We’ll learn today how to determine the polarity of communication and why it is needed. We will reveal the physical meaning of the quantity in question.
Chemistry and Physics
Once upon a time, all disciplines devoted to the study of the world were united by one definition. And astronomers, and alchemists, and biologists were philosophers. But now there is a strict distribution among the branches of science, and large universities know exactly what mathematicians need to know and what linguists need to know. However, in the case of chemistry and physics there is no clear boundary. Often they mutually penetrate each other, and it happens that they go in parallel courses. In particular, the controversy of communication is a controversial subject. How to determine whether this area of knowledge relates to physics or chemistry? On a formal basis - to the second science: now schoolchildren are studying this concept as part of chemistry, but they cannot do without knowledge of physics.
Atom structure
In order to understand how to determine the polarity of the bond, you first need to remember how the atom is structured. At the end of the nineteenth century, it was known that any atom is neutral in general, but contains different charges in different circumstances. Rutherford found that in the center of any atom is a heavy and positively charged nucleus. The charge of the atomic nucleus is always integer, that is, it is +1, +2, and so on. Around the nucleus there is an appropriate number of light negatively charged electrons, the number of which strictly corresponds to the charge of the nucleus. That is, if the nuclear charge is +32, then thirty-two electrons should be located around it. They occupy certain positions around the core. Each electron is, as it were, “smeared” around the nucleus in its orbital. Its shape, position and distance to the nucleus are determined by four quantum numbers.
Why does polarity arise
In a neutral atom located far from other particles (for example, in deep space, outside the galaxy), all orbitals are symmetrical about the center. Despite the rather complex shape of some of them, the orbitals of any two electrons do not intersect in one atom. But if our individual atom in vacuum encounters another (for example, enters a gas cloud), then it will want to interact with it: the orbitals of valence external electrons stretch toward the neighboring atom, merge with it. There will be a common electron cloud, a new chemical compound and, therefore, the polarity of the bond. How to determine which atom will take most of the total electron cloud for itself will be described below.
What are the chemical bonds?
Depending on the type of interacting molecules, the difference in the charges of their nuclei and the strength of the resulting attraction, the following types of chemical bonds exist:
- single electron;
- metal;
- covalent;
- ionic;
- van der Waals;
- hydrogen;
- two-electron three-center.
In order to wonder how to determine the polarity of a bond in a compound, it must be covalent or ionic (as, for example, in the NaCl salt). In general, these two types of bonds differ only in how much the electron cloud shifts toward one of the atoms. If the covalent bond is not formed by two identical atoms (for example, O 2 ), then it is always slightly polarized. In the ionic bond, the bias is stronger. It is believed that the ionic bond leads to the formation of ions, since one of the atoms "takes" the electrons of the other.
But in reality, completely polar compounds do not exist: just one ion very strongly attracts a common electron cloud. So much so that the remaining piece of balance can be neglected. So, we hope, it has become clear that it is possible to determine the polarity of the covalent bond, and the polarity of the ionic bond makes no sense. Although in this case, the difference between the two types of communication is an approximation, a model, and not a true physical phenomenon.
Determination of the polarity of communication
We hope that the reader has already understood that the polarity of the chemical bond is the deviation of the distribution in space of the general electron cloud from the equilibrium one. And the equilibrium distribution exists in an isolated atom.
Polarity Measurement Methods
How to determine the polarity of the connection? This question is far from unambiguous. To begin with, it must be said that since the symmetry of the electron cloud of a polarized atom differs from that of a neutral one, the x-ray spectrum will also change. Thus, the shift of the lines in the spectrum will give an idea of what the polarity of the bond is. And if you need to understand how to determine the polarity of a bond in a molecule more accurately, you need to know not only the emission or absorption spectrum. Find out:
- sizes of atoms involved in the bond;
- charges of their nuclei;
- what bonds were created at the atom before the occurrence of this;
- what is the structure of the whole substance;
- if the structure is crystalline, what defects exist in it and how they affect the whole substance.
The connection polarity is designated as the upper sign of the following form: 0.17+ or 0.3-. It is also worth remembering that one and the same type of atoms will have an unlike bond polarity in combination with various substances. For example, in BeO oxide, oxygen has a polarity of 0.35-, and in MgO - 0.42-.
Atom polarity
The reader may ask such a question: "How to determine the polarity of a chemical bond, if there are so many factors?" The answer is both simple and complex. Quantitative measures of polarity are defined as the effective charges of an atom. This value is the difference between the charge of an electron located in a certain region and the corresponding region of the nucleus. On the whole, this value rather well shows a certain asymmetry of the electron cloud, which occurs during the formation of a chemical bond. The difficulty lies in the fact that it is almost impossible to determine which region of an electron belongs to this particular bond (especially in complex molecules). So, as in the case of the separation of chemical bonds into ionic and covalent, scientists resort to simplifications and models. At the same time, those factors and values that influence the result slightly are discarded.
