Hi Monera! When writing electron configurations, you can write the 4f first. Some people like the numbers to count up, so they would write europium's electron configuration as [Xe]4f^(7)6s^(2). (Sorry, formatting is tough in comments!). Others prefer numbers to count down, so they would write the same electron configuration as [Xe]6s^(2)4f^(7). I show this a little more clearly in this video here: kzfaq.info/get/bejne/htqRntVq2aeqm4E.html at the 14:44 mark. The important thing to remember is you need to write your electron configurations the way your professor showed them to you. Some professors will accept any order, others are particular about the order. If you are unsure about what your professor wants, just ask! Helping students is what we love to do! I hope that helps and if you want more interactive help, check out the weekly live streams. You can join the chat and ask me questions in real time!

@@ChemHelpASAP okay , thanks!