In a Lewis diagram, how do I know which one to put into the middle? Please give a detailed explanation! Thanks so much!
In general put the element that has the most electrons that are to be shared in the middle, this is usually the element with the lowest electronegativity.
For example in the lewis diagram of Sulfuric acid the Sulfur should be put in the middle. All six valance electrons of the Sulfur will be at least partially transferred to the four Oxygen atoms. So the Sulfur should be draw in the middle surrounded by 4 Oxygen atoms.
The two hydrogen atoms are then attached to the 2 Oxygen atoms that get only 1 electron from the sulfur. ( note there are multiple ways of arranging the electrons or resonance structures )