the number of electrons in the outer energy level of an atom is called the number of valence electrons. the number of valence electrons gives the group number. that is, the number of valence electrons in the periodic table does not change from top to bottom or bottom to top. however, as you go from left to right, the number of valence electrons increases and begins to transform into a noble gas. for example, the atomic number of the element lithium is 3. when we list it according to the electron configuration, the number of electrons in the last layer is 1. therefore it is in group 1a.wikipedia
is the total number of electrons in an atom's final energy level. it is used to identify the groups of elements in the periodic table.*
number of electrons in the final orbital of an atom.
that of solid iron is +2. (bkz: i searched a lot, don't call the new generation)