Question Video: Recalling Electron Configuration of a Noble Gas Chemistry • 7th Grade

Video Transcript

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.