According to Dalton’s atomic theory, what are elements and what makes one element different from another?

Dalton was a chemist who discovered the “law of multiple proportion” in chemical reaction in early nineteenth century. For example, element of tin, he observed, interacted with oxygen to produce two different masses but in proportion 1:2. (Now, this is known to correspond to the formation of tin-monoxide and tin-dioxide, with one and two atoms of oxygen, respectively.)

From such and related studies, he proposed his theory of chemical elements: (a) that all elements are composed of ‘atoms’, the hypothetical indivisible constituents of matter of the ancient Greek thinkers, like, Democritus, and (b) that the elements differ from one another due to the atoms of different masses and sizes that constitute an element.

The modern theory of chemistry says that an element consists of an atom that consists of a heavy nucleus with positively charged ‘protons’ (and most often also the neutral particles called ‘neutrons’) at its center and, an equal number of negatively charged (and much lighter) ‘electrons’, distributed around it in discrete energy shells. Further, the elements differ from one another by the number of positively charged protons in their nucleus, the so-called atomic number, Z, of an atom. Finally, molecules are formed as well as chemical reactions take place, through the exchange or transfer of the electron(s) in the outermost shells of the reacting atoms (and/or molecules).