Valency is the combining capacity of an element, which is determined by how readily an atom gains, loses, or shares electrons to achieve a stable electron configuration, typically like that of a noble gas (usually a full outer shell). The number of electrons in the outermost shell dictates how an atom will participate in chemical bonding. Elements with a few valence electrons tend to lose them (forming positive ions), elements with nearly full outer shells tend to gain electrons (forming negative ions), and elements with partially filled outer shells often share electrons (forming covalent bonds).