Video: Noble Gases

In this video, we will learn how to compare the physical properties of noble gases and explain their inertness in terms of electron shell filling.

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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.

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