The electronic configuration of an element is 1s22s22p5. how many valence electrons does it have? 4 5 7 9

Okay so the electron configuration is . Look at the periodic table. There are 2 blocks you need to know for this question: the s-block, which is comprised of groups 1 and 2, and the p-block, which is comprised of groups 13 through 18. Because your configuration ends in , your element has to be in group 17, a halogen. Now, you need to know another thing - your s sub-level has 1 orbital and your p sub-level has 3 orbitals. Every orbital can fill up with 2 electrons. Now, the fact that the configuration ends in tells you that the s-orbital is filled, and your p sub-level is not completely filled. If it was, you would have 8 valence electrons, and it would end in . Now, because it ends in , it has one less valence electron, so in your element you have 7 valence electrons.

vapour or gas assuming atmospheriv pressure