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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).
The simple and easiest way to to see in which gruop it is. If Lithium is in group 1, then the valence shell will have only one electron. If aluminuim is in teh Group 3 then it will have 3 valence electrons. Check for yourself.
Just looks at the group number located at the top of each column, and the number there tells you the valence electrons for example every element in group 1 the very first column has 1 valence electron, and the column right next to it (group 2) has elements with 2 valence electrons and so on.
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