Question

a proper defination of "distance of closest approach."

Answer 1

The distance of closest approach of two objects is the distance between their centers when they are externally tangent. The objects may be geometric shapes or physical particles with well defined boundaries. The distance of closest approach is sometimes referred to as the contact distance.

Answer 2

Answer:

The minimum distance between the center of the nucleus & the alpha particle just before it gets reflected back through 180 ° is defined as the distance of closest approach r• ( also known as contact distance ).