Video Transcript
Which element is represented by Z and forms a Z2+ ion with an electronic configuration of 1s2 2s2 2p6?
An electronic configuration is an efficient way to show where electrons are in an atom or ion in terms of shells or energy levels. We can represent full electronic configurations as it is written in the question for the Z2+ ion by writing subshell terms in order from lowest to highest energy and using superscript labels to show how many electrons there are in each subshell. When it comes to an ion, electrons are either lost or gained depending on the sign of the charge. Since this is a two plus ion, this indicates two electrons were lost from Z, meaning the electron configuration for the Z atom should have two more electrons than the Z2+ ion.
The order of filling subshells follows the aufbau principle, which states that electrons will occupy the lowest energy orbitals available. According to the aufbau diagram that aligns with the aufbau principle, the two additional electrons the Z atom has should reside in the 3s subshell because the 3s subshell is the next lowest energy level after the 2p subshell is filled. That being said, the full electronic configuration for the Z atom is 1s2 2s2 2p6 3s2. Since each superscript indicates the number of electrons in each subshell, adding up all of the superscripts will give the total electron count for the Z atom.
Why is knowing the total electron count important? Using hydrogen as an example, the atomic number above each element on the periodic table is equal to the number of protons in the nucleus of an atom of that element. We also know for a neutral atom, the number of protons is equal to the number of electrons, which means once we know the total number of electrons, we can determine the atomic number and thereby determine the element. The total electron count for Z is 12 electrons, which equates to an atomic number of 12. Using the periodic table, we determine that magnesium has an atomic number of 12.
Therefore, “Which element is represented by Z?” The answer is magnesium.
