Reactivity with Metals
Master the formation of Metal Halides and the application of Fajans' Rules.
Halogens are the most electronegative elements on the periodic table, while metals are electropositive. This combination results in highly vigorous, spontaneous reactions. Halogens react with metals to form a massive class of compounds known as Metal Halides. Because halogens only need one electron to complete their octet, they consistently exhibit a -1 oxidation state when bonded to metals.
1. Formation and Reactivity Trend
All halogens react directly with metals. The reactivity of the halogens towards metals decreases down the group.
(Magnesium bromide: Mg is +2, Br is -1)
The Power of Fluorine: Because Fluorine is the most powerful oxidizing agent, it reacts with almost all metals. In fact, it often forces the metal to exhibit its highest possible oxidation state (e.g., Uranium hexafluoride, $UF_6$, and Bismuth pentafluoride, $BiF_5$).
2. Ionic vs Covalent Character (Fajans' Rules)
This is the most highly tested concept in this section. While we generally think of metal halides as purely ionic salts (like $NaCl$), many metal halides are actually highly covalent! The nature of the bond is determined by Fajans' Rules.
If a metal exhibits more than one oxidation state, the halide in the higher oxidation state will be more covalent than the one in the lower oxidation state.
Why? A highly charged metal cation (e.g., $+4$) is smaller and has a massive polarizing power. It strongly pulls the electron cloud of the halide ion toward itself, leading to covalent orbital overlap.
- $SnCl_4$ is more covalent than $SnCl_2$.
- $PbCl_4$ is more covalent than $PbCl_2$.
- $SbCl_5$ is more covalent than $SbCl_3$.
For a given metal, the covalent character increases as the size of the halide ion increases.
Why? A larger halide ion (like $I^-$) holds its outer electrons very loosely. It is highly polarizable, meaning its electron cloud is easily distorted by the metal cation to form a covalent bond.
$MI > MBr > MCl > MF$
- $AlF_3$ is purely ionic, while $AlCl_3$ is predominantly covalent.
Figure 1: Visualizing the difference between $SnCl_2$ and $SnCl_4$. The highly charged $Sn^{4+}$ cation severely distorts the chloride ion, leading to covalent character.
The shift from ionic to covalent bonding completely changes the physical properties of the compound. Ionic compounds form giant rigid crystal lattices, making them solids with high melting points (e.g., $SnCl_2$ is a solid, melting at $246^\circ\text{C}$). Covalent compounds form discrete molecules held together only by weak van der Waals forces, making them gases or volatile liquids (e.g., $SnCl_4$ is a fuming liquid, melting at $-33^\circ\text{C}$).
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