Now Lewis dot symbols, sometimes referred to as electron diagrams, are diagrams that represent the valence electrons of an atom or ion. Now when we talk about valence electrons, we look at it in terms of main group elements versus transition metals.
Now for main group elements, the elements that are groups 1A to 8A, the number of valence electrons that they possess is equal to their group number. So for example, if we're looking at aluminum, aluminum is in Group 3A, so it would have 3 valence electrons.
For transition metals, it's a bit different. For them it's the number of valence electrons equals their d electrons. Just remember, when we're looking at an element and trying to determine its number of valence electrons, you first have to look at it as being either a main group element or a transition metal.