Question

Which of the following elements has the least ionisation energy?

• A. $Li$
• B. $Cs$
• C. $Mg$
• D. $Ca$

Answer 1

Answer: (B)

$Cs$

On moving down the group $I.E$ decreases. So from $Li$ to $Cs$ I.E. decreases. Calcium and magnesium belong to alkaline earth metals i.e. group 2 so they have high I.E as it increases along a period when electrons are tightly held to nucleus in the same shell. In moving down new shell is added to the atom and valence electron is loosely held to nucleus so $Cs$ has least I.E.