In what sublevel and main energy level are an atom's Valence Electrons located?
The answer to this question will depend on how many electrons the atom has. I've included an image of a period table that's labeled so as to help you determine the main energy level and sublevel for the valence electrons of any atom.
First locate the element in question on the periodic table. The horizontal rows, or periods, are numbered to correspond with the main energy level. Hydrogen and helium for example, are in the first row so their valence electrons are in the 1st principal (main) energy level. The first energy level only has s sublevels, so H and He both have their valence electrons in 1s orbitals (sublevels). The possible sublevels for each principal level are:
2: s and p
3: s, p and d
4 and higher: s, p, d and f
For the 2nd principal level, Li and Be have their valence electrons in 2s orbitals and B through Ne have their valence electrons in 2p orbitals. This is a pattern within chemical families. Na has one valence electron in 3s sublevel, K has one in the 4s sublevel, and Rb has one in the 5s sublevel. Each element below Be has two electrons in the s sublevel of the principal level corresponding to its row. The same pattern holds for the 13th through 18th column or family - each element as you move to the right adds one additional electron to the p orbital of the corresponding row.
Notice that the fourth row has 10 elements between the s-block and p-block. In this row outer electrons are in 3d orbitals. 3d comes after 4s because it's lower in energy and electrons will occupy the lowest energy positions available. The next element after the end of the 3d sublevel is in the 4p position. You can see on the image 4d is below 3d but to the right of 5s, and so forth.
The two rows at the bottom are the f-block. They belong immediately after 6s and 7s, but are taken out and put below the rest of the table.
Using this periodic table I can locate any element and determine its valence electrons. For example, Br is in the 4th row and is the 5th element over in the p block, so it has five valence electrons in the 5p, meaning the p sublevel of the 5th principal energy level.