Question

is the central atom always the one in the center wih the least electronegativity?

Answer 1

It depends... In many organic molecules this isn't true. This is most frequently true in inorganic compounds involving alkali or alkaline earth metals and halogens.

In short, anything involving science concepts that says "always" or "never" in it that doesn't involve a mathematical fact or identity of some kind will be incorrect/false. Be very wary of these questions on multiple choice quizzes and tests. Nature frequently has the rare exceptions to the rule.

Answer 2

they ask me to draw this using the lewis structure H2O, NH3, CH4, CO2. I already did it and then I have to draw the actual molecule (the little spheres) and now they are asking me what is the central atom of each of them?

Answer 3

for exaple for h2o I can say that the central atom is oxygen?