To find the number of valence electrons in an atom one should

One should look at the vertical line it is in, or the group. Based on the group number (1-18), the number of Valnce electrons varies. For Groups one and two, the valence electrons are the same, one and two. Groups three through twelve are the transition metals, which do not always correspond to their group numbers. For thirteen through eighteen, simply count the group it is in, and subtract ten. For example, if Silicon is in group fourteen, subtract ten from fourteen to get four valence electrons.