In the basic CsCl (caesium chloride) crystal structure, ions form the corners of a cube and a ion is at the cube’s centre. The edge length of the cube is 0.40 nm. The ions are each deficient by one electron (and thus each has a charge of ), and the ion has one excess electron (and thus has a charge of ). (a) We have to find the magnitude of the net electrostatic force on the ion by the eight ions at the corners of the cube. (b) If one of the ions is missing the crystal is said to have a defect. We have to find the magnitude of the net electrostatic force exerted on the ion by the remaining ions.

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(a)

We not that relative to the ion at the centre of the cubic crystal there are a pair of ions which exert equal and opposite electrostatic forces on the ion. Therefore, the magnitude of the net electrostatic force on the ion due to the eight ions located at the vertices of the cubic lattice will be zero.

(b)

When one of the ions is missing there will be an imbalance of force on the ion. The magnitude of the force on the ion due to the remaining seven ions will therefore be equal to the force that the missing ion would have exerted on the ion.

It is given that the edge length of the cube is 0.40 nm. Therefore, the distance of the centre of the cube from any of the vertices of the cube will be

By Coulomb’s law we calculate the magnitude of the force exerted by two charges e at a distance d from each other. Therefore, the magnitude of the force on the ion due to the remaining seven ions will be