l. NaCl and KCl show metal excess defect due to anionic vacancies.
II. F-centres are the anionic sites occupied by unpaired electrons.
III. F-centres is derived from the German word 'Farbenzenter' for colour centre.
IV. f-centres impart yellow colour to the NaCl crystals.
Which of the above statement(s) is/are correct?

Alkali metals solid like KCl and NaCl show non-stoichiometric defect. When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The Cl^- ions get diffused to the surface of the crystal and combine with Na atoms to give NaCl. This happens by loss of electron by Na atoms to form Na⁺ ions. The released electron diffused into the crystal and occupy anionic sites. Hence, the crystal now has anionic sites. Therefore, the crystal now has an excess of  sodium. These anionic sites occupied by unpaired electrons are called,F-centres (derived from the German word Farbenzenter for colour centre). These F-centres impart yellow colour to the NaCI crystal.