Question

can anyone help me in explaining the metal excess defect in Non stoichiometric defects? i fail to understand it

Answer 1

i gather you know what structure of metals is, so look at it this way you take a huge pot and put in there lot of marbles, they will arrange in that matter so they occupy most of the space they can, and you will see that you have a vacant space between them. in that space you can put a smaller marble without destroying the structure they made, but also you can take one marble out and put in there a slightly bigger or smaller marble... if you want more explaining just ask...

Answer 2

Metal Excess Defect
.. Metal excess defect due to anionic vacancies: Alkali halides
like NaCl and KCl show this type of 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 diffuse to the surface of the crystal and
combine with Na atoms to give NaCl. This happens by
loss of electron by sodium atoms to form Na+ ions. The
released electrons diffuse into the crystal and occupy
anionic sites (Fig. 1.28). As a result the crystal now has
an excess of sodium. The anionic sites occupied by
unpaired electrons are called F-centres (from the German
word Farbenzenter for colour centre). They impart yellow
colour to the crystals of NaCl. The colour results by
excitation of these electrons when they absorb energy from
the visible light falling on the crystals. Similarly, excess of
lithium makes LiCl crystals pink and excess of potassium
makes KCl crystals violet (or lilac).

Answer 3

this is how it is explained in my book..can you explain on the basis of this @Kryten

Answer 4

forget fig 1.28...

Answer 5

what i gather in the first sentence it says you have anionic vacancies! so you deposit excess of sodium on the surface which creates a great positive potential, lets call it a need of sodium to get rid of it one electron and you have lets say an equilibrium in your NaCl crystal, so Cl will go to the surface cause there is an concentrational gradient in the crystal now and when Cl leaves his spot in structure it creates a vacancy which is a hole in structure dont know if this helped more...

Answer 6

okay so the sodium ion initially was inside the solid and when it went to the surface of solid, it turned into atom, how is that possible?

Answer 7

no it says that you deposit sodium on the surface so its from other source

Answer 8

oh so we deposit sodium ATOMS on the surface?

Answer 9

yap! it says that surface is treated with SODIUM VAPOR!

Answer 10

then why does Cl- go to the surface? that sodium atom is neutral..

Answer 11

cause sodium atom has a great need to get rid of his one electron and Cl ion is greatly attracted to such great supplier of electrons, on surface there is an excess of electrons which sodium wishes to get rid of (many sodium atoms) and chlorium must share its electrons in crystaline structure which he does not have to if he goes to surface, so the driving force is difference of potentials and when he goes to surface he leaves a blank spot

Answer 12

okay and can you help me out with this-
Metal excess defect due to the presence of extra cations at
interstitial sites: Zinc oxide is white in colour at room
temperature. On heating it loses oxygen and turns yellow.
heating 2
2
1
ZnO Zn O 2e
2
..... + + + -
Now there is excess of zinc in the crystal and its formula becomes
Zn1+xO. The excess Zn2+ ions move to interstitial sites and the electrons
to neighbouring interstitial sites.

Answer 13

oops something went wrong in copy paste..:)

Answer 14

the reaction mentioned above is ZnO ----> Zn 2+ + 2e- + 1/2 O2

Answer 15

on heating, it becomes