The physical meaning of the polarity of the compound
What is the physical meaning of the polarity of the bond? Consider one example. The hydrogen atom H enters both hydrofluoric acid (HF) and hydrochloric acid (HCl). Its polarity in HF is 0.40+, in HCl - 0.18+. This means that the total electron cloud deviates much more toward fluorine than towards chlorine. And it means that the electronegativity of the fluorine atom is much stronger than the electronegativity of the chlorine atom.
Atom polarity in a molecule
But a thoughtful reader will recall that, in addition to simple compounds in which two atoms are present, there are more complex ones. For example, to form one molecule of sulfuric acid (H 2 SO 4 ), it takes two hydrogen atoms, one sulfur, and as many as four oxygen. Then another question arises: how to determine the highest polarity of a bond in a molecule? First you need to remember that any connection has some structure. That is, sulfuric acid is not a heap of all atoms in one big pile, but a certain structure. Four oxygen atoms join the central sulfur atom, forming the likeness of a cross. On two opposite sides, oxygen atoms are attached to sulfur by double bonds. On the other two sides, oxygen atoms are attached to sulfur by single bonds and are “held” on the other side by hydrogen. Thus, in the sulfuric acid molecule, the following bonds exist:
Having determined the polarity of each of these connections from the reference book, one can find the greatest. However, it is worth remembering that if at the end of a long chain of atoms there is a strongly electronegative element, then it can “pull” the electron clouds of neighboring bonds onto itself, increasing their polarity. In a structure more complex than the chain, other effects are quite possible.
How is the polarity of the molecule different from the polarity of the bond?
How to determine the polarity of the connection, we told. What is the physical meaning of the concept, we have revealed. But these words are found in other phrases that relate to this section of chemistry. Surely readers are interested in how chemical bonds and the polarity of molecules interact. We answer: these concepts are mutually complementary and impossible separately. We will demonstrate this with a classic example of water.
In the H 2 O molecule, two identical HO bonds. Between them an angle of 104.45 degrees. So the structure of the water molecule is a kind of double-toothed fork with hydrogen at the ends. Oxygen is a more electronegative atom; it pulls the electron clouds of two hydrogens onto itself. Thus, with general electroneutrality, the prongs of the plug are slightly more positive, and the base is slightly more negative. Simplification leads to the fact that the water molecule has poles. This is called the polarity of the molecule. Therefore, water is such a good solvent, this difference in charges allows the molecules to slightly pull on themselves the electron clouds of other substances, separating the crystals into molecules and the molecules into atoms.
To understand why there is a polarity in molecules in the absence of charge, we must remember: not only the chemical formula of the substance is important, but also the structure of the molecule, the types and types of bonds that arise in it, the difference in the electronegativity of the atoms entering it.
Induced or Forced Polarity
In addition to its own polarity, there is also induced or caused by factors from the outside. If an external electromagnetic field acts on the molecule, which is more significant than the forces existing inside the molecule, then it can change the configuration of electronic clouds. That is, if an oxygen molecule pulls a cloud of hydrogen in H 2 O, and the external field is aligned with this action, then the polarization is enhanced. If the field interferes with oxygen, the polarity of the bond decreases slightly. It should be noted that a sufficiently large effort is required to somehow affect the polarity of the molecules, and even more to affect the polarity of the chemical bond. This effect is achieved only in laboratories and space processes. A conventional microwave only enhances the amplitude of vibrations of water atoms and fats. But this does not affect the polarity of the connection.
In which case does the direction of polarity make sense
In connection with the term that we are considering, it is impossible not to mention what the direct and reverse polarity is. If we are talking about molecules, then the polarity has a plus or minus sign. This means that the atom either gives up its electron cloud and thus becomes a little more positive, or, conversely, pulls the cloud toward itself and acquires a negative charge. And the direction of polarity makes sense only when the charge moves, that is, when current flows through the conductor. As you know, electrons move from their source (negatively charged) to the point of attraction (positively charged). It is worth recalling that there is a theory according to which electrons actually move in the opposite direction: from a positive source to a negative. But in general, it does not matter, only the fact of their movement is important. So, in some processes, for example when welding metal parts, it is important where exactly which poles are connected. Therefore, it is important to know how the polarity is connected: directly or in the opposite direction. In some appliances, even household ones, this also matters.