Group 17 Elements: The Halogens
Electronic Configuration, Mineral Occurrence, and High Reactivity.
Group 17 contains the most reactive non-metals in the entire periodic table. They are highly electronegative and exist as diatomic molecules in nature. Because of their extreme reactivity, you will never find them in a free, native state; they are always bound in compounds, primarily as salts.
1. Meet the Halogens
Group 17 of the periodic table consists of Fluorine ($F$), Chlorine ($Cl$), Bromine ($Br$), Iodine ($I$), Astatine ($At$), and the synthetically produced radioactive element Tennessine ($Ts$).
Collectively, these elements are known as the Halogens.
The name is derived from two Greek words: 'halos' meaning sea salt, and 'genes' meaning born or producer. Thus, Halogen literally translates to "salt-producer". This perfectly describes them, as their most common natural forms are salts found in sea water (like $NaCl$ and $MgCl_2$).
Figure 1: Halogens transition from highly reactive gases to a liquid (Br) and finally to solids down the group.
2. Electronic Configuration & Reactivity
All halogens have seven electrons in their outermost shell, with a general valence shell configuration of $ns^2 np^5$.
Because they are just one electron short of the next noble gas configuration ($ns^2 np^6$), halogens have a massive tendency to gain an electron. This gives them exceptionally high Electronegativity and Electron Gain Enthalpy (with Chlorine having the highest negative electron gain enthalpy in the entire periodic table). Consequently, they are immensely reactive and act as powerful oxidizing agents.
3. Occurrence in Nature
Because of their high reactivity, halogens do not occur in a free state in nature. They are found almost exclusively as halides.
A. Fluorine ($F$)
Found mainly as insoluble fluorides in the Earth's crust.
- Fluorspar (Fluorite): $CaF_2$
- Cryolite: $Na_3AlF_6$
- Fluorapatite: $3Ca_3(PO_4)_2 \cdot CaF_2$
B. Chlorine ($Cl$) & Bromine ($Br$)
The sea is the greatest storehouse of halogens.
- Sea Water: Contains chlorides, bromides, and iodides of Sodium, Potassium, Magnesium, and Calcium. The majority is $NaCl$ (approx 2.5% by mass).
- Carnallite: A solid mineral deposit found in dried-up seas, formulated as $KCl \cdot MgCl_2 \cdot 6H_2O$.
C. Iodine ($I$)
While found in sea water, Iodine is biologically concentrated by marine life and found in specific mineral deposits.
- Seaweeds: Various seaweeds contain up to 0.5% of iodine in their systems.
- Chile Saltpetre: While primarily Sodium Nitrate ($NaNO_3$), Chile saltpetre deposits uniquely contain up to 0.2% of Iodine in the form of Sodium Iodate ($NaIO_3$).
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