Video Transcript
In this video, we will learn about
the electrostatic attraction between ions that we call ionic bonding. We’ll also look at how these bonds
result in the distinctive properties of ionic compounds. An ion is the combination of a
nucleus containing protons and usually some electrons. Nuclei can also contain
neutrons. But since they only contribute
mass, they don’t really affect ionic bonding, so we won’t be looking at them in this
video.
A proton has a specific type of
charge, which we call positive charge, and we say one proton has a charge of one
plus. Meanwhile, electrons have equal and
opposite charge to protons, so they’re negatively charged with a charge of one
minus. Particles with opposite charges
attract one another via the electrostatic force, while particles with the same type
of charge repel. So protons will naturally attract
electrons and form atoms and ions.
By definition, an atom is neutral
overall, which means the number of protons in the nucleus must be equal to the
number electrons in the electron cloud. When a nucleus and electrons come
together, they can lose energy to their surroundings and become more stable. But that doesn’t mean that atoms
cannot get even more stable. Atoms can lose electrons and end up
with more protons than electrons, forming what we call cations. Or atoms can gain electrons and
have more electrons than protons, making them negatively charged anions. In some cases, it’s more stable if
we have cations and anions rather than atoms. To understand why, we need to zoom
in.
When electrons are added to a
nucleus, they start to fill out space and occupy what we call shells. The bigger the shell, the more
electrons it can contain. Electrons can’t occupy the same
space at the same time, so when electrons are added to the nucleus, the electrons
fill up space like tiers at a stadium. The first electron shell can
contain only two electrons, but the second shell can contain eight. And as shells get bigger, they can
fit more electrons.
Now, as an example, let’s have a
look at an atom of fluorine. Fluorine atoms have nine protons,
which means the nucleus of a fluorine atom has a charge of nine plus. And since atoms must be neutral, we
also have nine electrons. We can add the first two electrons
to the inner shell using dots to represent the electrons. And the remaining seven go into the
second shell, filling seven out of the available eight spaces. A lithium atom, on the other hand,
has only three protons and therefore three electrons. The first two electrons go in the
first shell, and the last one sits in the second.
But let’s examine this outer
electron for a moment. This outer electron experiences a
force of attraction to the nucleus, but it’s only three-plus charged. It also experiences a force of
repulsion from the inner electrons. Now, what about an electron in the
outer shell of the fluorine atom? This electron experiences a much
greater force of attraction to the nine-positive nucleus of the fluorine atom, while
still experiencing a similar degree of repulsion from the inner electrons and the
other electrons in the same shell. The electrons of the fluorine atom
aren’t going anywhere. But what about the lithium
atom?
If the lithium and the fluorine
atoms get close together, this outer electron of the lithium atom has two possible
configurations: being on the lithium atom, being weakly attracted to the three-plus
nucleus, or being on the fluorine atom forming a fluorine ion strongly attracted to
the nine-plus nucleus. The most stable configuration
happens when the electron hops over from the lithium to the fluorine forming F- and
Li+. F- is generally known as
fluoride. The F- and Li+ ions then attract
each other through electrostatic forces, forming LiF or lithium fluoride. The bond between the ions is known
as an ionic bond, and substances containing ionic bonds are known as ionic
compounds.
In cases where the force of
attraction between an outer electron and two different nuclei isn’t as clear, we get
covalent bonding where electrons are shared between atoms. However, we won’t be looking at
covalent bonding in this video. Instead, we’re going to look at how
to predict whether an atom will produce a cation or an anion and what charge it will
have.
Chemists have discovered that when
atoms form ions, they generally only lose or gain electrons in certain places. Generally, electrons in the inner
shells stay where they are. And we either see electrons being
added to the outer shell until it’s full, or we see the atom lose electrons forming
a cation. The inner shell, now exposed, is
considered the outer shell because it’s the shell furthest from the nucleus with at
least some electrons in it. If we like, we can remove the empty
shell entirely.
