The role of the two opposing tendencies in atom organization becomes clear when we study the relationship between the forces that form this entity. Each atom consists of a positively charged nucleus surrounded by negatively charged electrons in adequate number to balance the nuclear charge.

The existence of the atom depends upon forces acting between nucleus and electrons. One group is of coulombian nature. These are the electrostatic forces that account for the attraction between oppositely charged electrons and nuclei, and for the repulsion between electrons bearing similar charges. If such forces did not exist, electrons would wander irregularly and would not be retained around the nucleus.

Yet, if electrostatic forces were unopposed, electrons would be drawn closer and closer to the nucleus and would finally fall into it, thereby bringing about complete annihilation of all charges. The fact that electrons are not absorbed by the nucleus indicates the existence of a second, opposing force.

This second force is defined by quantum mechanics and the quantum theory of fields. Quantum mechanics ascribes a series of discrete energy levels to electrons within atoms. Radiation is emitted or absorbed only when electrons pass from one stationary level to another. The energy levels are relatively stable, and a state of minimum energy exists when each of the electrons of the atom is as close as it can be to the nucleus on the ground level.

The energy levels correspond to the orbits described by Bohr's theory which, although not entirely accurate, affords a good basis for understanding atomic properties. Bohr envisaged the electrons revolving around the nucleus in definite orbits, each orbit moving continuously in these states, the atom not emitting radiation. This differs from the older theory according to which the electrons can revolve around the nucleus on any orbit. Such casual orbit motions would lead to loss of energy by radiation. The electrons would come closer and closer to the nucleus and would, as already pointed out, finally be absorbed by it. The quantum theory of fields accounts for the absence of radiation and for electrons remaining in their particular orbits. However, the concept of stationary states fails to explain all the properties of the atom, particularly its chemical reactivity, by virtue of which different atoms combine to form molecules.

According to another tenet of the quantum theory, the Pauli Exclusion Principle, an orbit cannot be occupied by an indefinite number of electrons but, at most, by two electrons that spin in opposite directions. The orbits are arranged in shells, each shell having a definite level of energy. A shell is complete when it contains the maximum number of electrons compatible with the Pauli Principle. Complete shells consist of 2, 8, 18, etc., electrons. When an inner shell has its quota of electrons, additional electrons must occupy an outer shell. Consequently, instead of falling into the nucleus, the electrons in their lowest energy states will continue to revolve at a considerable distance from the nucleus.

As already indicated, if there were only electrostatic forces, the electrons would have long since fallen into their nuclei, neutralizing all electric charges. The universe would be in a state of maximum homotropy. No strong atomic forces would exist and no chemical reactions would take place. The intervention of quantum forces avoids this. It is apparent, then, that the organization of the atom results from the operation of two types of forces, electrostatic and quantum, the electrostatic serving to bring and keep nucleus and electrons together to constitute the atom, the quantum accounting for a motion of electrons which prevents their total annihilation and the neutralization of all electrical charges.

Homotropic And Heterotopic Forces In The Atom

We may now attempt to consider electrostatic and quantum forces in the atom in terms of homotropic and heterotropic trends. Let us hypothesize an atomic system in which only electrostatic forces are active and compare it with a real system which also has active quantum forces. Whereas the fictitious system will rapidly evolve towards a state of maximum homotropy, with annihilation of all charges, this will not occur in the real system. When the two systems have reached final states of equilibrium, the homotropy of the imaginary system will be greater than that of the real system. If the quantum forces that keep the electrons away from the nucleus in the real atom could be withdrawn, the electrostatic forces acting alone would bring about a state of complete annihilation, thus making available a certain amount of energy that had previously been preserved by the quantum forces. In this sense, it is apparent that the electrostatic attraction between nuclei and electrons is of a homotropic character, while quantum forces are heterotropic.