That is not the only error that you have:
C:\Users\Jacques\Documents\Arduino\sketch_oct01a\sketch_oct01a.ino: In function 'void setup()':
sketch_oct01a:14: error: size of array 'momentumOfparticles' has non-integral type 'float [i]'
float momentumOfparticles[MassOfparticle];
^
sketch_oct01a:17: error: array must be initialized with a brace-enclosed initializer
float MassOfparticle[i] = random(0,10);
^
sketch_oct01a:18: error: 'randomNub' was not declared in this scope
randomPick = randomNub[random(0,10)];
^
sketch_oct01a:21: error: array must be initialized with a brace-enclosed initializer
float VelocityOfparticles[i] = random(0,10);
^
sketch_oct01a:22: error: 'randomNub' was not declared in this scope
randomPick = randomNub[random(0,10)];
^
sketch_oct01a:28: error: invalid operands of types 'float [i]' and 'float' to binary 'operator*'
momentumOfparticles[i] = MassOfparticle * VelocityOfparticles[i];
^
sketch_oct01a:29: error: invalid types 'float[int]' for array subscript
Serial.println(MomentumOfParticles[i]);
^
exit status 1
size of array 'momentumOfparticles' has non-integral type 'float [i]'
Array size has to be constant of type integer:MomentumOfParticles[12]If you initialise the array like you did with particles[], you leave the brackets empty. The compiler will adjust the size of the array according to the number of elements.