A nonmetal ion of an unknown
element has the formula X3−. If atoms of X have seven protons,
what is the electronic configuration of X3−? (A) 1s2 2s2 2p3, (B) 1s2 2s2 2p6,
(C) 1s2 2s2 2p5, (D) 1s2 2s2, (E) 1s2 2s1.
Let’s start by determining the
amount of electrons in the unknown nonmetal ion X3−. The charge of an ion is equal to
the number of protons minus the number of electrons. The charge of ion X is negative
three, and we know that the atoms and ions of X will both have seven protons. Therefore, the number of electrons
equals seven minus negative three. So the X3− ion has 10
electrons. The aufbau principle states that
the electrons in an atom or ion fill the lowest energy subshells before they fill
higher energy subshells. Therefore, the 1s subshell, which
is lowest in energy, will be filled before the higher energy 2s and 2p
It’s important to remember that an
atomic orbital can only contain a maximum of two electrons. This means that any s subshell can
only hold up to two electrons because s subshells have only one orbital. However, any p subshell can hold up
to six electrons. This is because p subshells have
three orbitals, and three times two electrons per orbital is six electrons.
We’ve already determined that the
X3− ion has 10 electrons, so now let’s write its electronic configuration. The 1s subshell will contain two
electrons, so we begin by writing 1s2. The 2s subshell will also contain
two electrons, so we write 2s2. This leaves us with six electrons,
which will completely fill the 2p subshell.
When looking at the answer choices,
we can see that answer choice (B) matches the electronic configuration we wrote. In conclusion, the electronic
configuration of the nonmetal ion X3− is 1s2 2s2 2p6, or answer choice (B).