Video Transcript
The noble gases are a group of
unreactive, colorless, odorless gases with a variety of practical applications. In this video, we will learn how to
compare the physical properties and uses of noble gases and explain why they’re so
unreactive in terms of electron shell filling.
The noble gases are a family or a
group of elements on the periodic table. They’re found in the rightmost
column, known as group 18 or sometimes group eight, because depending on which row
you’re looking at in the periodic table, it is the 18th or eighth column over. This group is also sometimes
referred to as group zero because atoms of these elements need to gain or lose zero
electrons to fill their outer electron shell. The noble gases include helium,
neon, argon, krypton, xenon, radon, and oganesson. The last of these, oganesson, is a
synthetic element that has only been produced in laboratories in small amounts. So, for some questions or
comparisons, we might ignore it for simplicity sake.
The most notable quality of noble
gases is that they are inert, meaning they do not react with other atoms or
molecules. In fact, this inertness is where
they get their name. It’s often considered a noble
quality to remain calm and unreactive in the face of provocation. So these unreactive gases earned
the name noble gases. It’s worth noting that there are
some compounds that can be formed between noble gases and other elements at extreme
conditions. But these compounds are
exceptions. For the most part, noble gases will
not react, which begs the question, why are these gases so unreactive? Well, to answer that question, we
can look at their electron shells.
Negatively charged electrons
surround the nucleus of an atom in layers called electron shells. The first innermost shell can hold
two electrons. The second electron shell can hold
eight electrons. The next two shells work a little
differently than the first two. But as a simplification, we say
that the third electron shell can hold eight electrons, and the fourth can hold
18. Interestingly enough, there is a
connection between the number of electrons in a shell and the number of elements in
a row of the periodic table although we usually only use this pattern up to element
20. We should note that beyond the
third and fourth period of the periodic table, more electrons can be added to the
third and fourth electron shells. The third electron shell can
eventually hold 18 electrons, and the fourth electron shell can eventually hold 32
electrons. Again, the values indicated here of
eight and 18 are simplifications.
The first electron shell holds two
electrons. The two electrons found in a helium
atom, the second element over in the row, will fully fill the first shell. The second electron shell holds
eight electrons. The third through 10th electrons
found in neon, the eighth element over in the second row, will fully fill the second
shell. Following this pattern, we can see
that argon’s 18 electrons fill the first three energy shells. While we tend to use this model of
electron shells only up to calcium, element 20, as a simplification, we can say that
all noble gases have full outer electron shells. In fact, the column or group of an
element will tell you how many electrons are in its atoms’ outermost electron
shell.
Atoms of group one elements like
lithium, known as the alkaline metals, have one electron in their outer shell. Atoms of group two elements like
beryllium, known as the alkaline earth metals, have two electrons in their outer
shell. And atoms of the halogens, the
elements in the group second from the right, have an outer electron shell that is
one electron away from being full, in the case of fluorine, seven electrons. Up to element 20, calcium, this
pattern will hold. The number of elements over in the
row is the number of electrons in the outermost shell of an atom of that
element.
We’ve learned a lot about electron
shells, but we still don’t know why a full outer electron shell would make a gas
unreactive. Let’s look at an example to answer
this question. Magnesium is a silver-colored
metal, often purchased in the form of a flat, flexible wire. If we hold magnesium metal in a
flame, it will release an intense white light. And the burned portion of the wire
will become a gray, crumbled, ash-like substance. That new substance is magnesium
oxide. The magnesium metal has combined
with the oxygen from the air in a one-to-one ratio by atom to make a new
molecule.
Magnesium in group two has two
electrons in its outer shell. Oxygen in group six has six
electrons in its outer shell, two electrons away from having a full outer shell. When magnesium and oxygen combine
to form magnesium oxide, magnesium donates two electrons to oxygen, emptying its
outer shell, resulting in a smaller, now full outermost shell and filling oxygen’s
outermost shell as well. The two ions that result, Mg2+ and
O2−, are then bound by an electrostatic attraction, forming a compound with the
chemical formula MgO. Atoms tend to form ions and
compounds that result in full outermost electron shells, an observation called the
octet rule, referring to the octet of eight electrons that fill the second or third
electron shells.
