The Lanthanides and Actinides

Introduction

Look at almost any printed periodic table and you will notice two rows of elements floating on their own underneath the main block. They look like an afterthought, but they are not. These are the lanthanides and the actinides , and they belong inside periods 6 and 7. They are placed underneath simply so that the table fits neatly on a page. This page explains where they really belong, why they are separated and why they matter in modern technology.

Core explanation

Where they really belong. Period 6 begins with caesium (atomic number 55) and barium (56). The next element, lanthanum (57), starts a run of fifteen elements that ends with lutetium (71). Only after these does the period continue with hafnium (72) in Group 4. In the same way, period 7 starts with francium (87) and radium (88), then runs through the fifteen actinides from actinium (89) to lawrencium (103) before continuing with rutherfordium (104).

Why they are moved. If all thirty of these elements were placed in their correct positions, periods 6 and 7 would each be 32 elements long. The table would become very wide and thin — about twice as wide as it is tall. That shape is hard to print in a book or show on a classroom wall. So chemists cut out the two strips of fifteen elements and place them underneath. Many tables mark the gap with a small note such as "57–71" and "89–103" to show where the strips fit back in. The order of atomic numbers is not broken; it has just been folded.

Why they are so alike. In each strip, the elements have very similar chemical properties. Moving from one lanthanide to the next, the extra electron goes into an inner part of the atom rather than into the outer shell. As a result, the outer electrons are almost the same from one element to the next, and so their chemistry is almost the same too. This is why the lanthanides are hard to separate from each other. Together these two rows are called the f-block .

The lanthanides. These are shiny, reactive metals. They tarnish in air and react slowly with water. They are sometimes called "rare earth elements", but most are not especially rare — cerium is more common in Earth's crust than copper. What makes them difficult to obtain is that they occur mixed together in the same ores and must be separated by long, careful processes.

The actinides. All actinides are radioactive . Only thorium and uranium occur naturally in large amounts; most of the others are made in nuclear reactors or particle accelerators and exist only in tiny quantities. Some, such as uranium and plutonium, are used as nuclear fuels.

Step-by-step reasoning

To place a lanthanide or actinide correctly:

1. Find its atomic number, for example neodymium, 60. 2. Check whether it lies between 57 and 71 (lanthanide) or between 89 and 103 (actinide). 3. 60 lies between 57 and 71, so neodymium is a lanthanide. 4. Lanthanides belong in period 6, so neodymium is in period 6, sitting between barium and hafnium in the full-width table.

Visual explanation

Imagine the full periodic table as a long strip of paper 32 boxes wide. Cut two slices of fifteen boxes out of the bottom two rows, push the rest of the table together, and lay the slices underneath. In the interactive periodic table, highlight the f-block to see the two strips glow and trace where they slot back in.

Real-world analogy

It is like a long footnote in a book. Instead of breaking up a paragraph with a large block of detail, the author moves the detail to the bottom of the page and leaves a small marker in the text. The information still belongs in the paragraph; it has only been moved for neatness.

Real-world example

Your mobile phone and earbuds contain lanthanides. Neodymium is used to make very strong, small magnets for speakers, electric motors and wind turbines. Europium and terbium help produce the red and green colours in some screens and energy-saving lamps. Among the actinides, americium is used in tiny amounts in many household smoke detectors.

Why?

Why do the lanthanides behave so alike? Their chemistry is controlled mainly by their outer electrons. As atomic number increases along the row, new electrons go into an inner level, so the outer arrangement barely changes. Similar outer electrons mean similar reactions, which is why these elements are found together and are hard to separate.

Common misconception

"The two bottom rows are extra elements that do not fit into any period." They do fit: the lanthanides are part of period 6 and the actinides are part of period 7. They are drawn separately only to keep the table a manageable shape.

Worked example

Question: Uranium has atomic number 92. Which strip does it belong to, and in which period does it really sit?

Reasoning: The actinides run from 89 to 103. 92 lies within this range, so uranium is an actinide. The actinides belong in period 7.

Answer: Uranium is an actinide and belongs in period 7.

Quick check

1. Which period do the lanthanides belong to? Answer: Period 6, between barium (56) and hafnium (72).

Exam focus

Be ready to explain in one or two sentences why the lanthanides and actinides are placed below the table: to avoid a very wide table, not because they are outside the periods. Know the atomic number ranges 57–71 and 89–103, and that all actinides are radioactive.

Advanced insight

In more advanced chemistry, the lanthanides and actinides are described as filling f sub-shells, which hold up to fourteen electrons. Filling an f sub-shell from empty to full spans fifteen elements, which is why each strip is fifteen elements long. Some chemists argue that lutetium and lawrencium, rather than lanthanum and actinium, belong in Group 3, and different published tables show slightly different choices.

Summary

The lanthanides (57–71) and actinides (89–103) are two strips of fifteen elements that belong inside periods 6 and 7. They are printed below the main table so that it is not too wide. Within each strip the elements are chemically very similar because their outer electrons hardly change. Lanthanides are reactive metals used in magnets and screens; actinides are all radioactive, and some are nuclear fuels.

Practice questions

1. Give the range of atomic numbers of the actinides. Answer: 89 to 103. 2. Explain why the lanthanides and actinides are shown below the main periodic table. Answer: If placed in their true positions, periods 6 and 7 would be 32 elements wide, making the table too wide to print or display conveniently, so the strips are moved underneath. 3. State one property shared by all the actinides. Answer: They are all radioactive. 4. Why are the lanthanides difficult to separate from one another? Answer: Their outer electron arrangements are almost identical, so they have very similar chemical properties and occur mixed together in the same ores. 5. Cerium has atomic number 58. Is it a lanthanide or an actinide, and which period does it belong to? Answer: It is a lanthanide and belongs in period 6.