# Video: Selecting the Electron Configuration of the Element Most Likely to Be an Atomic Gas

Which of the following is the configuration of the element that exists as single gaseous atoms at 0°C and 1 atm? [A] 1s² [B] 1s² 2s¹ [C] 1s² 2s² 2p¹ [D] 1s² 2s² 2p² [E] 1s² 2s² 2p⁶ 3s² 3p⁵.

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### Video Transcript

Which of the following is the configuration of the element that exists as single gaseous atoms at zero degrees Celsius and one atmosphere? A) 1s², B) 1s² 2s¹, C) 1s² 2s² 2p¹, D) 1s² 2s² 2p², or E) 1s² 2s² 2p⁶ 3s² 3p⁵.

The phrase configuration of the element is shorthand for the electron configuration of an atom of that element in its ground state. What we′re looking for is an electron configuration which corresponds with something we′d expect to be a gas at zero degrees Celsius and one atmosphere. So the boiling point would have to be lower than zero degrees Celsius. And the material would have to be relatively unreactive. Simple atomic gases only form from elements that are unreactive because reactive elements will either pair up or form more complicated structures in their elemental state.

So where in the periodic table do we know there′s a group of elements that are known for being unreactive? The noble gases in group 18 are renowned for not reacting or only reacting under extremely harsh conditions. Helium is at the top of group 18 followed by neon, argon, krypton, xenon, and a couple of others. Atoms of group 18 elements have full valence shells, making them unreactive. Group 18 elements form atomic gases under ordinary conditions and they are the only elements to do so. So our electron configuration has to match the electron configuration for an atom in its ground state of a group 18 element. So let′s have a look at the options.

The electron configuration 1s² tells us there are two electrons in the first electron shell. For this configuration, the first electron shell is also the valence shell. Now, the first electron shell can only accommodate two electrons. So in this case, we have a full valence shell. We can work out which element this configuration corresponds to by working out the total number electrons first, in this case, two electrons in the 1s subshell. This means there must be two protons in the nucleus of an atom with this configuration since atoms are neutral. The number of protons in a nucleus is the same as that nucleus’s atomic number, in this case, atomic number two. The element with atomic number two is helium, which is a group 18 element. So the electron configuration we′re looking for is 1s².

But let′s have a look at the others just in case. For the electron configuration 1s² 2s², assuming it refers to an atom, we have one out of the eight electrons necessary to fill the valence shell. The second electron shell can fit a total of eight electrons. An atom with this electron configuration under these conditions would likely form something other than an atomic gas. So it′s not a correct answer. It is, in fact, the standard configuration for a lithium atom. Now, lithium forms a metallic structure, not an atomic gas at zero degrees Celsius and one atmosphere. So we can be sure that B is not the correct answer.

An atom with configuration 1s² 2s² 2p¹ has three electrons in its valence shell. This two is short of the full eight required in order to form a group 18 electron configuration. An atom with this configuration would be an atom of boron. Boron is a solid under these conditions, not an atomic gas.

An atom with electron configuration 1s² 2s² 2p² only has four out of the eight electrons required to fill the valence shell. An atom with this configuration would be an atom of carbon, which under these conditions would form a solid like graphite or diamond.

An atom with the last electron configuration would have seven electrons in its valence shell. The rule of thumb is that an octet of electrons in a valence shell is most stable. And seven electrons is one short of eight. Now, in the third electron shell, there are 3D orbitals. But these aren′t accessible at normal energy levels. An atom with this configuration would be an atom of chlorine. Under these conditions, chlorine atoms would quickly pair up, forming a Cl₂ molecule.

So of the five electron configurations given, the one that corresponds to an atom of an element in its ground state that would exist as a single gaseous atom at zero degrees Celsius and one atmosphere is 1s².

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