometimes also refer to
the outer shell as the valence shell because the valence shell contains valence
electrons. So a simple definition of
valence electron is an electron in the outer shell of an atom or ion. It’s generally also required
that valence electrons be able to participate in bonding, but this simple
definition will do.
Atoms of hydrogen have one
valence electron but so do lithium atoms because the two electrons in the inner
shell are not part of the valence shell. Broadly speaking, the number of
valence electrons of an atom determines its chemical character. When atoms bond and share their
electrons, they form what we call a covalent bond. Shared electrons are attracted
to each nucleus, and these electrons help fill the outer shells of each
atom. The co- in covalent means
together or sharing or joint, and “valent” refers to the outer electrons
involved.
The simplest type of covalent
bond is the single bond. In a single covalent bond, two
electrons are shared. In a double covalent bond, four
valence electrons are shared. And in a triple covalent bond,
six electrons are shared. Quadruple bonds where eight
electrons are shared are possible, but only between much larger atoms or in
highly exotic circumstances. In general, you’ll only need to
worry about single, double, and triple bonds. But how do we know how many
covalent bonds different atoms are likely to form?
Fluorine atoms have seven out
of the maximum of eight valence electrons available because the second shell can
only fit eight electrons. A fluorine atom can get that
one extra electron from another atom, like another fluorine atom. An atom of oxygen has six
valence electrons, so it needs to form a double bond or two single bonds to fill
it. And an atom of nitrogen has
only five out of the maximum of eight valence electrons. So it needs to form a triple
bond, a double and a single bond, or three single bonds to fill its outer
shell.
Meanwhile, a carbon atom has
four electrons in its outer shell. So we’d expect a carbon atom to
form a total of four bonds in order to fill its outer shell. However, carbon generally
doesn’t form quadruple bonds. In fact, you can assume that a
triple bond is the best a carbon atom can do. Instead, you’ll commonly see
carbon atoms sharing electrons with multiple other atoms, for instance, forming
four single bonds, two single bonds and one double bond, two double bonds, or
one single bond and one triple bond.
If we were reacting pure carbon
to start with, the marked electrons would be from the carbon and the unmarked
electrons would be from the other atoms. We can look at the first 10
elements on the periodic table and look at their number of valence
electrons. This is what we get. What’s more interesting is the
number of covalent bonds these atoms will form or the charge of the ions they’ll
form when they react. Atoms of hydrogen tend to form
one single bond, while helium atoms don’t form bonds at all.
Atoms of lithium and beryllium
tend to react and give up their electrons to form Li+ and B2+ ions. And the combining power of
these atoms increases up to carbon and then decreases again until we get to
neon, which doesn’t form bonds at all. We call these numbers the
valency, the combining power of that type of atom. But it will be very annoying if
we have to go around and remember the valency for every single atom and deduce
it from first principles.
Instead, there’s a rule of
thumb that will help us out. And it’s called the octet
rule. The first electron shell can
fit two electrons. The second can fit eight. The third shell can fit 18
electrons. However, those last 10 slots
are a little different, and they are only filled in larger atoms. So as a simplification, we
often say that the third shell also contains only eight electrons. This is handy because it helps
us formulate the octet rule, which tells us that an atom will tend to react to
achieve eight electrons in its outer shell. This will tend to mirror the
electron configuration of one of the noble gases.
Atoms of hydrogen and helium
are exceptions because the first electron shell only contains two electrons. However, the octet rule does
account for the most common bonding behavior of these elements, lithium to
silicon, and a significant proportion of the behavior of these elements, between
phosphorus and radon. It doesn’t make sense to apply
the octet rule to the noble gases since they already have full outer shells. And some synthetic elements are
simply too unstable to worry about how they’ll react. And things are a little too
complicated for the d-block and f-block elements for the octet rule to
apply.
The takeaway from this is that
atoms will tend to react so they have eight electrons in their outer shell. But if we look at some of these
elements, we see that not all of them bond covalently. We can roughly divide the
periodic table into metals and nonmetals, and we tend to only see covalent
bonding between nonmetal elements.
And we tend to see ionic
bonding between metallic and nonmetallic elements. And between metals and metals,
we see metallic bonding. Fluorine molecules and hydrogen
chloride are example of nonmetal–nonmetal pairings with covalent bonds. And for metal–nonmetal, we have
the examples of sodium chloride and lithium fluoride. And we see metallic bonding in
alloys like brass, which is a mixture of copper and zinc.
The last thing we need to look
at is how we represent covalent bonds because sometimes we can’t draw all the
electrons or all the shells. Here are a few of the ways you
might see covalent bonds represented. Here are some of the features
you might see in various electron shell diagrams. The symbols for the nuclei may
be written as element symbols or as charges or numbers of protons. And electrons can be drawn with
different shapes and colors in order to indicate where they might’ve come from,
for instance, dots and crosses. And for complicated diagrams,
it’s sometimes easier if the valence shell is the only shell drawn and the inner
shells are left out.
Meanwhile, electron dot or
Lewis dot diagrams are useful for condensing all this information into a much
easier-to-draw form. Element symbols are used for
the nuclei. Dots are used for the
electrons. The electrons are arranged in
pairs on sides of the element’s symbol resembling the valence shell. And you can recognize bonding
pairs by the dots between element symbols. And the other pairs of dots are
known as lone pairs.
Double bonds and triple bonds
simply require more bonding pairs. And it’s quite common to
substitute the bonding pairs with the right number of lines, one line per two
electrons. But the most common way to see
covalent bonds is as lines without any lone pairs on the atoms. Each type of diagram is useful
for a different purpose. Now it’s about time we had some
practice.
How many electrons are shared
in a double bond between two oxygen atoms?
Oxygen is an element, and we
can find information about the element on the periodic table. The atomic number of the
element oxygen is eight. This means that oxygen atoms
contain eight protons. And since atoms are by
definition neutral, we have eight electrons as well, eight electrons to balance
out the charge of the eight protons. The question asks about
electrons shared in a double bond between two oxygen atoms. Now, oxygen is a nonmetal, so
we’d expect a certain type of bonding. And the word “shared” gives us
the start of it, co- in covalent.
Now the “valent” part in
covalent refers to the valence electrons. Valence electrons are simply
those electrons in the outer or valence shell of an atom or ion. An oxygen atom has eight
electrons and the first two fill the first electron shell. And the remaining six occupy
the second electron shell, but the second electron shell can fit a maximum of
eight electrons. We can find more electrons to
fill that available space in our second oxygen atom. Since the valence shells are
the only ones of interest here, I’m going to remove the inner shell.
If the atoms get closer
together, some of the electrons are shared between the two nuclei, helping to
produce a more stable configuration. Since this involves sharing of
valence electrons, we have a covalent bond. And since there are four
electrons involved, we’re dealing with a double covalent bond. The quick way round is just to
remember that a double covalent bond contains four electrons, a single contains
two, and a triple contains six. So how many electrons are
shared in the double bond between two oxygen atoms? Four.
Now let’s have a look at the
key points. A covalent bond is a chemical
bond involving shared valence electrons. We can predict the number of
covalent bonds that would form between different atoms using the octet rule of
thumb, which tells us that atoms tend to react to acquire eight valence
electrons in total. And nonmetals, either with
themselves or other nonmetals, tend to bond covalently. And the main types of covalent
bond are single bonds involving two shared electrons, double bonds involving
four, and triple bonds involving six.