Balance.
Balance.
Balance.
Balance
The principle is known as movement.
Balance.
Balance.
Balance.
balance
balance
Balance
symmetrical balance
balance
In the nucleus of the atoms of the elements concerned
I believe the principle of design you are referring to is Balance. This is because the concept of balance is supposed to create a visual equilibrium and it's mainly because our eyes and mind are always trying to focus and create a sense of symmetry and coherence. The reason is because our minds want to work out some stability.
balance
Refer to the related links for more information about 5 elements that are an exception to the Aufbau principle.