Group 17 Elements — Core Principles

Core Principles

Group 17 elements, known as halogens (F, Cl, Br, I, At), are highly reactive non-metals with an $ns^2np^5$ outer electronic configuration, driving their strong tendency to gain one electron. This makes them powerful oxidizing agents.

Their reactivity decreases down the group, with fluorine being the most reactive. Key trends include increasing atomic radii, decreasing ionization enthalpy and electronegativity (F being the highest), and increasing melting/boiling points.

An important anomaly is that chlorine has a more negative electron gain enthalpy than fluorine, and $F_2$ has a lower bond dissociation enthalpy than $Cl_2$ and $Br_2$. Halogens form hydrides (HX), which show increasing acidic strength down the group (HF is a weak acid due to H-bonding).

They also form various oxides (mostly unstable) and interhalogen compounds ($XX'$, $XX_3'$, $XX_5'$, $XX_7'$), which are generally more reactive than the parent halogens. Important compounds like chlorine and HCl have significant industrial applications, including water purification, bleaching, and chemical synthesis.

Oxoacids of halogens (e.g., $HClO_4$) exhibit increasing acidic strength with increasing oxidation state of the halogen.

Important Differences

vs Group 16 Elements (Chalcogens)

Aspect	This Topic	Group 16 Elements (Chalcogens)
Valence Electrons	7 ($ns^2np^5$)	6 ($ns^2np^4$)
Tendency	Strong tendency to gain 1 electron	Tendency to gain 2 electrons
Common Oxidation State	-1 (Fluorine always -1)	-2 (Oxygen always -2, except in $OF_2$)
Electronegativity	Very high (highest in respective periods)	High, but generally lower than halogens
Reactivity	Extremely reactive non-metals	Reactive non-metals (O, S, Se, Te) and metalloids (Po)
Bond Dissociation Enthalpy (Diatomic)	Anomaly: $F_2 < Cl_2 > Br_2 > I_2$	Generally decreases down the group (e.g., $O_2$ vs $S_2$)

Halogens (Group 17) and Chalcogens (Group 16) are both p-block non-metals, but differ significantly in their electron configurations and reactivity. Halogens have 7 valence electrons and a strong drive to gain one, leading to exceptionally high electronegativity and powerful oxidizing capabilities. Chalcogens, with 6 valence electrons, typically gain two electrons. This difference results in halogens being generally more reactive and having a primary oxidation state of -1, while chalcogens commonly exhibit -2. The anomalous behavior of fluorine, particularly its electron gain enthalpy and bond dissociation enthalpy, further distinguishes Group 17.