Atoms can have more electrons than protons, resulting in anions, while cations have fewer electrons than protons. The number of electrons in a neutral atom is equal to the number of protons, represented by the atomic number (Z). The electron configuration of an atom, determined by the Aufbau principle, dictates its position in the periodic table. In the examples discussed, atom A with 15 protons and 18 electrons is not in the same group as atom B with 20 protons and 18 electrons, as they belong to different periods due to their differing proton counts. However, both atoms can have the same electron configuration, with the binding of electrons to the nucleus varying based on the number of protons. Atoms C and D, both with 13 protons and 13 electrons but differing neutron counts, are identified as isotopes of the same element.
