Which electronic configuration describes how the electrons are arranged in the shells of a sodium ion? (A) 2, 8, 8, 5; (B) 2, 8, 2; (C) 2, 8, 1; (D) 2, 8; or (E) 2, 1.
Electrons are the negatively charged particles that surround the nucleus of an atom or ion. The electronic configuration is a series of numbers that gives the number of electrons in each electron shell of the atom or ion. For example, in choice (E), the electronic configuration 2, 1 signifies that there are two electrons in the first innermost electron shell and one electron in the second electron shell. A key piece of information to know when determining an electronic configuration is the maximum number of electrons of each electron shell.
The first electron shell closest to the nucleus can hold two electrons. The second electron shell can hold eight electrons. And when dealing with electron configurations up to the element calcium, element number 20, we say that the third electron shell can hold eight electrons. Beyond the element calcium, more electrons can be added to the third shell up to 18 total, but we will use a capacity of eight electrons as a simplification for this problem.
Note that sometimes we use e− to signify an electron with its negative charge. It’s also important to know that electron shells will fill outward, meaning, for example, that there need to be two electrons in the first shell before any appear in the second, eight electrons in the second shell before any appear in the third, and so on. If we find the total number of electrons in the atom or ion and fill the shells from inside to outside in this way, we can determine the atom or ion’s electronic configuration.
So, how many electrons are in a sodium ion? Well, if we find sodium in the leftmost column of the periodic table, we see that its atomic number is 11. That means that there are 11 protons in any sodium atom. That also means that there will be 11 electrons in a neutral atom of sodium. Note that the question is not asking about a neutral atom of sodium. It is asking about a sodium ion or a charged particle of sodium. However, we can examine the electronic configuration of the neutral atom to learn more about the electronic configuration of the sodium ion. If we look at the arrangement of electrons in a sodium atom, we will find that two electrons fill the first shell, eight electrons fill the second shell, and there’s one electron left to go in the third shell.
But the question remains. How does the configuration of electrons in a sodium ion differ from the configuration of electrons in a sodium atom? For questions like this that ask about an ion instead of an atom, we need to know that ions tend to form in a way that results in a complete outer shell of electrons. In the case of our sodium atom, the easiest way for it to end up with a full outer shell of electrons is for it to lose its only electron in its third electron shell. This means that sodium ions will have only 10 electrons. We notate the sodium ion as Na+. The plus denotes the positive charge that the ion has. Since it lost an electron with a negative charge, the resulting charge of the ion is positive.
In this question, we lost an electron to form an ion with a single positive charge. Depending on the element in question, arriving at a full outer shell of electrons may involve adding or removing one or more electrons. For example, the formation of an oxygen ion, which in its atomic state has six electrons in its outermost shell, involves the addition of two more electrons, filling the second shell with eight electrons and resulting in an oxygen ion with a charge of two minus, written as O2−. Returning to the question at hand, our sodium ion with one fewer electron than the sodium atom will have an electron arrangement as shown, with two electrons in the first full shell and eight more electrons in the second full shell.
For this sodium ion, the electronic configuration that matches this electron arrangement is answer (D), 2, 8, meaning there are two electrons in the first shell and eight in the second shell. A tempting yet incorrect answer is answer (C), 2, 8, 1. This is the electronic configuration of a neutral sodium atom. However, the question is specifically asking about a sodium ion, which involves one fewer electron and a full outer electron shell. So answer (C) is incorrect.
Answer (B), 2, 8, 2, would be the electronic configuration of the element with 12 electrons, a magnesium atom. The three electrons in answer (E) correspond to the three electrons of a lithium atom. And while it might be reasonable to assume that answer (A), 2, 8, 8, 5, corresponds to element number 23 on the periodic table, vanadium, in actuality, electron configurations beyond the third energy level work a little bit differently than how we learned. We don’t need to learn those exceptions right now, but suffice it to say that 2, 8, 8, 5 is not the electronic configuration of a vanadium atom, and certainly not the electronic configuration of a sodium ion.
As we’ve just determined, the electronic configuration that describes how the electrons are arranged in the shells of a sodium ion is answer (D), 2, 8.