Answer:
1.33 Å
Explanation:
Given that the edge length , a of the KCl which forms the FCC lattice = 6.28 Å
Also,
For the FCC lattice in which the anion-cation contact along the cell edge , the ratio of the radius of the cation to that of anion is 0.731.
Thus,
.................1
Also, the sum of the radius of the cation and the anion in FCC is equal to half of the edge length.
Thus,
...................2
Given that:

To find,

Using 1 and 2 , we get:

<u>Size of the potassium ion = 1.33 Å</u>
solution:
You need to find the frequency, and they have already given you the wavelength. And since you already know the speed of light, you can use formula (2) to answer this problem. Remember to convert the nano meters to meters because the speed of light is in meters. 

Explanation:
Contributing structures are the resonating structures which are formed due to the delocalization of electrons in a molecule.
The azide ion that is
, is a symmetrical ion, all of whose contributing structures have formal charges.
Lone pair of central nitrogen atom in azide ion is in conjugation with the neighboring nitrogen atoms.
Contributing structures of azide ion are drawn in the image attached.
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The answer is covalent bond