Video Transcript
Which picture shows the arrangement
of electrons in an atom of oxygen?
What we’ve been given are five
electron shell diagrams. In the middle is a drawing of a
nucleus, much, much bigger in relation to the atom than it would be in real
life. The red circles with p are protons,
and the white circles with n are neutrons. The black circles are electron
shells, which can fit a limited number of electrons. And the blue dots are the
electrons.
What the question is asking for is
the picture that shows the arrangement of electrons in an atom of oxygen. Oxygen is an element that we can
find on the periodic table. There, we see that the atomic
number of oxygen is eight. This means that every atom or ion
of oxygen has eight protons in its nucleus. Atoms are neutral, which means we
need an equal number of electrons and protons. The first thing we can do is check
all of our diagrams depict eight protons and eight electrons.
All the nuclei look identical, and
they each have eight protons. This means we’re dealing with
nuclei of oxygen, and we can proceed to the next test. The easiest way to count out
electrons is to work out the electron configuration of each diagram. We do this by counting the
electrons in each shell, starting with the first shell.
In the first diagram, there are two
electrons in the first electron shell, there are two in the second, and there are
four in the third. This is the right number of
electrons, but we’ll come back to this in a moment. The second diagram has
configuration two, six. And the third
diagram has configuration zero, eight. Remember, the zero still matters
because it’s an inner shell. The fourth diagram has
configuration eight. And the last diagram has
configuration four, four.
Each electron shell has a fixed
maximum number of electrons that it can fit. You can fit two electrons in the
first electron shell and up to eight in the second. The other principle we’re going to
use to find the answer is that electrons occupy the most stable space. For electron shell diagrams, that
means the lowest available shell.
So, in the first diagram, we can
see that there are two electrons in the lowest available shell. That’s good. However, there are four electrons
in the third shell when there’s still space in the second. So, this is not the correct
diagram. In the second diagram, there are
again two electrons in the first electron shell. And the remaining six out of the
eight electrons fill the second shell as they should. So, this is the correct
diagram. But let’s look at the other three
just in case.
In the third diagram, there are
eight electrons in the second shell when there are two available spaces in the
first. So, this is not the correct
configuration. For the fourth diagram, all the
electrons in the first electron shell, which is six too many. And in the last diagram, there are
two too many electrons in the first electron shell. Meaning that the picture that shows
the correct arrangement of electrons in an atom of oxygen is the one with eight
protons in the nucleus, two electrons in the first shell, and six in the second.
