Question Video: Identifying the Number of Electrons in the Outermost Electron Shell of an Atom

How many electrons are in the outermost electron shell of fluorine?

07:25

Video Transcript

How many electrons are in the outermost electron shell of fluorine?

Electrons are the negatively charged particles that surround the nucleus of an atom or ion. They’re sometimes abbreviated as e−. Electrons are arranged in layers, referred to as shells or energy levels. In the picture here, we can see the arrangement of electrons of an atom of lithium with two electrons in its inner shell and one more in its outer shell. Each electron shell has a certain number of electrons that it can hold before it fills up. Any additional electrons will be added to the next shell. For example, the innermost shell can hold only two electrons. So for lithium, the third electron rests in the second shell.

Since electrons tend to settle in the space with the lowest possible energy, the shells will fill from the inside out. We can display this arrangement of electrons in a picture as shown or to save time, we can also write it as a sort of list, called an electron configuration. The electron configuration for an atom of lithium is written as 2,1, indicating that there are two electrons in the first shell and one more electron in the second. As a blueprint for solving this question, we can first determine the number of electrons in an atom of fluorine. Then, by referencing the number of possible electrons in each shell, we can determine how the electrons of fluorine are spread out across different shells and find the number of electrons in the outermost shell.

First, how many total electrons are there in an atom of fluorine? We can answer this question by looking at the periodic table. If we find fluorine in the column second from the right, we can see that its atomic number is nine. This means that there are nine protons in the nucleus of a fluorine atom. The question doesn’t refer to an ion or indicate that there is a charge, so we can assume that we’re dealing with the element fluorine as a neutral atom. Since there is no charge, the number of protons and electrons will be the same, nine of each. There are nine positively charged protons because the atomic number of fluorine is nine. So there must be nine negatively charged electrons to balance out the overall charge of the atom.

For the next step of the problem, we need to ask ourselves, how are these nine electrons arranged? To answer this question, we need to know how many electrons each electron shell can hold. The first electron shell can hold two electrons, the second electron shell can hold eight electrons, and the third electron shell can hold eight more electrons. Beyond the third shell, electrons arrange themselves a bit differently. So while the shells increase in size from there, for our purposes, we can focus on the first three shells. Knowing that the electron shells fill from the lowest energy level to the highest or, in other words, from the inside to the outside, how will the nine electrons of a fluorine atom arrange themselves?

Well, the first two electrons go in the first shell, filling it completely, seven more electrons to nearly fill out the second shell, and no more electrons remaining for any shell after that. We can write the electron configuration for fluorine as 2,7, indicating that there are two electrons in the innermost shell and seven electrons in the next shell out. We can ignore any shells with zero electrons. Based on this arrangement, we can answer the question and say that there are seven electrons in the outermost electron shell of fluorine. We can also use the patterns of the periodic table to our advantage to answer this question another way.

Note that the number of elements in each row of the periodic table, also known as a period, corresponds to the number of electrons in a full electron shell. There are two elements in the first row, eight elements in the second row, and eight more elements in the third row. As we move to the right on the periodic table, starting from any element, we will see that the atomic number increases by one. That also means that the number of protons increases by one and that the number of electrons in a neutral atom of that element will increase by one as there must be an additional negative charge to balance out the positive charge of the added proton.

If we start with the element lithium, which has one electron in its outer shell, and move over seven spaces to neon, adding an electron each time, we will find that we have filled an electron shell. Since each row of the periodic table represents an electron shell, elements on the left-hand side of the periodic table, like lithium, will have one electron in their outermost electron shell, whereas elements all the way on the right-hand side of the periodic table, like neon, will have full outer electron shells. So we can use the column or group of an element to determine the number of electrons in its outermost shell, sometimes referred to as valence electrons.

Elements from the fourth column of the periodic table, like carbon, have four electrons in their outermost electron shell or four valence electrons. Fluorine is in the seventh column of the periodic table, so it has seven electrons in its outermost shell, matching our answer from earlier. Note that when we include the 10 columns of d-block transition metals, which would appear in between and below beryllium and boron, it increases the number of columns in those rows of the periodic table by 10. This is why we will sometimes see fluorine’s column be referred to as column 17.

However, that does not mean that there are 17 electrons in the outermost electron shell of fluorine. If we consider just the elements in that row of the periodic table, we can see that fluorine is the seventh element over, meaning it has seven electrons in its outermost shell. Other elements in the same column, such as chlorine, also have seven electrons in their outermost shell.

Knowing about the outermost electron shell can tell us a bit about how an element will behave. As fluorine is one electron away from a full electron shell, it will tend to form an F− ion, adding an electron and filling its outermost electron shell. Lithium, with only one electron in its outermost electron shell, will frequently give up its only electron, giving it a full outer shell as well. Gaining or losing an electron will turn the atom into an ion, with a negative or a positive charge. Gaining or losing electrons will also change the electron configuration, as there is now a new number of electrons in the outermost shell. However, as a neutral atom, we can say that fluorine has seven electrons in its outermost electron shell.

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