Question

Define a unit cell. Which colour is shown by NaCl crystal due to formation of F-centre? Why does fluorine show anomalous behaviour in ‘17 group’ elements?

Hint:

The anomalous properties of the first element in any p-block group (like Fluorine) are a consistent trend. The main reasons are always small size, high electronegativity, and the non-availability of d-orbitals.

Answer 1

Unit Cell and F-centre


Unit Cell Definition: A unit cell is the smallest repeating structural unit of a crystalline solid. When repeated in three dimensions, it generates the entire crystal lattice.
Colour due to F-centre: When an NaCl crystal is heated in an atmosphere of sodium vapour, it develops a yellow colour. This is due to the formation of F-centres, which are anionic sites occupied by unpaired electrons. These electrons absorb energy from the visible region of light and get excited, imparting colour to the crystal.

Anomalous Behaviour of Fluorine
Fluorine, the first member of Group 17, shows anomalous behaviour compared to other halogens. This is due to its:

Small atomic size: It is the smallest atom in its group.
Highest electronegativity: It is the most electronegative element in the periodic table.
Low F-F bond dissociation enthalpy: Due to repulsion between the lone pairs on the small fluorine atoms.
Absence of d-orbitals: In its valence shell, which restricts it to an oxidation state of -1 and prevents it from forming polyhalide ions or expanding its octet.