<P> The octet rule is a chemical rule of thumb that reflects observation that atoms of main - group elements tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas . The rule is especially applicable to carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium or magnesium . </P> <P> The valence electrons can be counted using a Lewis electron dot diagram as shown at the right for carbon dioxide . The electrons shared by the two atoms in a covalent bond are counted twice, once for each atom . In carbon dioxide each oxygen shares four electrons with the central carbon, two (shown in red) from the oxygen itself and two (shown in black) from the carbon . All four of these electrons are counted in both the carbon octet and the oxygen octet . </P> <P> Ionic bonding is common between pairs of atoms, where one of the pair is a metal of low electronegativity (such as sodium) and the second a nonmetal of high electronegativity (such as chlorine). </P>

How does one determine the number of valence electrons an atom has