Metal and nonmetal elements form ionic bonds when they combine to form compounds. The ionic bond involves a transfer of electrons. The metal atom loses one or more electrons to form a positive ion (cation). The nonmetal atom gains one or more electrons to form a negative ion (anion). The attraction between these two oppositely charged ions is known as ionic bond.
 

There are two questions in this Question Group. Each question is very similar to one another. They are also very interactive and difficult to capture its essence outside of the actual Concept Builder. The question below provides a sense of what you could expect.
 

Version 1:
Construct the electron shell diagram for an atom of potassium (19K). Then identify the number of valence electrons in the potassium atom. Then complete the following sentence:
 
For potassium to become an ion, it will _____________ (gain or lose) ________ (a number) electrons and become a ___________ (ion formula) ion.
 

All About Electrons, Shells, and Valence Electrons

We understand electrons to be located in regions of space surrounding the nucleus that we refer to as electron shells. There are a series of concentric electron shells surrounding an atom's nucleus, each of varying size. The shells themselves contain orbitals. You can think of orbitals as electron homes. The electrons are positioned inside of these orbitals which are located inside of the shells. The orbitals can have varying shapes and orientations in space. For instance, the s orbitals are spherically shaped. The p orbitals are shaped like a cylinder that has been pinched at its midpoint. 

That's all background. Here's what you need to know to nail this question. These electron shells hold electrons. But since each shell contains a different number of orbitals, the electron-holding capacity  is different for different electron shells.  Each successively larger shell can hold more electrons. So the innermost shell has a single s orbital and can hold at most 2 electrons. The next biggest shell has a single s orbital and three p orbitals and can hold at most 8 electrons. The third shell, and those that are larger, can hold more electrons but only 8 of those electrons will be housed inside of s- and p-type orbitals.

This is all important because the electrons that are involved in bonding atoms together are usually the electrons located in the s- and p-type orbitals. In fact, we refer to these bonding electrons as valence shell electrons. Valence shell electrons are the s- and p-electrons that are located in the outermost occupied electron shell of an atom. All the other electrons are considered inner shell electrons. But the valence electrons are the outer shell electrons ... located in s and p orbitals. There can be at most 8 of these electrons in any given shell of an atom.

 

Electron Shell Diagrams

An electron shell diagram is a visual means of representing the electrons present in the inner shells and outer shell of an atom of an element. Such a diagram represents the shells of electrons surrounding the nucleus as concentric circles. Drawing an electron shell diagram involves showing how electrons occupy those shells. Use the description above to do so. Determine the number of electrons first.  For neutral atoms, the number of electrons will be equal to the atomic number for that element. Place those electrons in shells until you have used up all the electrons. Begin with the innermost shell - it only gets 2 electrons as discussed in the previous section. The next 8 electrons (electron #3-#10) will be placed in the second electron shell. You will note there are 8 places in the diagram to place those 8 electrons. The next 8 electrons (electron #11-18) go in the third shell; again there are 8 places to put these. If there are still more electrons to place in shells, then start placing them in the fourth electron shell.

This method works really well for the first 20 element. After electron #20, it gets a bit more complicated but that's a story for another day ... and for another Concept Builder. 

 

Noble Gas Stability

Now you might be wondering - what's the big deal with these diagrams? That's a good question and you're going to find out right now. When you consider all the elements in the Periodic Table, there's an entire group that is non-reactive and seldom forms bonds with other elements. Do you know what that group is? It's the noble gas family in the 18th group (column) of the table. You might think of these as elements that are very content being by themselves. That is, they don't typically form compounds. And that is because they are stable. In terms of electrons, they are satisfied.

It seems that this noble gas satisfaction is envied by all the other elements on the Periodic Table. Chemists have observed that when elements form compounds by bond formation, they do so in a way that allows the atoms to have a similar configuration of electrons as that of the Noble Gas elements. So what do the Noble Gas elements have that makes them so enviable? The answer:  8 valence shell electrons! Noble gas elements have a full octet of outer shell electrons. That's what every element hopes for - a full octet of valence shell electrons.

So how does an element like fluorine or oxygen become as happy as a Noble Gas with a full octet of electrons? Once you draw the electron shell diagram, you can easily see the answer in the diagram. (And that's why they're so useful.)  For instance, you will notice that there are 7 electrons in the outermost occupied electron shell of fluorine. And so fluorine will be happy if it can gain 1 more electron to have a full octet of outer shell electrons. If it does, then it becomes a 1- ion: the F- ion. Similar reasoning can be applied to oxygen. The electron shell diagram shows 6 valence shell electrons. And so oxygen will be happy if it can gains 2 more electrons to have a full octet of outer shell electrons. If it does, then it becomes a 2- ion: the O2- ion.

So non-metals such as fluorine and oxygen gain electrons in order to achieve a full octet of valence shell electrons. But what do metals such as sodium and magnesium do? An electron shell diagram will help you to understand the answer. A sodium atom has a total of 11 electrons - 2 in the innermost shell, 8 in the second shell, and one lone electron in its valence shell. To fill its valence shell, it would need to acquire 7 electrons - a difficult task for a metal like sodium that is a known electron-loser. So unlike fluorine and oxygen, sodium acquires a full octet by losing electrons and emptying out its outermost shell. So if sodium loses 1 electron, then it's third shell is empty and its valence shell becomes the second shell. That shell already has a full octet and so sodium loses 1 electron and becomes the Na+ ion with a full second electron shell.  Pretty creative, huh?

Here's very similar reasoning for magnesium. Magnesium has a total of 12 electrons - 2 in the innermost shell, 8 in the second shell, and two electrons in its valence shell (third shell). Magnesium acquires a full octet by losing 2 electrons and emptying out its outermost shell. So if magnesium loses 2 electrons, then it's third shell is empty and its valence shell becomes the second shell. That shell already has a full octet and so magnesium loses 2 electrons and becomes the Mg2+ ion with a full second electron shell.

When it comes to ion formation and ion charges, it is helpful to think of atoms as desiring to obtain the same number of electrons as the nearest noble gas. Chlorine has 17 electrons; it would like to gain 1 electron to become like Argon with its 18 electrons. Sulfur has 16 electrons; it would like to gain 2 electrons to become like Argon with its 18 electrons. Potassium has 19 electrons; so it loses 1 electron to become like Argon ... with 18 electrons. And since Calcium has 20 electrons, it would like to lose 2 electrons to become like Argon with its 18 electrons. When ionic bonding occurs, a metal (an electron loser) transfers one or more electrons to a nonmetal (an electron gainer) in order for both to be as happy as a Noble gas.

 

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