Question

Which of the following is the wrong statement?
(A) All the actinide elements are radioactive.
(B) Alkali and alkaline earth metals are $s$ block elements.
(C) Chalcogens and halogens are $p$ block elements.
(D) The first member of the lanthanide series is lanthanum.

Answer 1

Hint : Chalcogens are the elements that belong to group $16$ in the modern periodic table. Halogens are the elements that belong to group $17$ in the modern periodic table. Actinides are the elements with atomic numbers from $90$ to $103$ in the modern periodic table. Lanthanides are a group of fifteen elements with atomic numbers from $57$ to $71$ in the modern periodic table.

Complete Step By Step Answer:
All the actinides are radioactive and they release energy on radioactive decay. Radioactivity of actinides is caused by their nuclear instability.
Alkali metals are the elements of group $1$ and they belong to the $s$ block in the modern periodic table. This is because in these elements the last electron enters into $s$ subshell. Alkaline earth metals are the elements of group $2$ and they belong to the $s$ block in the modern periodic table. This is because in these elements the last electron enters into $s$ subshell.
Chalcogens are the elements of group $16$ and they belong to the $p$ block in the modern periodic table. This is because in these elements the last electron enters into $p$ subshell. Halogens are the elements of group $17$ and they belong to the $p$ block in the modern periodic table. This is because in these elements the last electron enters into $p$ subshell.
The first member of the lanthanide series is the element cerium.
Therefore, option D is the correct answer.

Note :
The nucleus of a radioactive actinide element undergoes radioactive decay and releases alpha particles, gamma rays, beta particles or neutrons to gain stability. Actinides are used in defense operations, nuclear reactors and also nuclear weapons. The element plutonium is used in nuclear reactors and also in nuclear bombs.