Question

Which of the following elements is placed just after the actinide series?
(A) Rutherfordium.
(B) Unnilpentium.
(C) Unnilquadium.
(D) Ununbium.

Answer 1

We know that actinides are group three elements. Actinide derives its name from the first element in the series Actinium. It is represented with the chemical symbol $An$ and the series actinoid or actinides elements comprises the $15$ metallic chemical elements from atomic numbers $89$ to $103$ (actinium to lawrencium)

Complete answer:
The term ‘actinide series’ is derived because of the first element of the series; Actinium. $An$ is the symbol used to refer to any of the elements representing the Actinide series. All the actinide series are radioactive in nature and release a large amount of energy on radioactive decay. Actinides are the second series of the F block elements and the first series of the f block elements are called lanthanides which include elements with atomic numbers starting from $57$ to $71.$ The former, i.e. actinides, are radioactive in nature whereas lanthanides are non-radioactive except promethium.
Having lower ionization energy, actinides are electropositive as compared to lanthanides and are most reactive. Actinides can react with hot water as well as react with oxidizing agents to form a passive coating, halides and hydrides. Actinides are also known to be strong reducing agents. Actinide series starts with actinium $\left( Z=89 \right)$ and contains fourteen elements from thorium $\left( Z=90 \right)$ to lawrencium $\left( Z=103 \right).$ This is followed by Rutherfordium $\left( Z=104 \right).$ There Rutherfordium $\left( Z=104 \right)$ also known as Unnilquadium.
Therefore, correct answer is option A and C. i.e. Rutherfordium & Unnilquadium.

Additional Information:
In actinides, there is steady decrease in the ionic radii with the increase in the atomic number and this is known as actinide contraction i.e. the nuclear charge increases as we move from left to right but the electrons are also added in the $n-2$ subshell and the effect of increased nuclear charge dominates over the imperfect shielding by the $5f$ electrons and thus, the $5f$ electron subshell contracts and thus, the decrease is very small.

Note:
Remember that the ionic radii or atomic size of tri positive actinide ions tend to decrease steadily from $Th$ to $Lw$ because of the increasing nuclear charge and electrons entering inner $\left( n\text{ }2 \right)\text{ }f$ orbital Hence this gradual reduction in the size with an increase in atomic number is called actinide contraction and occurs similarly to lanthanide contraction. Contraction may be larger along the period because of the poor shielding by $5f$ electrons.