Question

The correct statement(s) about actinides is/are:
1. The 5f electrons of actinides are bound less tightly than the 4f electrons
2. The trans uranium elements are prepared artificially
3. All the actinides are radioactive. 
4. Actinides do not exhibit actinide contraction

Answer 1

Correct Answer - Option 3 : All the actinides are radioactive. 

Concepts:

• Elements whose f-orbital getting filled up by electrons are called f-block elements.
  • They are divided into lanthanoids and actinoids. 
• The first series of elements are called lanthanides and include elements with atomic numbers beginning from 57 and ending at 71. These elements are non-radioactive.
• The second series of elements are called actinides and include elements with atomic numbers beginning from 89 and ending at 103. They are radioactive.
• Actinides are the elements whose atomic number is from 90 to 103.
• The members of the series are Thorium, Protactinium, Uranium, Neptunium, Plutonium, Americium, Curium, Berkelium, Californium, Einsteinium, Fermium, Mendelevium, Nobelium, Lawrencium.

Explanation:

• All the actinide elements are radioactive, hence the statement All the actinides are radioactive is correct.
• In the actinoids, the last electron goes into the 5f orbital.
• Now, the 5f orbital due to its large size and diffused nature has a very poor shielding effect.
• As a result of the poor shielding effect, much of the nuclear charge leaks from the electron cloud, as a result, the outer electrons face more nuclear charge of the nucleus.
• As the atomic number increases, the nuclear charge increases and as a result, the ionic radii or the atomic radii decrease from Thorium to Lawrencium.
• This is known as Actinoid contraction. Thus, the statement Actinides do not exhibit actinide contraction is incorrect.
• The poor shielding effect of 4f electrons causes Lanthanide contraction is Lanthanides.
• Comparing it to 5f orbital, as 5f orbitals are larger in size, its shielding effect is much poor, as a result of which the electrons in 5forbitals are less tightly bound. Also, it is much farther than the nucleus compared to 4f orbitals.
• Hence, the statement: The 5f electrons of actinides are bound less tightly than the 4f electrons is also correct.
• The transuranium compounds are the elements with an atomic number greater than 92.
• The transuranium compounds are all radioactive and are man-made elements.
• Hence, the statement: The trans uranium elements are prepared artificially is also correct.
• Hence, the correct options are: 1,2 and 3.