Chemistry help! it's not that hard, but..?

Question

Please explain this. We had a test, i made a tremendousy terrible grade on it. We got to correct our answers, and make a study guide based on the test. I don't know how to explain these.



~~An element has the electron configuration [Kr] 4d10 5s2 5p2. The element is a?



It's a metal, but how do i explain the differences in groups --noble gas, metal, nonmetal, etc.??

~~The electron configuration for Cr2+ is...

The answer is [Ar]3d4


I keep getting [Ar] 4s2 3d2... what am i doing wrong?

Last one, for now.

~~ Which of the following frequencies corresponds to light with the longest wavelength?

The answer is either 4.12 x 10 5 s-1

Or 3.00 x 10 3 s-1



How do you find it?



Thank you, if i have any help.

Thank you!!

Answer 1

1. The bottom elements in Group 4 (14) are metals.
2. Take all the s electrons off first when you make a transition metal ion.
3. Smaller frequency corresponds to longer wavelength.