The actinoids - class-XII

Terbium (Z = 65) is a lanthanoid and all others are actinoids. Lanthanoids have atomic numbers from 58 to 71. Actinoids have atomic numbers from 90 to 103.

4f and 5f orbitals are not equally shielded. 5f are poorly shielded as 5f orbitals are away from the nucleus.

LANTHANIDES

LANTHANIDES

Lu, Gd and La belongs to lanthanide series and Th belongs to actinide series.

We have 15 elements in actinides. They are from atomic number 89 to 103 Actinium, Thorium, Protactinium, Uranium, Neptunium, Plutonium, Americium, Curium, Berkelium, Californium, Einsteinium, Fermium, Mendelevium, Nobelium, Lawrencium. Hence, from these list we could say that Actinium is the first and Lawrencium is the last element in actinides.

Actinides are also called the 5f series.

$5f$ orbitals are bigger than $4f$ orbitals, in other words, $5f$ orbitals are more diffused thereby having the poor shielding effect. Hence have a more effective nuclear charge thereby having a small size. Hence actinide contraction is more than lanthanide contraction.

The assertion regarding the order of complex formation is correct based on charge density. The reason is also factually correct regarding bonding, but it does not explain why the specific order of complex formation occurs, as that is primarily driven by the ionic charge and size (electrostatics).

The two rows that are generally placed underneath the main Periodic Table are called the lanthanide series and the actinide series.

These two rows are produced when electrons are being added to f orbitals.

Therefore,
this block of elements are referred to as the "f block".

Actinide series starts with actinium (Z=89) and contains fourteen elements from thorium (Z=90) to lawrencium (Z=103). This is followed by Rutherfordium (Z=104). There Rutherfordium (Z=104) also known as Unnilquadium.

Actinoids are highly electropositive elements, and their compounds are predominantly ionic in nature due to the large size of the ions and the relatively low electronegativity of the metals.

Uranium (U) exhibits a wide range of oxidation states, with +6 being the highest and most stable in compounds like UF6 or UO2(2+).

Am shows +2, +3, +4, +5, +6, +7 oxidation states.

Oxidations are the polyatomic cations contain one or more oxygen atoms. Actinides have a strong tendency towards the complex formation and form oxocations like oxides and hydroxides and are basic in nature.

The actinide contraction is due to imperfect shielding of $5f$ electron. The valence shell electronic configuration of actinoids is $\displaystyle 5f^{1-14}6d^{0-1}7s^2$. With an increase in atomic number, the positive charge on the nucleus increases by $+1$, and one more electron enters in the same $5f$ subshell. There is imperfect shielding of one electron by another electron in the same $5f$ subshell. Due to this, the atomic and ionic radii show a gradual decrease with increase in atomic number.

The name of californium was derived from the name of the state and University of California.

Synthetic elements, particularly those with high atomic numbers, are often produced through nuclear reactions and are found within the actinide series and beyond (transuranic elements).

Lanthanides and actinides resemble in electronic configuration:

Because of higher effective nuclear charge and smaller size of atoms the actinides have higher charge density and because of that they have greater tendency to form complexes than the lanthanides.

Actinoids shows larger oxidation states due to poor shielding of both $4f$ and $5f$ orbital electrons as a result these orbital have almost similar energy and hence take part in bond formation. Also, there is very small energy between $5f, 6d$ and $7s$ subshells.

Actinides show larger oxidation states due to poor shielding of both $4f$ and $5f$ orbital electrons, as a result, their orbitals have almost similar energy and hence take part in bond formation. Thus the energy gap between $5f, 6d$ and $7s$ become very small.