3, protons are positive and there are 3 positive atoms visible
Atomic structure applies to just that, atoms in isolation. Witness looking at atomic spectra using laboratory discharge tubes to ionise low pressure gases of atoms of the elements you want to examine for energy level structure (sodium and the yellow D line "doublet").Crystal structure applies to vast collections of atoms/molecules/ions in complicated geometrical arrays which form lattices. Simple cubic, body centred cubic, face centred cubic being a few "simple" examples.
I attached the requested diagram.
<em>In the case of the magnetic field in a bar</em> by convention, the direction of the field is taken out of the north pole and towards the south pole of the magnet. These types of images are commonly made of some ferrous material.
<em>In the case of the horseshoe </em>magnet, the highly concentrated magnetic field is distinguished between its legs. In the figure it is shown in a contribution from North to South, again by agreement, however outside the two poles, the magnetic field falls rapidly. A horseshoe magnet is basically a bent bar magnet.
(a) The efficiency of an engine is defined as the ratio between the work done by the engine and the heat it takes in:

The engine in this problem does a work of

and it takes in

of heat, therefore its efficiency is

(b) The heat taken by the machine is 4000 J; of this amount of heat, only 1100 J are converted into useful work. This means that the rest of the heat is wasted. Therefore, the wasted heat is the difference between the heat in input and the work done by the engine: