How do i tell the number of valence electrons present in an element just by looking at a periodic table?

i have a question on my test that deals with this, but i don't know what to do in order to find this out. any is appreciated! i can post the question, if that would your explanation.

Two nitro no2 groups are chemically bonded to a patch of surface. they can't move to another location on the surface, but they can rotate (see sketch at right). it turns out that the amount of rotational kinetic energy each no2 group can have is required to be a multiple of ε, where =ε×1.010−24 j. in other words, each no2 group could have ε of rotational kinetic energy, or 2ε, or 3ε, and so forth — but it cannot have just any old amount of rotational kinetic energy. suppose the total rotational kinetic energy in this system is initially known to be 32ε. then, some heat is removed from the system, and the total rotational kinetic energy falls to 18ε. calculate the change in entropy. round your answer to 3 significant digits, and be sure it has the correct unit symbol.

Why is nh3 classified as a polar molecule

Will mark brainliest how many electrons can be held in the energy level n = 4?