I’ve shown this in one example, but
bear in mind that most elements commonly exhibit either one or the other behavior:
gain electrons to fill a shell or lose electrons to empty a shell. Since it’s electrons in the outer
shell that exhibit this special behavior, they get a special name: valence
electrons. And the outer shell of an atom or
ion is known as the valence shell. However, there’s one more
complication: not all the shells contain the same number of electrons; it’d be quite
difficult to remember. So fortunately, there’s a little
trick.
The first electron shell can fit
two electrons, while the second can fit eight. But the third electron shell can
actually fit 18 electrons. However, those last 10 slots only
get filled when atoms get bigger. So sometimes we say that the third
electron shell contains only eight. This is useful 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. And the final electron
configuration will tend to mirror that of a noble gas.
When we pair up elements and use
the octet rule, we can predict the chemical behavior of these elements very well
although with lithium to boron, they tend to lose electrons to form the electron
configuration of helium, which has two electrons in its outer shell. And we can use the octet rule to
predict a significant proportion of the chemical behavior of these elements. It doesn’t make sense to apply the
octet rule to the noble gases since their atoms already have full outer shells. And some synthetic elements are so
rare and unstable that we don’t need to worry about how the rule applies. And the electronic behavior of d-
and f-block elements is a little too complicated for the octet rule to be
reliable.
We can look at a group of many of
the elements and see how many electrons short of a full shell their atoms are or how
many electrons they’re likely to lose. When they react, atoms of the
alkali metals in group one tend to lose one electron. This gives them a full outer shell,
an electron configuration mirroring a noble gas, and a charge of one plus. For the alkaline Earth metals,
having two electrons in their outer shell, they’re likely to lose those two and form
two-plus ions. Atoms of the elements in group 13,
otherwise known as group three, tend to form three-plus ions, losing three
electrons. Although the closer to the top of
the periodic table the element is, the more likely it is to bond covalently instead
of ionically.
Elements in group 14, like carbon
and lead, will form four-plus ions under some circumstances and four-minus ions
under much rarer ones. And again, elements closer to the
top tend to bond covalently more than they do ionically. On the other hand, atoms of
elements in group 15 tend to gain three electrons to form three-minus anions. Those of the group 16 elements tend
to gain two electrons forming two-minus ions. And the halogens, the elements in
group 17 also known as group seven, tend to be found as one-minus ions.
This still leaves us the question
of which elements reliably bond ionically and which will bond covalently. We can crudely divide the elements
on the periodic table into metallic elements and nonmetallic elements. And as a general rule, we only see
ionic bonding when we see a nonmetal and a metal in the same compound. Nonmetals in pure form or paired
with other nonmetals tend to bond covalently. And we see metallic bonding for
pure metals and alloys. Sodium chloride and lithium
fluoride are good examples of nonmetal–metal pairings which exhibit ionic
bonding. Fluorine gas and hydrogen chloride
exhibit covalent bonding. And we see metallic bonding in pure
metals or in alloys like brass which is a mixture of copper and zinc.
The next thing we need to look at
is how we represent ionic bonds. One of the most direct ways you can
represent ionic bonding is using electron shell diagrams. There are many ways of doing these
diagrams, so here are some of the variants. The nuclei may be represented using
the respective element symbol or as a charge or a number of protons. The overall charge can be placed
outside brackets or attached to the symbol in the middle. Electrons may be displayed with
other symbols to indicate where they might have come from. And the inner or nonvalence
electron shells may be left out entirely.
We can even simplify things further
using Lewis dot or electron dot diagrams. In these diagrams, the expectation
is the nuclei will be labeled with elements symbols. But the charges could be on the
element symbol or outside some brackets. And valence electrons are dotted
around elements’ symbols paired up where possible. Now, before we go on to any
examples, we need to look at two more things. And that’s the structures that ions
form and the properties that arise because of those structures.
Unlike in covalent bonding, ions
attract each other in all directions, and a single ion of one charge can attract
multiple ions of the opposite charge. When those ions arrive, then they
start attracting positive ions again and the layers build up. And before long, you’ve got a
crystal. This regular structure is what we
call a lattice. But of course, this drawing is only
two-dimensional, and atoms and ions are three-dimensional. This kind of three-dimensional
structure is what we mean when we draw these two-dimensional diagrams. These lattices can conceivably go
on for almost forever.
