Lesson 6: Reactivity Trends (Lab Work)
Now let's consider something that is more directly observable. Let's look at the chemical reactivity of some of the elements and relate that reactivity to some of the properties we have just talked about.
Reactivity means just what it says. A highly reactive element reacts very easily, maybe even violently, with lots of other elements or compounds.
In your lab work for this lesson, you will make observations comparing the reactivities of some of the elements by observing the reactions of a few. The sections on this page pertain directly to your lab work for this lesson. If your schedule permits, it would be best to do this portion of the lesson after doing the lab work. The "Post-Lab Discussions" of parts I and II of the lab work can be done on-line using the computers in the lab immediately after completing your observations in each part.
Pre-lab Comments | Post-Lab Discussion: Metal Reactivity
Post-Lab Discussion: Nonmetal Reactivity
Pre-Lab Comments
You will compare the reactivities of the metallic elements Na, K, Mg, and Ca to each other when they are added to water. You will need to get the instructor to help you because some of these elements react quite violently with water and require special precautions. So take the time now to observe the reaction of these four elements with water by doing the first part of exercise 13 in your workbook. Leave the questions until after you perform the second part of the experiment and read the Post-Lab Discussion section.
Next, you should do part 2 of this experiment which deals with the reactivity of nonmetals. You will compare bromine with iodine by observing how each reacts with the chemical 1-octene. There aren't as many of the nonmetals and it is also more difficult to come up with good simple reactions for you to work with to make these comparisons. Consequently, we will have to deal with fewer observations and more inferences. So take some time now to do part 2 of this experiment.
Post-Lab Discussion: Metal Reactivity
Let's take a look at your observations for the reactions of these four metals and take a look at what kinds of trends you were able to observe and see what kinds of projections and explanations we can come up with.
Let's take a look at how those elements compare with one another on the periodic table. Sodium and magnesium are both in period 3. In period 4 we have potassium and calcium. In group IA are sodium and potassium and in group IIA are magnesium and calcium. We have 2 elements in each of 2 groups and also 2 periods, so we can make some comparisons. |
Within period 3, what happened? Which was more reactive? Was it sodium or magnesium? It was sodium. The more reactive metal was on the left and the least reactive was on the right. In period 4, what about the comparison of potassium with calcium? Well, again the most reactive was on the left and the reactivity got less as you went across. The vertical comparison in both group IA and group IIA is that the reactivity increased as you went down the group. So the reactivity of metals decreases as you go from left to right and it increases as you go down on the periodic table.
Relating Reactivity of Metals to Atomic Structure
Let’s consider how metals react. Metals react by losing electrons. They have a low ionization energy so it's fairly easy for them to lose electrons.
As you go across a period, the nuclear charge will increase; the number of energy levels will stay the same, so there is a stronger and stronger attraction for the electrons. The electrons are being held more tightly as you go across a period. It becomes more and more difficult to lose electrons and consequently the reactivity of the metals decreases as you go from left to right across the periodic table. As you go down the periodic table, the nuclear charge increases but so does the number of shielding electrons. Consequently the dominant factor is that we have more and more energy levels and the electrons are further and further away from the nucleus. Thus it is easier for those electrons to come off. |
Those are the reasons for the pattern of reactivity that is seen for the metals.
Does this trend work for elements beyond the ones we have just looked at? You can check this out by taking a look at the reactions in the Periodic Table video available for viewing in the lab.
Post-Lab Discussion: Nonmetal Reactivity
You should have noticed in this case that bromine reacted more readily than iodine. Notice that this is the opposite of what we found with the metals. With the metals, the element that was further down on the periodic table was more reactive. But with nonmetals over on the right side of the periodic table, the element which is further up is most reactive. This points out that there is something fundamentally different about the nonmetals compared to the metals. There is a fundamental difference between the way that metals and nonmetals react.
Relating Reactivity of Nonmetals to Atomic Structure
Nonmetals usually react by gaining electrons, rather than by losing electrons like the metals do. Let’s review how atomic structure affects the ability to gain electrons. From your observations in the lab you know that as you go down a nonmetallic group in the periodic table, the elements become less reactive. You also know that as you go down a group on the periodic table, the number of energy levels is the most predominant factor. If an electron comes into an atom that has a large number of energy levels, it will be further away from the nucleus and not be attracted as strongly as it would be in a smaller atom with fewer energy levels. For example, iodine is attracting an electron into its fifth energy level. Bromine is attracting an electron into its fourth energy level. Bromine does a better job of attracting electrons, and thus is more reactive, because it allows the new electron to get closer to the nucleus where the force of attraction is stronger. Following this line of reasoning and extending it to other atoms, we would expect chlorine to be even more reactive and fluorine to be even more reactive still.
The reactivity of the nonmetals ties in well with the concept of electron affinity and the tendency to gain electrons. With nonmetals the greater the tendency to gain electrons, the more reactive it is. This argument should hold true whether we are talking about nonmetals within a family or within a period. As you go across a period, there is a greater nuclear charge and thus the electrons should be attracted more readily by elements that are further to the right and the tendency to gain electrons will increase. Thus the reactivity of the nonmetals should increase as you go from left to right across the periodic table, up to but not including the inert gases.