Geometric distribution is a special case of Pascal distribution when r= 1 In Bernoulli's experiment, the probability of success is p, if ξ represents the number of successful occurrences for the first time, ξ is a discrete random variable, and only a positive integer is taken, and the power of (k- 1) of p (ξ = k) is multiplied by p (k= 1). P< 1), the random variable ξ is said to obey the geometric distribution. Its expectation is 1/p, and its variance is (1-p)/(the square of p).

The state of electron motion is described by wave function ψ, |ψ |? It represents the probability of electrons appearing in a unit volume somewhere in the extranuclear space, that is, the probability density. Electrons in different motion states have different | ψ |. Of course not. The higher the density, the more events occur, and vice versa.

If the density of black dots is used to represent the probability density of each electron, then |ψ|? The bigger and denser the black spots, the higher the probability density, and vice versa. The small black dots distributed outside the nucleus are like a negatively charged cloud, surrounding the nucleus. People call it an electronic cloud.