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Home  / GENERAL CHEMISTRY Textbook / Chapter 12. Periodic Law

Chapter 12. Periodic Law

Mendeleev systematized the chemical elements by grouping them according to chemical and physical properties. The periodic law formulated by Mendeleev in 1869 states that the properties of chemical elements are not arbitrary but are in periodic dependence on their atomic mass. According to the scientific thinking of those years, the proposed systematization looked quite paradoxical. 

After the discovery of atomic structure and ascertainment of the fact that atomic mass is concentrated in the nucleus of the atom, the periodic law's paradoxicality further increased. Research on the ionization potential of elements showed that the nuclei of atoms are surrounded by layers of electrons and that the electrostatic interactions between electrons and nuclei exceed the mass ones by a factor of 1040. As a result of the work of Moseley, the periodic law was finally defined as "the nuclear charge is the most important property of an element that determines its chemical properties" (see R. Dickerson, H. Gray, G. Haight, Basic Principles of Chemistry, v.1, p.312).

After the discovery of the periodic law and the adoption of the Lewis rules by the scientific community, ascertaining the physical meaning of the discovered laws and rules became a priority, as did the answers to the following questions: 

Why does charge determine the chemical properties of elements, as well as complex substances which are formed from these elements?

Why do the properties of elements change periodically while the most important property - the nuclear charge - increases linearly?

What is the mechanism of the nuclear charge's influence on the chemical and physical properties of elements?

Why is the periodicity of changing of the physical and chemical properties for the 2nd and3rd periods 8 elements, and why does a basic chemical property of elements - valence - have the periodicity of 4 elements?

However, instead of finding answers to these questions, the main forces of the scientific society after Moseley's discovery were used to confirm the correctness of the periodic law. During studies, it was shown that increasing the nuclear charge causes the number of electrons in the outer layer of atom, atomic radius, the element valence according to hydrogen and the first ionization energy to change periodically, along with almost all other physical and chemical properties of not only the elements themselves but even, for example, of their compounds with hydrogen and oxygen. Famous scientists L. Nilsson, P. Lecoq de Boisbaudran and K. Winkler who discovered, respectively, scandium, gallium and germanium, which were predicted by Mendeleev on the basis of the periodic law discovered by him, were called by Mendeleev "strengtheners" of the periodic law.

The traditions formed in chemistry after the discovery of the periodic law, the philosophical debates about reductionism, and, most importantly, the paradoxicality of this law are some of the possible explanations for the enthusiasm of the scientific society in "strengthening" the periodic law without elucidating its physical nature. On the other hand, during the "quantum-mechanical euphoria," Descartes announced "... that the physical meaning of all chemical laws is clarified in the framework of quantum mechanics, and the difficulty is only that the strict application of these laws leads to such complex equations that it is impossible to solve them." In other words, the return to the question of the physical meaning of the periodic law became possible only when a period of "quantum-mechanical disappointment" came, in the 80th years of the last century.

In order to clarify the physical meaning of the periodic law, we used the same experimental data that "strengthened" it. From the experimental data mentioned above we have selected three properties of atoms which change periodically with an increasing in nuclear charge. These properties are the first ionization potential, data on the electron affinity and the number of electrons in the outer layer of atoms. The periodicity of changing of these atom properties coincided with the periodicity of changing of the chemical and physical properties of elements (for example, for the 2nd and the 3rd periods - 8), indicating that these three properties of atoms determine the chemical and physical properties of elements and compounds formed from them. We looked for answers to the following questions: 

1) Why do these three properties of atoms change periodically? 

2) How do these three characteristics influence the chemical properties of the elementary substances and complex substances derived from them?

