What is the electronic configuration of a calcium ion Ca2+? (A) 1s2 2s2 2p6 3s2 3p6. (B) 1s2 2s2 2p6 3s2 3p6 4s2. (C) 1s2 2s2 2p6 3s2 3p6 4s2 3d2. (D) 1s2 2s2 2p6 3s2 3p4 4s2. (E) 1s2 2s2 2p6 3s2 3p4.
The electronic configuration describes how many electrons an atom or ion has and how they are arranged in the electron shells and subshells. When writing an electronic configuration, each subshell label contains a coefficient which is the principal quantum number 𝑛, which identifies the electron shell; a letter which tells the type of orbitals in the subshell, which can be s, p, d, or f orbitals; and a superscript which tells the number of electrons in the subshell.
The periodic table can be divided into blocks which represent the subshells. Because each element on the table has one more electron than the one before, the number of elements across one subshell block represents the maximum number of electrons that that subshell can hold. All s subshells contain a single s orbital and therefore can have a maximum of two electrons. And we can see that the width of the s block is two elements wide. All p subshells contain three p orbitals within them and therefore can have a maximum of six electrons. And we can see that the width of the p block is six elements wide. d subshells can hold up to 10 electrons, and f subshells can hold up to 14. We will not be concerned with f subshells in this video.
The Aufbau principle states that electrons fill the lowest-energy subshells before they fill the higher-energy ones. The following energy level diagram shows the relative energy values of the first seven subshells. The order of subshells on the periodic table matches the order of subshells in this energy level diagram.
Calcium is the 20th element on the periodic table. Therefore, a calcium atom has 20 electrons. Let’s write the electronic configuration of a calcium atom first. We start by filling the 1s subshell with two electrons and write 1s2. The next lowest energy subshell is 2s. And we fill it with two electrons and write 2s2. Then, we fill the 2p subshell with six electrons and write 2p6. Now, we can move on to the 3s and 3p subshells and write 3s2 and 3p6. Finally, we can fill the 4s subshell with two electrons and write 4s2.
So this electronic configuration, which contains 20 electrons, is the electronic configuration of a neutral calcium atom. However, to answer the question, we need to write an electronic configuration for the calcium ion, which has a charge of two plus. A charge of two plus indicates that the calcium atom lost two electrons when forming the calcium ion. Therefore, the calcium ion has 18 electrons. These two electrons are lost from the valence electron shell, which in calcium is the fourth electron shell. Therefore, the electronic configuration of the calcium ion is 1s2 2s2 2p6 3s2 3p6, or answer choice (A).