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.