Question

a) State the position of cerium (z=58) and uranium (z=92) in periodic table?
b) Write general electronic configuration of lanthanoids. What are transuranic elements?

Answer 1

Hint: a) Cerium and uranium are part of the F-block elements. F-block elements include Lanthanides and Actinides series. These are also known as Inner-Transition Elements. Cerium is present in lanthanide series and uranium is present in actinide series.
b) Lanthanide series contain 15 elements starting from Lanthanum (z=57) and ending at Lutetium (z=71). The general configuration for them is $ (n-2){{f}^{0-14}}(n-1){{d}^{0-1}}n{{s}^{2}} $ . Transuranic is composed of two words, Trans and uranic, Trans means “beyond” and uranic is used for uranium. So, transuranic elements are those which are beyond uranium in the actinide series.

Complete Step By Step Solution
a) Cerium has atomic no. 58 and it is present in the F-block. F-block contains two series lanthanides and actinides. The Lanthanide series has elements ranging from atomic no. 57 to atomic no. 71. So, cerium is the second element in lanthanide series with lanthanum to its left and praseodymium to its right.
Uranium is present in the actinide series of F-block. This series contains 15 elements ranging from actinium (z=89) to lawrencium (z=103).Uranium is the fourth element of the actinide series. It contains protactinium to its left and neptunium to its right. Actinides are of great importance because it includes radioactive elements. F-block is present separately at the end of the periodic table with two series. The position of cerium and uranium can be easily located.
b) Lanthanides are a part of F-block elements which are also known as inner-transition elements because they form transition series with the transition elements of d-block. The distribution of electrons for lanthanides is $ (n-2){{f}^{0-14}}(n-1){{d}^{0-1}}n{{s}^{2}} $ , f, d and s are orbitals which can contain maximum up to 14, 10 and 2 electrons respectively. These are also known as 4f elements. Most of them are non-radioactive except promethium. There is a decrease in atomic and ionic radii from lanthanum to lutetium. This is called the lanthanoid contraction. These are soft metals and are good conductors of heat and electricity.
Transuranic elements are the elements present beyond uranium in the actinide series, out of these only neptunium and plutonium have been observed in nature. Transuranic elements are unstable and radioactive in nature. These are produced in nuclear reactors by bombarding natural elements with neutrons which are produced in nuclear fission reactions. These elements do not occur naturally.

Note
Study the F-block elements properly. These have properties similar to d-block elements. Lanthanide series is known as 4f series and actinides series is known as 5f series due to their respective electronic configurations. They are separately placed below the main periodic table. There is an oxidation state of $ +3 $ present in both the series. They are highly electropositive and reactive in nature.