Periodic Trends in Valency

Valency is an element's combining capacity — a measure of how many bonds (or how many univalent atoms such as hydrogen or chlorine) one atom of the element can typically combine with. For the representative (s- and p-block) elements, valency follows a clean, predictable pattern tied directly to the group number and the number of valence electrons:

• Groups 1 to 14 — valency generally equals the number of valence electrons, which for these groups equals the group number itself for groups 1–2, and (group number $-$ 10) for groups 13–14. So Group 1 elements ($Li, Na, K, \ldots$) show valency $1$; Group 2 elements ($Be, Mg, Ca, \ldots$) show valency $2$; Group 13 elements ($B, Al, \ldots$) show valency $3$; Group 14 elements ($C, Si, \ldots$) show valency $4$.
• Groups 15 to 17 — valency generally equals $8$ minus the number of valence electrons, because these elements more commonly complete their octet by sharing or gaining just enough electrons to reach eight, rather than by losing five, six or seven. So Group 15 elements ($N, P, \ldots$, 5 valence electrons) show valency $3$ ($8-5$); Group 16 elements ($O, S, \ldots$, 6 valence electrons) show valency $2$ ($8-6$); Group 17 elements — the halogens ($F, Cl, \ldots$, 7 valence electrons) — show valency $1$ ($8-7$).
• Group 18, the noble gases, generally show valency $0$. Their valence shell is already completely filled (an octet, or a duplet for helium), so under ordinary conditions they have essentially no tendency to gain, lose or share electrons and do not combine with other elements at all (only under extreme conditions do a few heavier noble gases such as xenon form a handful of compounds, which is why "generally zero" rather than "always zero" is the precise statement). …