What is the element with the electron arrangement 2,8,7?
the electron configuration of an element shows how many electrons are present. Add them to get the atomic number. 2+8+7 = 17.
Alternatively, there are 3 shells of electrons so the element is in period 3, and the last shell has 7 electrons so it is in Group (17 or VIIA). Trace to where the period number and group number intersect on the periodic table and you will get to chlorine.