4.6 Lanthanides & Actinides

This topic introduces the lanthanides and actinides — the two rows of elements placed separately at the bottom of the periodic table. They are also called the f-block elements.

Definition

Lanthanides

Lanthanides, also known as "rare earth elements", are a series of 14 elements located at the bottom of the periodic table. They include elements with atomic number 57 to 71.

Simple explanation: Lanthanides are the first row of elements placed separately at the bottom of the periodic table.

Actinides

Actinides are another series of 14 elements located just below the lanthanides. They include elements with atomic number 89 to 103.

Simple explanation: Actinides are the second row of elements placed separately at the bottom of the periodic table.

Main Idea

The lanthanides and actinides are placed separately at the bottom of the periodic table to keep the table compact. They belong to the f-block because their valence electrons lie in the f sub-shell.

• Lanthanides: atomic numbers 57 to 71 (14 elements)
• Actinides: atomic numbers 89 to 103 (14 elements)

Simple explanation: These two series are taken out of the main table and placed below it so the table doesn't become too wide.

Core Concept

Detailed Explanation

Lanthanides (Rare Earth Elements)

Atomic numbers: 57 to 71

Examples: Lanthanum (La, 57), Cerium (Ce, 58), Praseodymium (Pr, 59), Neodymium (Nd, 60), Promethium (Pm, 61), Samarium (Sm, 62), Europium (Eu, 63), Gadolinium (Gd, 64), Terbium (Tb, 65), Dysprosium (Dy, 66), Holmium (Ho, 67), Erbium (Er, 68), Thulium (Tm, 69), Ytterbium (Yb, 70), Lutetium (Lu, 71).

• Also called "rare earth elements" — though they are not actually rare in the Earth's crust.
• They are metals with similar chemical properties.
• They are used in magnets, lasers, and phosphors.
• Most lanthanides are stable (non-radioactive), with some exceptions like promethium.

Actinides

Atomic numbers: 89 to 103

Examples: Actinium (Ac, 89), Thorium (Th, 90), Protactinium (Pa, 91), Uranium (U, 92), Neptunium (Np, 93), Plutonium (Pu, 94), Americium (Am, 95), Curium (Cm, 96), Berkelium (Bk, 97), Californium (Cf, 98), Einsteinium (Es, 99), Fermium (Fm, 100), Mendelevium (Md, 101), Nobelium (No, 102), Lawrencium (Lr, 103).

• All actinides are radioactive.
• Uranium and plutonium are used in nuclear reactors and nuclear weapons.
• They are metals with similar chemical properties.
• They are mostly synthetic — only a few (like uranium and thorium) occur naturally in significant amounts.

Why They Are Called f-Block Elements

The book states:

> "Lanthanides and actinides are known as f-block elements since their valence electrons lie in the f sub-shell."

Simple explanation: The last electron in these elements goes into the f sub-shell, so they are called f-block elements.

Position in the Periodic Table

Simple explanation: The f-block elements are placed separately to keep the periodic table from becoming too wide.

Intermediate Level

Why Are They Placed Separately?

If the lanthanides and actinides were placed in the main body of the periodic table, it would become too wide — 32 columns instead of 18. So they are placed separately at the bottom to keep the table compact.

Lanthanides — Detailed

• Atomic numbers: 57–71
• 14 elements
• Valence electrons: in the 4f sub-shell (mostly)
• Properties: soft, silvery metals; similar chemical properties; high melting points
• Uses: magnets, lasers, phosphors, catalysts, alloys
• Radioactivity: mostly stable (promethium is radioactive)

Actinides — Detailed

• Atomic numbers: 89–103
• 14 elements
• Valence electrons: in the 5f sub-shell (mostly)
• Properties: silvery metals; radioactive; similar chemical properties
• Uses: nuclear energy, nuclear weapons, research
• Radioactivity: all are radioactive

Comparison of Lanthanides and Actinides

f-Block and the Periodic Table

• f-block elements are placed separately because they would make the table too wide.
• They belong to Group 3 conceptually, but are shown separately.
• Lanthanides fill the 4f sub-shell.
• Actinides fill the 5f sub-shell.

Important Points

• - Lanthanides: 14 elements, atomic numbers 57 to 71, also called rare earth elements.
• - Actinides: 14 elements, atomic numbers 89 to 103.
• - Both are f-block elements (valence electrons in f sub-shell).
• - Placed separately at the bottom of the periodic table.
• - Lanthanides: mostly stable; used in magnets, lasers, phosphors.
• - Actinides: all radioactive; used in nuclear energy and weapons.
• - Lanthanides fill the 4f sub-shell; actinides fill the 5f sub-shell.

Remember

• - Lanthanides = 57–71 = 14 elements = rare earth elements.
• - **Actinides** = 89–103 = 14 elements.
• - Both are f-block elements.
• - Placed separately at the bottom of the periodic table.
• - Actinides are all radioactive.
• - Lanthanides are mostly stable.
• - Lanthanides fill 4f; actinides fill 5f.

Common Mistakes

• 1. Thinking lanthanides and actinides are d-block elements. They are f-block elements.
• 2. Thinking lanthanides are radioactive. Most lanthanides are stable; actinides are all radioactive.
• 3. Confusing the atomic number ranges. Lanthanides: 57–71; actinides: 89–103.
• 4. Thinking "rare earth" means they are actually rare. They are called rare earth elements, but they are not that rare in the Earth's crust.
• 5. Thinking all actinides occur naturally. Most actinides are synthetic; only a few (like uranium and thorium) occur naturally.
• 6. Forgetting why they are placed separately. They are placed separately to keep the periodic table compact.

Quick Revision

• Lanthanides: series of 14 elements with atomic numbers 57 to 71; also called rare earth elements; mostly stable.
• Actinides: series of 14 elements with atomic numbers 89 to 103; all radioactive.
• Both are f-block elements because their valence electrons lie in the f sub-shell.
• Placed separately at the bottom of the periodic table.
• Lanthanides fill the 4f sub-shell; actinides fill the 5f sub-shell.
• Uses: lanthanides in magnets, lasers, phosphors; actinides in nuclear energy and weapons.

60-Second Revision

• Lanthanides: 57–71, 14 elements, rare earth elements.
• Actinides: 89–103, 14 elements.
• Both are f-block elements.
• Placed at the bottom of the periodic table.
• **Actinides** = all radioactive.
• Warning: lanthanides are not all radioactive; actinides are.