The entire mass of the atoms is centered at the nucleus of the atom, so it becomes heavy and has a positive charge (+).
Why are the central atoms shown as positively charged?
Due to the havey mass of the central atom!
In centre of atoms there is nucleus and in nucleus,there are protons(positively charged) and neutrons(neutral).mass of protons are 1.00782 amu and mass of neutrons are 1.00836 amu.Due to positivity of protons,it is positively charged.