On descending Group 18 of the periodic table, what happens to the boiling points of the elements in their standard states?
Group 18 of the periodic table can be found to the far right. The names of the elements in Group 18 as we descend are helium, neon, argon, krypton, xenon, radon, and oganesson. A boiling point is the temperature at which a substance transitions from a liquid to a gas. And the standard state of an element is its expected chemical state at one bar of pressure.
Since oganesson is actually a synthetic element only made in tiny amounts, its boiling point has not been measured. So, for the purpose of this question, it will be ignored. For the other elements, the key feature we need to understand is the forces holding the atoms together. The key feature of elements in Group 18 is that they have full valence shells.
For instance, helium has two electrons. Being in the first group, these two electrons fully fill the first electron shell. The same is true for neon, where its 10 electrons fill the first and second electron shells. This means in their standard states all Group 18 elements are very unreactive. Although, as you descend the group, some of the elements do start to pair up with oxygen and fluorine under extreme conditions.
This extremely low reactivity means that in their gas state all the Group 18 elements are monatomic. Being monatomic means there are very weak forces between the atoms. This means that in a liquid state it doesn’t take a great deal of energy to break these weak interatomic forces. So, the boiling points of the elements in Group 18 are generally quite low. Because there are no sources of polarity or ionic behaviour, the only forces that exist between atoms of Group 18 elements is dispersion forces.
Dispersion forces arise from very small fluctuations in the positions of electrons in the atom, leading to one side being slightly negative and one side slightly positive. This causes the electrons in neighbouring atoms to shift as well, causing small dipole-dipole interactions. This is why dispersion forces are sometimes known as instantaneous dipole-induced dipole interactions.
While the relationship is quite complex, the general rule is that dispersion forces are stronger when the atoms have more electrons. We can predict the general trend in boiling points down Group 18 by looking at the number of electrons atoms of each element have. We can work this out by looking at the atomic number of each element. The number of electrons should be equal to the atomic number.
Helium has two electrons, neon 10, argon 18, krypton 36, xenon 54, and radon 86. And just for interest’s sake, oganesson has a 118. On this basis, we can predict that the boiling point of Group 18 elements as you descend the group, increase, So, the answer to the question what happens to the boiling points of Group 18 elements as you descend the group is that they increase.
An alternative way of coming to this answer would be to look up the boiling points of each element. Helium has a boiling point of near absolute zero at minus 269 degrees Celsius. As we descend the group, the boiling points gradually increase, from minus 246 degrees Celsius for neon to minus 62 degrees Celsius for radon.
You may find it interesting to know that even though oganesson has not been made in large enough quantities to allow its boiling point to be measured, it has been predicted to be about 77 degrees Celsius. So, oganesson would be a liquid at room temperature, the only Group 18 element to do so. Whichever way you went about it, the answer to our question is that the boiling points of the Group 18 elements as you descend the group increase.