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The Diagonal Relationship, Metallic Character, and Boiling Point

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  • 0:05 Other Periodic Relationships
  • 0:36 Diagonal Relationship
  • 1:18 Metallic Character
  • 3:53 Boiling Point
  • 4:50 Lesson Summary
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Lesson Transcript
Instructor: Kristin Born

Kristin has an M.S. in Chemistry and has taught many at many levels, including introductory and AP Chemistry.

What may have once seemed like a bunch of random squares with letters in them is now shaping up to be one very organized chemistry reference. In this lesson you will discover three other trends that are found on the periodic table: the diagonal relationship, trends in metallic character, and trends in boiling point.

Other Periodic Relationships

With 118 different elements, the periodic table can be a bit overwhelming. The key to decoding this potpourri of letters and numbers is understanding how it is organized. Earlier, I deconstructed the periodic table by explaining several of the major relationships and trends among the elements and their positions on the table. This last segment will touch on a few more of these trends: the diagonal relationships and trends in the metallic character and boiling point.

The Diagonal Relationship

First, we are going to discuss the diagonal relationship.

Remember those periodic trends - atomic radius, ionization energy, and electronegativity? Usually, the trend moving down on the periodic table tended to be the exact opposite of the trend moving across. For example, the atomic radius tended to increase as you moved down a group and tended to decrease as you moved across a period. This opposition lends itself to a few diagonal relationships. For example, lithium and magnesium, beryllium and aluminum, and a few others will have similar atomic radii, similar ionization energies, and many other similar physical and chemical properties.

Electrons in metals get excited easier, causing the energy to return as visible light or heat.
Metal Electrons Get Excited Easier

Metallic Character

Next we will examine the metallic character of different elements. When you hear the word metal, a few words or images should come to mind: shiny, silvery, good conductor of heat and electricity, and able to bend without cracking. What you may not know about metals is why they have all of these properties. Which subatomic particle do you think is responsible for the characteristics of metals? If you guessed electrons, then you are correct. Electrons can take most of the credit for the action in chemistry.

So what about these electrons is causing metals to be so shiny and conductive? The key here is how attached those electrons are to the nucleus. The weaker the attraction, the more delocalized they are, meaning they don't feel a strong attraction to the nucleus and they're free to wander about. So if I have a hunk of copper, those nuclei may have a strong attraction to the inner electrons, but the outer ones - the valence electrons - are loosely held. What this means is they can move about throughout the hunk of copper as much as they please with very little resistance. This ability of electrons to move about is the reason why metals are so conductive of heat and electricity. After all, electricity is pretty much just moving electrons.

Why, then, are metals shiny? Remember how light is produced? Energy in, electrons get excited, electrons 'fall' back down, and energy leaves. Well, because metals don't have a very strong 'hold' on their electrons, those electrons can get excited a lot easier. Even if I were to shine a dim light on a piece of aluminum, it has enough energy to excite the loosely bound electrons, and then they return that energy back. (Keep in mind, not all of it's returned in the form of visible light; some of it's converted to heat.)

Elements become less metallic as you move to the right on the periodic table.
Metallic Elements Periodic Table

Now that we have an understanding of what metallic character is and why it exists, we can make some sense of the trend in metallic properties of the elements. Elements that have a weaker hold on their electrons are going to have more metallic character. So metallic character decreases from left to right across a period and increases as you move down a group. The elements on the right - the nonmetals - have such a strong attraction to their electrons because of the greater numbers of protons in the nucleus pulling in the electrons that they are very dull and very poor conductors of heat and electricity. This is why they are often used as insulators.

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