ionisation energies q (1 Viewer)

[anonymoushehe]

can someone explain to me why 18 is C? I was stuck between choosing D and C but wasn’t too sure why C was the answer?

 

[Luukas.2]

Since element B has the highest first IE amongst the four consecutive elements, it must be a noble gas. Thus:

• element A is a halogen (group 17) and forms an A^- anion
• element C is in group 1 and forms a C⁺ cation
• element D is in group 2 and forms a D²⁺ cation

Forming a D³⁺ cation requires much more energy than is needed to form D²⁺. Since an atom of element D has 2 electrons in its valence shell, the D²⁺ cation has emptied that valence shell and its outermost electron shell is now a complete / filled shell that was formerly an inner shell. We can predict that the 2nd IE of D was perhaps double its 1st IE, whereas its 3rd IE will be much higher, perhaps five to ten times that of the 2nd IE, and so the formation of the D³⁺ cation is very unlikely.