Now we have a good working model of the atom. Since elements can contain very different numbers of electrons then we expect there to be significant differences in the chemical reactivity of atoms.
Some examples follow.
We see that each have 6 electrons and 6 protons, but one has 6 neutrons and one has 7 neutrons. Since each has the same number of electrons, each will behave the same chemically. In the early 1900's Niels Bohr developed a theory for the atom which was based upon the assumption that the electrons circled the nucleus in orbits much like the planets revolve about the sun. This led to further understanding of some concepts, but the basic assumption was incorrect. The current model of the atom was developed independently by Schrodinger and Heisenberg in 1927 and is now called Quantum Mechanics.