Inter Atomic Force As A Function Of Nucleic Spacing
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Inter Atomic Force As A Function Of Nucleic Spacing

Interatomic Force
The bonding force F, between atoms may be expressed approximately as follows:

F(r) = A/rM - B/rN (N > M) -----------(1)
Where r, is the center-to-center spacing between atoms and A, B, M, and N are constants that vary according to the type of bond.
A/rM represents the attractive force while B/rN represents the repulsive force.

(a) Express the equilibrium spacing r0 in terms of A, B, M, and N.
(b) Derive another form for F(r) in which the only constants are r0, A, M, and N.
(c) From the equation for F(r) derived in (b) calculate the following:
(i) The spacing r1 for which F is maximum
(ii) The value of the maximum force Fmax.

The strings:

(a) S7P7A72 (Dynamic - Equilibrium)

(b) S7P3A31 (Force - Pull)

(c)(i) S7P1A12 (Containership -Length)
(c)(ii) S7P3A31 (Force-Pull)
The math:
Interatomic Force
Pj Problem of Interest is of type force (electrostatic). Problem is primarily of type force since interatomic force underlies problems (a), (b) and (c).

F(r) = A/rM - B/rN (N > M) -----------(1)

At the equilibrium spacing r0, the resultant interatomic force is zero, that is the attractive force balances the repulsive force.
So, F(r0) = A/(r0)M - B/(r0)N = 0
So, A/(r0)M = B/(r0)N

So,(r0)N-M = B/A -------(2)

So, equilibrium spacing, r0 = (B/A)(1/N-M)-------(3)

From equation (2) B = A(r0)N-M

Let A(r0)N-M replace B in equation (1) :

Then, F(r) = A/rM[1 - ((r0)N-M/rN-M)]-------(4)

(c)(i) Let r1 be the spacing for which F is maximum
Then the derivative of F at r1, dF(r1)/dr = 0.

Using F(r) of equation (4):

dF(r)/dr = -MAr-(M+1) + NA(r0)N-M(r-(N+M))

dF(r1)/dr = 0, implies:

NA(r0)N-M((r1)-(N+M)) = MA(r1)-(M+1)

So, (N/M)(r0)N-M = [(r1)-M]/[(r1)-N]

So,(N/M)(r0)N-M = (r1)N-M

So, The spacing r1, for which F is maximum:

r1 = r0(N/M)(1/N-M).

(c)(ii) Substitute the expression for r1 in equation 4 to get the expression for the value of maximum Force, Fmax.

The attractive forces in interatomic bonds are primarily electrostatic. So, the expression A/rM for the attractive force is usually assumed to be the same as the forces between electric charges. Consequently, Coulomb's Law applies and the force is inversely proportional to the square of the spacing. So M is usually = 2. The value of N is more dependent on the type of bond. For metallic bonds, N ranges from 7 to 10. For ionic and covalent bond, N ranges from 10 to 12.


The point . is a mathematical abstraction. It has negligible size and a great sense of position. Consequently, it is front and center in abstract existential reasoning.
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