How can I predict how many electrons are in the outermost electron cloud of an element by using the periodic table?
You can tell much about the valence shell electrons of an element based on its position on the periodic table. The vertical rows are called groups and they tell how many and what kind of valence shell electrons are in a particular element. The group 1 elements (H, Li, Na, etc.) have one electron in the valence s shell. The group 2 elements (Be, Mg, Ca, etc.) have two electrons in the valence s shell. Groups 13-18 elements (except He) correspond to having 1-6 electrons in the valence p shell. Groups 3-12 elements have d shell electrons and the lanthanide and actinide series at the bottom of the table have f shell electrons. So, as an example, Mg is a group 2 element and therefore has 2 electrons in its valence shell (3s2, to be specific). Also, Br is a group 17 element and has 7 valence shell electrons (4s2,4p5, to be specific).
check Approved by eNotes Editorial