# Question Video: Determining the Electronic Configuration of an Ion Given Its Charge and Proton Number Chemistry • 10th Grade

A nonmetal ion of an unknown element has the formula X³⁻. If atoms of X have 7 protons, what is the electronic configuration of X³⁻?

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### Video Transcript

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 subshells.

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).