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

Answer: (a) Smallest repeating unit of a crystal lattice; NaCl turns yellow. (b) Small size, highest electronegativity, low F–F bond energy, no d orbitals.

(a) Unit cell: The smallest repeating structural unit of a crystalline solid which, when repeated in three dimensions, generates the entire crystal lattice.

When NaCl is heated in sodium vapour, anion vacancies trap electrons (F-centres); these absorb visible light and NaCl appears yellow.

(b) Fluorine differs from the other halogens because:

1. it has the smallest atomic and ionic size;
2. it has the highest electronegativity (4.0);
3. the F–F bond dissociation enthalpy is low (due to repulsion between lone pairs on small atoms), making F₂ very reactive;
4. it has no vacant d orbitals in its valence shell, so it cannot expand its octet and shows only the −1 oxidation state.

Hence HF is a liquid with hydrogen bonding and a weak acid, and fluorine forms only one oxoacid (HOF).