So by knowing about the contents of
the outermost electron shells of atoms, we can predict what reactions will happen
and what compounds will be formed. And again, it’s worth noting that
beyond the element calcium on the periodic table, electron shells work a little
differently. So, the octet rule is not as
useful. Back to our noble gases, knowing
that the noble gases already have full outer electron shells, it makes sense that
they would be unreactive. Other elements react to gain more
energetically favorable arrangements of their electrons by filling outer shells. But the noble gases already have
the most favorable arrangement. In fact, about one percent of the
atmosphere surrounding our magnesium oxide reaction is the noble gas argon sitting
idly by, not involved in the reaction. Other noble gases are present in
the atmosphere in much smaller amounts as well.
As an analogy, we can think of
students with trading cards, where a full set has eight cards. Students with one or two cards
might be convinced to trade away their cards, while students with six or seven cards
will try hard to get the last cards in the set. Students who already have a full
set of eight trading cards will be extremely unlikely to make any trades that would
break up their complete set.
Another thing to know about the
structure of noble gases is that they are monoatomic gases. This means that samples of noble
gases will be made up of units of one atom of the element. Another type of gas: diatomic gases
have molecules of two atoms bonded together. Noble gases are monoatomic gases,
as are the gaseous forms of many other elements. The diatomic gases include hydrogen
gas, nitrogen gas, oxygen gas, and the gaseous forms of most halogens. Noble gases also have low boiling
points. As monoatomic gases, they have
relatively weak intermolecular forces compared to diatomic gases, ions, and polar
compounds.
The main attractive forces are
called London dispersion forces. These forces occur between nonpolar
atoms or atoms with an even enough distribution of electrons to prevent a partial
charge from forming. However, the atoms can develop
short-lived partial charges, notated here as a lowercase 𝛿+ and lowercase 𝛿−, when
electrons group together. These electron groupings can emerge
spontaneously or because of the electron grouping in a nearby atom. The presence of these nearby
opposite partial charges causes a brief electrostatic attraction. However, that attraction is much
weaker than the attraction between particles with stronger charges or preexisting
partial charges such as ions or atoms of a polar substance. Because this process describes
particles with short-lived charges or poles, we also call these forces instantaneous
dipole–dipole interactions.
Dipole is a word for a particle
with partial charges. The weak intermolecular forces mean
that noble gases have very low boiling points. The weaker the attractive force
between particles, the easier it is for the particles to separate into the gaseous
state when heated and the lower the boiling point. All noble gases have low boiling
points, from helium’s boiling point a few degrees above absolute zero to radon’s
boiling point at negative 62 degrees Celsius. One pattern to note is that the
boiling point within the group increases as we go down the periodic table. Why is this?
A moment ago, we talked about how
the intermolecular forces in a noble gas are due to the grouping of electrons to
form temporary partial charges. Well, as we move down the periodic
table and increase the number of electrons in the atom, we will increase the number
of electrons in these spontaneous groupings, increasing the intensity of the
temporary partial charge as well as the strength of the resulting electrostatic
attraction. If the attractive forces are
stronger, the particles are more tightly held in place, and it will be harder for
them to evaporate into gases raising the boiling point. The boiling point increases down
the group because more electrons leads to stronger attractions, which creates a
higher boiling point.
The characteristics of noble gases
make them very useful for real-world applications. Argon is used in light bulbs and
lamps. The nonreactive noble gas prevents
the filaments inside the bulb from oxidizing or corroding, which would change the
chemical makeup of the inside of the bulb and potentially lead to a hazardous
fire. Helium, whose boiling point is
negative 269 degrees Celsius near absolute zero, is used in low-temperature
cryogenics as it remains a gas at extremely low temperatures. The so-called neon signs you see on
storefronts are electrified tubes of low-pressure noble gases. The color of the light depends on
the noble gas used, so it’s only truly a neon light if it emits a reddish orange
light. The most common use is the simple
helium balloon.
