Question

In the diagram, how are valence electrons illustrated? How many valence electrons does each element have?

Answer 1

Well, I would think that the electrons on the outside being DARKER are a good sign that they are important.

And in fact they are; the valence electrons are crucial to effective chemical bonding, and are usually the sole participants in chemical reactions (as opposed to core participation).

Thus, there are `bb4` valence electrons in silicon atom, and `bb5` in phosphorus atom.

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For simplicity, you can find valence electrons for main-group elements (i.e. not transition metals and not `f`-block elements) via their group number if it is less than `10`, or their group number minus `10` if the group number is greater than `10`.

So, in principle (at least for non-transition metals and non-`f`-block elements), one can easily look across the periodic table and predict that oxygen has `16 - 10 = 6` valence electrons, or boron has `13 - 10 = 3` valence electrons.