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Lanthanoids & Actinoids: Configuration & Comparison

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

The two rows separated out from the periodic table are called the inter-transitional metals. In this lesson, we will learn about these two series and how they compare to each other.

Periodic Table

If we were to draw the periodic table without the two rows separated below then it would be very wide and difficult to read.


Inter transitional metals appear in two lines at the bottom of the periodic table.
periodic table


This is why when you look at the periodic table, you will find two lines below the rest of the table. These are called the inner-transitional metals. These two lines are separated from the rest of the table because it is easier to draw it this way; if they were in the 'normal' place in the table, then it would be wide and unwieldy.

The top of these two rows is called the lanthanoids, while the bottom row is called the actinoids.

Lanthanoids Properties

Lanthanoids include 14 elements, with atomic numbers 58-71:

  • Cerium: Xe 4f1 5d1 6s2
  • Praseodymium: Xe 4f3 6s2
  • Neodymium: Xe 4f4 6s2
  • Promethium: Xe 4f5 6s2
  • Samarium: Xe 4f6 6s2
  • Europium: Xe 4f7 6s2
  • Gadolinium: Xe 4f7 5d1 6s2
  • Terbium: Xe 4f9 6s2
  • Dysprosium: Xe 4f10 6s2
  • Holmium: Xe 4f11 6s2
  • Erbium: Xe 4f12 6s2
  • Thulium: Xe 4f13 6s2
  • Ytterbium: Xe 4f14 6s2
  • Lutetium: Xe 4f14 5d1 6s2

These elements are also called the rare earth elements. They can all be found naturally on earth, and they're all radioactively stable except promethium, which is radioactive. One interesting property of the lanthanoid elements is a trend called lanthanide contraction.

Lanthanoid Contraction

As we look at the periodic table, we can see a trend in atomic radius. In general, across a row the atomic radius decreases as the atomic number increases. And as we go down a column the atomic radius increases.

But when we look at columns 4-12, something funny happens when we compare the second and third elements in the columns. Suddenly, the atomic radius is almost the same!

Let's look at the atomic radius of column 3:

  • Scandium: 2.09
  • Yttrium: 2.27
  • Lanthanum: 2.74

As expected, the atomic radius increases as we go down the column. Now let's look at the atomic radius of column 4:

  • Titanium: 2.00
  • Zirconium: 2.16
  • Hafnium: 2.16

Wait, zirconium and hafnium have the same atomic radius - what happened to the trend? Now hafnium has a much lower atomic radius than expected; in fact, it appears not to have increased at all. This sudden, steep drop in the changes in atomic radius is called the lanthanide contraction.

Shielding Effect

In order to understand this steep drop, let's quickly review why the atomic radius decreases as we increase the atomic number across a row. At first glance we would expect the atomic radius to increase as we increase the atomic number, because we are increasing the number of electrons in the atom. But we are also increasing the number of protons in the nucleus. Since electrons and protons are attracted to each other, this increased attraction pulls the atom closer towards the nucleus, thus decreasing the atomic radius.

Each orbital can shield the electrons from the protons, to an extent. The s-orbital is best at doing this, while the f-orbital is not very good at doing this. The elements in the lanthanoid series have an f-orbital. Thus, these electrons are shielded even less than previous elements, so they get pulled in even more, hence the sudden drop in atomic radius.

Actinoid Properties

The actinoid series also includes 14 elements, with the atomic numbers 90-103:

  • Thorium: Rn 6d2 7s2
  • Protactinium: Rn 5f2 6d1 7s2
  • Uranium: Rn 5f3 6d1 7s2
  • Neptunium: Rn 5f4 6d1 7s2
  • Plutonium: Rn 5f6 7s2
  • Americium: Rn 5f7 7s2
  • Curium: Rn 5f7 6d1 7s2
  • Berkelium: Rn 5f9 7s2
  • Californium: Rn 5f10 7s2
  • Einsteinium: Rn 5f11 7s2
  • Fermium: Rn 5f12 7s2
  • Mendelevium: Rn 5f13 7s2
  • Nobelium: Rn 5f14 7s2
  • Lawrencium: Rn 5f14 7s2 7p1

Several of these elements are not found naturally on the earth, but have only been created synthetically in a laboratory. Since all of these elements have an atomic number above 83, they are all considered radioactively unstable.

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