The valency of an element (how many outer electrons are in an atom of the element) was treated as one of the basic properties of classifying elements for many reasons. First of all, it determined the chemical reactivity of an element to other elements. For example, fluorine is very reactive with most elements mostly due to its need to have one more valence (outer) electron needed in complete its shell (outer electron layer) in order to achieve stability (fluorine has 7 valence electrons and needs 8 to achieve stability to not react chemically anymore). It is so reactive that it can cause respiratory irritation if inhaled and even death if a person inhaled a lot of it. In comparison, carbon is less reactive than fluorine due to its need for 4 valence electrons to complete its shell. It is so unreactive compared to fluorine that a person could ingest pure carbon (in the form of graphite or charcoal) without negative consequences. The valency of an element also determines the nature of the bonds it would have with other atoms. In fluorine, due to its high valency, it forms one ionic bond (fluorine steals electrons from metals and the metal and fluorine are attracted to each other due to an imbalance of charge between them) with metals and a nonpolar covalent bond (atoms share electrons without creating a dipole (like magnetic poles for atoms/molecules)) with most nonmetals. In comparison, carbon can form between 1 chemical bond (as in the case of carbon monoxide) to a maximum of 4 chemical bonds (as seen in methane). In carbon monoxide (CO), the bond type is a polar covalent bond (atoms share electrons with one holding more electrons than the other, creating a positive charge on one end (the carbon atom)and negative charge on the other (the oxygen atom), making a dipole). In methane, however, the bond type between hydrogen and carbon is a nonpolar covalent bond due to the symmetry of the methane molecule (shape of molecules is partially due to valency as well because of electron orbitals (an area where an electron is likely to be))