Atoms have a nucleus with protons and neutrons. There are rings of electrons circling the nucleus. Electrons have a negative charge, protons have a positive charge, and neutrons have a neutral charge. The amount of electrons on one ring depends on how many rings there are. For example, the inner most ring of an atom with three rings has the maximum of two electrons, the middle ring has a max of eight electrons, the third ring has a max of 18 electrons. All atoms strive to have their outer most ring have eight electrons. That is why they bond to other elements, like how sodium bonds too chlorine because sodium's outer ring has one electron and chlorine's outer ring has seven electrons, forming sodium chloride. Also, the thing that determines elements from one another is its amount of protons. For example, carbon has six protons and nitrogen has seven protons. I hope this helped