If the outermost electron of an atom is represented by the electron configuration 3s1, it means that the electron is located in the third energy level (n = 3) and the s orbital within that energy level. The "1" indicates that there is only one electron in that orbital.
The atomic number of an element corresponds to the number of protons in the nucleus of an atom. In this case, since the electron configuration 3s1 is described, it means that the atom has one electron in the 3s orbital. The number of electrons in a neutral atom is equal to the number of protons, so the atomic number of the element with this electron configuration is 1.
Therefore, the element with the outermost electron represented by 3s1 is hydrogen (H), which has an atomic number of 1.