It can be pretty easy for students to draw arrows for periodic trends. But, can they really explain why the trends change across and down the periodic table? This is a wonderful class activity that really reinforces periodic trends, and will have your students describing trends in depth.

Timing: One Class Period

Prior Knowledge: Students should be familiar with reading the periodic table, and have some familiarity with the periodic trends.
Materials:

  • Lab instruction handout with reference tables
  • Student lab report handout

This activity is pretty simple, but it packs a punch for reinforcement of periodic trends. Students will use group I elements and period 3 elements to track the trends atomic radius, ionization energy, and electronegativity. The graphs created will allow students to visualize trends which they will use to verbalize and explain the trends.

Instructions:

1. Have students complete the table on their handouts for group 1 elements & period 3 elements. Tip: If a student doesn’t have access to colored pencils, you can suggest using a pencil and a pen.

2. Have students meet in their groups and discuss data. You will want to have six groups – one per periodic trend. You can do counting heads 1-6, 1-6, etc. Then all of the ones meet, all of the twos meet, etc. 

3. Assign each group a trend and group or period to graph. For example, one group will graph on their whiteboard atomic radius across period 3. Another group will graph electronegativity down group 1, and so on. Be sure to remind them that you are looking for all of the characteristics of a well-designed graph. – title, titled x and y axis with units, appropriate units.

4. One member of each group will present the group’s graph to the class. Before they present, I tell the class that they will not only present the graphing results, but they will explain why and what causes this trend. Give the group a little time to discuss and make notes.

5. See the discussion below for things that we are looking for as explanations during presentations. Ask questions during presentations to lead them to these conclusions.

6. If you have time, have everyone do a gallery walk as a group to look at other group graphs a little closer.

7. On the back of their lab reports, have students write a conclusion on each trend including an explanation of why the trend occurs the way that it does.

Discussion:

So why does the atomic radius decrease going across the period, and how does this affect other periodic trends? Things that we want students to be able to discuss and verbalize during presentations.

Start with atomic radius. It is easier to explain periodic trends if we first have students understand atomic radius, and then relate other trends to atomic radius. This helps my students so much better than just memorizing the trend and how to draw the arrows.

As we move across a period, elements will have the same number of energy levels. So, what changes? The number of protons. As the number of protons increases by one, and the number of energy levels holding the negative electrons does not increase, the positive nuclear force has a stronger hold on the electrons – pulling them in closer. I don’t go into “effective nuclear charge” or “shielding” in this lesson – just looking at the basics.

As we move down a group, elements will have the same number of valence electrons but will be gaining an energy level with each step down the group. An increase in energy levels causes the positively charged protons to be further from the outer electrons. The atomic radius increases going down the group. I always tell my students – “Well, duh, this makes sense! – easy.”

Relating first ionization rate and electronegativity to atomic radius.

Since atomic radius decreases across the period, pulling the electrons closer to the nucleus, the electrons are harder to remove – takes more energy. Ionization energy will then increase going across the period, and less energy will be needed to remove electrons when going down the group because they are further from the positive protons – decrease going down the group.

Since atomic radius decreases across the period, pulling the electrons closer to the nucleus, the electrons are harder to remove – takes more energy. An atom’s ability to attract electrons, electronegativity, will then decrease going across the period because they are being held tighter by the positive nuclear force. Electronegativity will increase going down a group because the electrons will be easier to attract.

If you have time, this is a great activity to let the students do a gallery walk to see everyone’s graphs and make notes on their lab reports!

Enjoy!

This activity is included in our Periodic Table & Periodic Trends Unit:

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