There would be 6 electrons placed on the third energy level.
A advantage is less car accidents and a disadvantage is, in order to keep the roads clear of ice is a chemical. Like salt. Which is bad for the animals.
The momentum p of a moving particle is the product between its mass, m, and tis velocity, v:

In our problem, we know

and

, and using the relationship mentioned above, we can find the mass m of the particle:
It is an example of liquid. if thats what you are asking for...
Answer:
From the second law of motion:
F = ma
we are given that the force applied on the block is 20N and the block accelerates at an acceleration of 4 m/s/s
So, F= 20N and a = 4 m/s/s
Replacing the variables in the equation:
20 = 4* m
m = 20 / 4
m = 5 kg