A phenomenological answer to the first question came from the experimental data on the ionization energy, which showed that the internal electron layers of all elements contain the same number of electrons (two electrons in the layer which is the closest to the nucleus and eight electrons in the each of the other inner layers). Correspondingly, the increase in the number of electrons in atoms which occurs with the increasing of nuclear charge (because the number of electrons in an atom is equal to the number of protons in the nucleus) leads to the  number of electrons in the high layer of atoms with a linear increase of nuclear charge changing periodically. The increase of nuclear charge of an atom per proton unit with the simultaneous attaching of an electron to the outer electron layer occurs with an energy gain. The approach of a new electron to the nucleus of an atom creates a gain in energy but simultaneously expends energy to allow the electron to approach the electrons located in the atom, starting with the electrons of the outer layer. The largest energy gain occurs when a new electron enters an existing outer layer of an atom. Bohr showed that the model in which electrons surrounding the nucleus revolve around it in concentric circles lying in the same plane, has an analytical solution. This model is examined in the book "How Chemical Bonds Form and Chemical Reactions Occur" (p.28). According to the model, an electron can join a hydrogen atom in an existing high layer, and but cannot do the same with a helium atom, as observed in the experiment: the affinity of the helium atom for the electron has a negative value.

Our calculation of the Bohr model showed that an electron will join an atom with a nuclear charge of 3 proton units, not in its existing outer layer but by beginning to form a new outer electron layer, which is observed in the experiment. The calculations assumed that the electrons and nuclei are particles between which Coulomb forces are exerted. The quantitative match-up of the theory and the experiment is the principal explanation of the physical nature of the layered structure of the atom and, correspondingly, the physical nature of the periodic change of the number of electrons in the outer layer of atoms and the periodic change of the ionization potentials and electron affinities of atoms for an electron (see Chapter 4. Chemical bond).

After clarification of the physical nature of the periodic change of the number of electrons in the outer layer, the time came to find out why the valence of the elements of the second and third periods changes with a period of 4 elements, while the number of electrons in the high layer and properties depending on it (the first ionization potential and affinity energy) change with a period of 8 elements. We were able to answer the last question in the course of clarifying the physical nature of the formation of covalent chemical bonds (Chapter «G - theory of chemical bonding").

The physical meaning of the valence rules

We have found that the valence is determined by the number of electrons in the outer layer of the element and by the number of electrons which can be taken by this atom into already-existing electron shells with an energy gain. According to the G-theory of chemical bonding:

  1. Only the electrons located on the outer electron shell of bonded atoms participate in the formation of chemical bonds.
  2. Only one electron of the outer shell can form one bond.
  1. Two electrons - one from each atom - participate in the formation of chemical bonds between two atoms. These two electrons are bonding electrons.
  2. After bond formation, both bonding electrons turn out to be in the outer shells of bonded atoms. Therefore, in the course of the bond's formation, the number of electrons in the already-existing outer shell of bonded atoms increases by one electron.
  3. The number of chemical bonds which can be formed by an atom may not be less than the number of electrons located in the outer shell of this atom. For the atoms of the second and third periods, this restriction exists for the atoms with less than 5 electrons in the outer shell, i.e. to atoms of Li, Na, Be, Mg, B, Al, C and Si.
  4. The number of bonds which can be formed by atoms of the second and third periods may not be greater than the number of electrons located in the outer shells of these atoms. According to the first ionization energy data, the maximum number of electrons that can be on the outer shells of atoms of the second and third periods is 8. This restriction holds for atoms with more than 4 electrons in the outer shell, i.e. to atoms of N, P, O, S, F, Cl, Ne and Ar and, is described by the size 8.

The physical meaning of the periodic law

Physical meaning is a causal relationship between phenomena. The periodic law is paradoxical because it seemed impossible even to suppose any causal relationship that could explain the periodic dependence of the physical and chemical properties of elements and the compounds formed from them on the charges of their nuclei surrounded by a swarm of electrons.

Why do chemical and physical properties change periodically?

Physical properties of substances are determined by bonds between molecules, and chemical properties are determined by the reactive capacity of molecules. In the framework of the theory of elementary interactions, it is established that the chemical properties of substances and their reactive capacity are determined by the difference in energies of the bond which is disrupted in the course of reaction and the bond which is formed in the course of the reaction. After clarifying the physical nature of chemical bonding, it was found that the chemical bond energy is determined by the first ionization potential of the atom. Since the first ionization energy changes periodically, chemical and physical properties of substances also change periodically (see Chapter "Chemical reactions").

Knowledge of the nature of chemical bond formation (G-theory of chemical bond), the nature of the valence and theory of elementary interactions have led to an understanding of the physical meaning of the periodic law.