Since helium has such a low
relative atomic mass and ideal gas particles take up the same volume of space
regardless of mass, helium as a gas has an extremely low density, allowing balloons
of helium to float in the air. You could get an even more buoyant
balloon if you filled it with the even lighter-weight and lower-density hydrogen
gas. But the more reactive hydrogen gas
could ignite and explode. For a birthday party, it seems like
it would be best to go with helium. Unfortunately, the Earth’s supply
of helium is dwindling, and it has a variety of important uses, namely, as a coolant
in medical imaging devices and supercomputers. So, perhaps, buoyancy isn’t the
most important thing helium has to offer.
Now that we’ve learned about the
atomic structure, characteristics, and uses of noble gases, let’s do some practice
to apply what we’ve learned.
Which of the following is the
electron configuration of a noble gas? (A) 1; (B) 2,8,2; (C) 2,4; (D) 2,6;
or (E) 2,8.
An electron configuration lists the
number of electrons in each electron shell of an atom. For example, choice (E) 2,8
indicates that there are two electrons in the first shell and eight electrons in the
second shell. To answer this question, we need to
know that the first electron shell holds two electrons. The second electron shell holds
eight electrons. And as a simplification, we say
that the third electron shell can hold eight electrons, and the fourth electron
shell can hold 18 electrons. The other important piece of
information to remember is that noble gases have full outer electron shells.
So, which of the choices here has a
full outer electron shell? Choice (A) only has one out of the
two electrons needed to fill the first shell. Since electron shells fill from the
inside out, the last number represents the outer electron shell. Choice (B) only has two out of the
eight electrons needed to fill the third electron shell. Of the three remaining choices,
which one has the correct number of electrons to fill the second electron shell? The correct answer is choice (E)
2,8 as there are eight electrons needed to fill the second electron shell.
Another way of solving this problem
involves identifying the element that each electron configuration corresponds to and
identifying the one that is a noble gas. The element whose atoms have just
one electron, element number one, is hydrogen. The element whose atoms have 12
electrons is magnesium. Element number six, whose atoms
have six electrons, is carbon. Element number eight is oxygen. Finally, the element whose atoms
have 10 electrons is neon. Neon is the only noble gas among
the five elements here, so it is the correct answer. Choice (E) gives us the correct
electron configuration for neon.
For questions like this, we should
verify that the electron configuration with 10 electrons is indeed the electron
configuration for neon and not an incorrect electron configuration that happens to
have 10 electrons. Neon’s 10 electrons completely fill
the first and the second electron shells. Since atoms tend to form ions and
compounds that result in a full outer electron shell, and the noble gases already
have full outer electron shells, the result is that noble gases are very
unreactive. So, which of the following is the
electron configuration of a noble gas? That’s choice (E) 2,8.
Now that we’ve done a little
practice, let’s review the key points of the video. The noble gases can be found in the
rightmost column of the periodic table, known as group 18 or sometimes group eight
or group zero. The noble gases have full outer
electron shells, which makes them very unreactive as a full outer electron shell is
already the most energetically favorable arrangement of electrons. The noble gases are monoatomic
gases, meaning they’re made up of units of a single atom.
Also, the attractive forces between
the atoms of a noble gas are relatively weak when compared to the forces in diatomic
gases or in substances with charges or partial charges. As a consequence of having weak
intermolecular forces, the boiling points of noble gases are quite low as the
particles are more able to evaporate into the gaseous state. Other notable characteristics of
noble gases include the fact that they are odorless and colorless. And lastly, they have a variety of
uses in the real world, including cryogenics, imaging, lighting, and balloons.
