why is it useful to know the number of valence electrons in an atom
Valence electron The number of electrons in an atom's outermost valence shell governs its bonding behavior. Therefore, elements whose atoms can have the same number of valence electrons are grouped together in the periodic table of the elements. As a general rule, a main group element (except hydrogen or helium) tends to react to form a closed shell, corresponding to the electron configuration s2p6. This tendency is called the octet rule, because each bonded atom has eight valence electrons including shared electrons.
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Vein Veins are less muscular than arteries and are often closer to the skin. There are valves in most veins to prevent backflow.