Study Guides/Chemistry/Difference Between Lanthanoids and Actinoids
Study Guide · Chemistry

Difference Between Lanthanoids and Actinoids

At the very bottom of the modern Periodic Table, there are two separated rows of elements. These are the f-block elements (Inner Transition Metals). The top row consists of the Lanthanoids, and the bottom row consists of the Actinoids.

Question (Click to Flip)

Why are they placed at the bottom of the table?

Answer

If inserted into the main body of the periodic table, they would make the table too wide and ruin the symmetry of the groups. They are placed at the bottom to maintain a compact shape.

Card 1 of 1 free previews

Key Facts

Uranium and Plutonium, the most famous actinoids, are the primary fuels used in nuclear reactors and atomic weapons.

Elements beyond Uranium (Atomic number 92) are called 'Transuranic elements'. They do not exist in nature and are entirely man-made.

1. The Lanthanoids (4f Series)

  • Position: These 14 elements follow Lanthanum (Atomic Numbers 58 to 71).
  • Electron Filling: The extra electrons are filled progressively into the deep 4f orbital.
  • Oxidation States: They primarily show a very stable +3 oxidation state. Sometimes +2 and +4 are shown, but higher states are not possible.
  • Radioactivity: Except for Promethium (Pm), all lanthanoids are non-radioactive and relatively stable.
  • Complex Formation: They have a low tendency to form complex compounds.

2. The Actinoids (5f Series)

  • Position: These 14 elements follow Actinium (Atomic Numbers 90 to 103).
  • Electron Filling: The extra electrons are filled progressively into the 5f orbital.
  • Oxidation States: While they also show a +3 state, they frequently exhibit much higher oxidation states like +4, +5, +6, and +7 (e.g., Uranium). This is because the 5f, 6d, and 7s energy levels are very close to each other.
  • Radioactivity: All actinoids are radioactive. The heavier ones are highly unstable and artificial (made in labs).
  • Complex Formation: They have a very high tendency to form complex compounds.

3. The Contraction Effect

Both series experience a steady decrease in atomic size across the row.

  • Lanthanoid Contraction: Caused by the poor shielding effect of the 4f electrons.
  • Actinoid Contraction: The contraction here is much greater from element to element because the shielding effect of the 5f electrons is even poorer than that of the 4f electrons.

Questions and Answers

Why are they placed at the bottom of the table?+

If inserted into the main body of the periodic table, they would make the table too wide and ruin the symmetry of the groups. They are placed at the bottom to maintain a compact shape.

More in Chemistry

⚗️

How Many Valence Electrons Does Nitrogen Have?

Learn how many valence electrons a Nitrogen atom has. Understand its atomic number, electron configuration, and bonding capacity.

⚗️

Oxidation Number of Nitrogen in NO2

Learn how to calculate the oxidation number (or oxidation state) of Nitrogen in NO2 (Nitrogen Dioxide). The correct answer is +4.

⚗️

Normality — Definition, Formula, and Relation to Molarity

Normality definition in chemistry: gram equivalents of solute per litre of solution. Formula N = Molarity × n-factor. Examples with acid-base and redox reactions.

⚗️

Nuclear Charge — Atomic Number, Zeff, and Shielding

Nuclear charge = Z × e (protons × charge). Effective nuclear charge Zeff = Z − S. Learn shielding effect, Slater's rules, and periodic trends. Class 11 chemistry.

⚗️

Number of Neutrons in 1.7g of Ammonia

Number of neutrons in 1.7g of ammonia (NH₃) = 4.215 × 10²³. Calculation: 0.1 mol NH₃ × 6.022 × 10²² molecules × 7 neutrons per molecule. Step-by-step solution.

Study Smarter with Shinyu.ai

Turn this guide into revision flashcards, a practice exam, or an AI-generated podcast — free, no signup required.