Where exactly is an orbital? I understand they are a volume where we expect to find electrons 90% of the time but I don't understand where their shapes are and fit on an atom, and the different forms of orbitals confuse me a little. Can someone explain me where they are with the help of an image? Pardon me if my question isn't very clear

95% of the time (or any other percentage you choose), the electron will be found within a fairly easily defined region of space quite close to the nucleus. Such a region of space is called an orbital. You can think of an orbital as being the region of space in which the electron lives

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)

atomic mass = 39

number of neutrons = 20

number of protons= 19

electronic configuration = 1s2 2s2 2p6 3s2 3p6 4s1

the number of electron in first shell = 2

number of electrons in n=3 shell = 8