Which Halogen Is the Darkest?

Astatine is the darkest halogen.

Halogens, elements found in Group 17 of the periodic table, exhibit a fascinating trend in their physical properties, including their color, as one descends the group. This trend sees the elements become progressively darker, moving from gases to liquids to solids at room temperature.

The Darkest Halogen: Astatine

While many are familiar with common halogens like chlorine and iodine, it is astatine (At) that holds the distinction of being the darkest. Positioned directly below iodine in Group 17, astatine is a highly radioactive and rare element.

• Color Trend Down the Group: As you move down Group 17, the halogens generally display a deepening of color:
  • Fluorine (F₂): Pale yellow gas
  • Chlorine (Cl₂): Greenish-yellow gas
  • Bromine (Br₂): Reddish-brown liquid
  • Iodine (I₂): Shiny grey-black solid crystals, which produce a purple vapor when sublimed.
  • Astatine (At): Predicted and observed to be a black solid.

The increased number of electrons and the stronger intermolecular forces (London dispersion forces) as atomic mass increases down the group lead to this progression from light-colored gases to darker, more dense solids. Astatine's position at the bottom of the naturally occurring halogens in Group 17 aligns with this trend, making it the one with the deepest color.

Halogen Colors Overview

To illustrate the color variation among halogens, consider the following table:

The transition from the pale yellow of fluorine to the black of astatine clearly demonstrates how the color intensifies with increasing atomic number within the halogen group.

For more information on the properties of halogens, you can explore educational resources such as this overview of Group 17 elements.