A neutral, or stable, atom is composed of an equal amount of three components: protons, neutrons, and electrons. Protons have a positive electrical charge, neutrons are neutral, and electrons have a negative charge. The most simple atom to describe is the hydrogen atom, which has one proton and one neutron that exist within the nucleus. The hydrogen atom also has an electron; which balances the positive charge in the nucleus created by the proton. Elements comprised of atoms having positive charges are the metals: alkali earth, and transition metals. Scientists describe the physical behavior of these elements as the metallic character; which is a comparison of each elements charge, size, and mass. These three variables are generally set for each atom, but may vary yielding two or three electronic states for a particular element (e.g. Iron +2, and Iron +3). Hydrogen atoms, and all metals, seek out electrons to balance that net positive charge. We will discuss the source later. The electron, once found, orbits the nucleus at a distance that represents a balance between the mass of the nucleus (protons + neutrons) and the velocity of the electron; which is exactly like a satellite circling earth. The subsequent neutral orbit is an orbital.
The basic atom, which is hydrogen, may become another element through fusion; which is combining two hydrogen atoms to form one atom and some radioactive byproducts [nucleosynthesis]. During fusion between two hydrogen atoms, 1) one atom gains a proton, 2) the other forms byproducts, 3) a new type of hydrogen (an isotope) called deuterium forms which has two protons. Similar processes fusing hydrogen with other elements sequentially form heavier elements; which have a resultant peroidic change in mass. Chemists refer to this change as a change in mass, because a proton and neutron have measurable masses and their addition to the nucleus makes heavier atoms. Further, it is periodic because progressively heavier elements are created by adding protons to different nuclei one at a time, sequentially starting with the hydrogen atom. The second consequence of adding protons is that the net charge of the nucleus also goes up, therefore as counterbalance electrons are also added to the atom in new orbitals. Chemists reflected the resulting change in mass by organizing the elements from lightest (1) to heaviest (178). However, because adding protons and neutrons is balanced by electrons, changes in geometry and electronegativity also occur.
Electrons are added to the atom in a very periodic manner. Simply put, there is a pattern to it, and the Aufbau Principle shows this pattern in a simple flow chart [AufBau]. The orbitals, in which electrons reside, are pathways bounded by an outer shell that is simply an outer limit in space for those orbitals. Four types of shells exist: s, p, d, and f shells. Each shell has different amount of electron paths, or an electron capacity. The capacities are: s = 2, p = 6, d = 10, and f = 12 electrons. The shells are filled sequentially, (with a slight twist) until thirty electrons exist, then the shell sequence repeats itself again. The repetition reflects the lowest energy state for each successive electron, the stable state mentioned earlier. The shells in the first repetition are referred to as 2s, 2p, and so on, 3s, 3p, and so on in the third repetition. The Aufbau principle allows us to map the addition the electrons, and understand that effect on atomic geometry and electronegativity.
The changes in geometry and electronegativity depend directly on the amount of electrons in the outer shell. The geometry (size and shape) of an atom depends upon the mass of its nucleus, which must increase with addition of protons, the subsequent amount of electrons, and proportion of electrons in the outer shell. The mass to size relationship of a nucleus should be self explanatory. One might assume that because mass is added that size must also necessarily always increase too. But this is not quite true The subsequent amount of electrons may vary about the atom creating charged species (e.g. H+). The resulting species, shrink when charge goes up by the loss of an electron, and expand when the net charge goes down through gaining an electron. The transition metals exhibit this character by having different oxidation states: e.g. Fe++, Fe+++. An increase in the net charge causes the nucleus to exert more force on the remaining electrons with in the outer shell, thus pulling it inward and shrinking the outer limit of the atom. If the outer shell of the atom is full, then that atom is electronegative. If the outer shell is not full, then the degree of electronegativity depends upon which shell is partially filled and to what extent it is filled.
The changes in electronegativity depend directly on which type of shell is the outer shell and the amount of electrons in the outer shell.Species with net negative charges may be considered electron acceptors, and net negative charges as electron donors.