So, unlike covalent compounds,
ionic compounds do not have a molecular formula. Instead, we find the repeat unit
that describes the whole lattice. And the unit formula for the ionic
compound is the same as its empirical formula, the simplest ratio of the elements in
the compound.
Lastly, we come to the properties
of ionic compounds. There’s a huge variety of ionic
compounds from simple ones, like sodium chloride, to more complicated ones, like
ammonium nitrate. But generally speaking, they’re
solids at room temperature and have high melting points due to the strong ionic
bonding. And many of them dissolve in
water. Water, being polar, has slightly
positive and slightly negative bits, which helps stabilize ions in solution. Now, let’s have some practice.
Which of these statements explains
why atoms of neon do not form ionic bonds? (A) Atoms of neon have delocalized
electrons. (B) Atoms of neon must gain or lose
four electrons to form a stable octet in their outer shell. (C) Atoms of neon already have a
stable octet in their outer shell. (D) Atoms of neon form dense
metallic structures instead. Or (E) Atoms of neon share
electrons to form covalent structures.
Neon is an element to be found in
group 18 otherwise known as group eight of the periodic table. The atomic number of neon is
10. This means that an atom of neon
will contain 10 protons in its nucleus. And since atoms are by definition
neutral, we’ll need 10 electrons as well. So our atoms of neon consist of a
nucleus with a 10-plus charge surrounded by 10 electrons in an electron cloud. Ionic bonds are electrostatic
attractions between positively and negatively charged ions. Atoms can lose or gain electrons to
form positive or negative ions.
We need to look at the five
statements and find the one that accounts for the fact that atoms of neon do not
form ionic bonds. The first thing it will be helpful
to remember is that neon is one of the noble gases, which are well-known for not
forming any bonds at all, with some exceptions. However, generally speaking, neon
goes around as single atoms. Statement (A) suggests that atoms
of neon have delocalized electrons. When considering bonding, we
generally think about metallic bonding when we consider delocalized electrons. But neon, being a monatomic gas,
doesn’t have delocalized electrons, so we can dismiss this answer.
The next statement says that atoms
of neon must gain or lose four electrons to form a stable octet in their outer
shell. The outer shell is sometimes also
known as the valence shell. The word octet in this statement
should hint towards the octet rule, which tells us that atoms tend to react to
acquire eight electrons in their outer shell. To continue with this, we need to
put the electrons in our neon atom into their shells. The first electron shell can fit a
maximum of two electrons, leaving us with eight. And the second electron shell can
fit all those eight electrons and no more. So we’ve accounted for the
locations of all 10 electrons.
Eight electrons is an octet, and
the second electron shell for a neon atom is the outer shell. So we don’t need to lose or gain
electrons in this shell to form an octet. However, if, for instance, we were
dealing with atoms of silicon with electron configuration 2, 8, 4, we could lose
four electrons to have 2, 8 or gain four electrons to have 2, 8, 8. Statement (C) says that atoms of
neon already have a stable octet in their outer shell. This is true. The octet of electrons in the neon
atom is particularly stable, so we don’t see neon reacting to lose or gain electrons
and form ionic bonds.
However, just in case, let’s have a
look at the other two answers. We’ve already identified that neon
is nonmetallic. It’s a monatomic gas generally, so
the fourth answer is not correct. And atoms of neon won’t form
covalent structures and share electrons because they already have a full outer
shell. So the statement that explains why
atoms of neon do not form ionic bonds is that atoms of neon already have a stable
octet in their outer shell.
To finish off, let’s have a look at
the key points. An ionic bond is the electrostatic
attraction between two oppositely charged ions. A positively charged ions is known
as a cation, and a negatively charged ion is known as an anion. When atoms react, they may gain or
lose electrons to form anions or cations. We can use the octet rule to
predict whether an atom is likely to form an anion or a cation since atoms tend to
react to acquire eight valence electrons.
Ionic structures, structures
produced by ionic bonds, consist of lattices of cations and anions in three
dimensions. And for ionic compounds, the unit
formula is the same as the empirical formula, the simplest ratio of elements in the
compound.
