What is the period 3 element whose 2+ ion is isoelectronic with neon?

Question

How can I find the solution?

Thanks

Answer 1

It's tough to explain without a board but look at Mg. Period 3 has Ar as the "end" element. So if you want it to be isoelectronic with Ne, the element must be in period 3 but lose electrons which makes it Na, Mg, Al etc. Mg is the only one of those that forms a 2+ ion to make it isoelectronic with Ne. .