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Trends in Group 17 (Group VIIA or Halogen) Elements

Properties of Group 17 (VIIA) Elements

Period Name
Atomic Number (Z) Simple Electronic Configuration Atomic Radius
1stIonization Energy(IA(kJ/mol) Melting point (oC) Boiling point (oC) Physical Appearance at STP (0oC, 1atm)
2 Fluorine
9 2,7 68 3.98 1690 -223 -187 pale yellow gas

3 Chlorine
17 2,8,7 99 3.16 1260 -102 -35 greenish-yellow gas

4 Bromine
35 2,8,18,7 114 2.96 1150 -7.3 59 reddish-brown liquid

5 Iodine
53 2,8,18,18,7 133 2.66 1020 114 183 purple solid

6 Astatine
85 2,8,18,32,18,7 2.2 899* Astatine doesn't occur naturally (longest lived isotope's half-life=8hours).
It resembles iodine in chemical properties but is more metallic

Trends in Properties of Group 17 (VIIA) Elements

  • All Group 17 (VIIA) elements have 7 valence electrons (7 electrons in the highest energy level).

  • Atomic radius increases down the Group as successive 'electron shells' (energy levels) are filled.

  • Electronegativity (the relative tendency shown by a bonded atom to attract electrons to itself) decreases down the group as the elements become more metallic in nature. (Typically, metals have low electronegativity, little ability to attract electrons, while non-metals have high electronegativity, greater ability to attract electrons). The reactivity of Group 17 (VIIA) elements is related to the element's ability to attract electrons, so the greater the electronegativity, the more reactive the Halogen. So, chemical reactivity of Group 17 (VIIA) elements decreases down the Group, from the most reactive (Fluorine) to the least reactive (Iodine).

  • Down the Group, first ionization energy (the energy required to remove 1 electron from the gaseous atom) decreases. As the atomic radius increases and the electron is further from the nucleus it is less attracted to the nucleus (electron is said to be 'shielded').

  • Melting point and boiling point increase down the Group as the elements become more metallic in nature.

  • There is a gradation in color going down the group, the elements become darker in colour as they become more metallic in nature. Similarly there is a gradation in physical appearance at STP, from gas to liquid to solid, as the elements become more metallic in nature.

*Rothe, S. et al. Measurement of the first ionization potential of astatine by laser ionization spectroscopy. Nat. Commun. 4:1835 doi: 10.1038/ncomms2819 (2013).

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