How to determine the polarity of the connection? Forward and reverse polarity

We will learn today how to determine the polarity of the connection and why this is necessary. We will reveal the physical meaning of the quantity under consideration.

Chemistry and Physics

Once upon a time, all the disciplines devoted to the study of the world around us were united by one definition. And astronomers, alchemists, and biologists were philosophers. But now there is a strict distribution across sections of science, and large universities know exactly what mathematicians need to know, and what - to linguists. However, in the case of chemistry and physics there is no clear boundary. Often they mutually penetrate each other, and sometimes they go parallel courses. In particular, the polarity of the connection is a controversial object. How to determine whether this area of knowledge is related to physics or chemistry? According to a formal indication - to the second science: now schoolchildren are studying this concept as part of chemistry, but without knowledge of physics they can not do.

The structure of the atom

In order to understand how to determine the polarity of the connection, we first need to recall how the atom is constructed. At the end of the nineteenth century, it was known that any atom is neutral in general, but contains different charges in different circumstances. Rezerfod established that at the center of any atom is a heavy and positively charged core. The charge of an 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 nuclear charge. That is, if the charge of the nucleus is +32, thirty-two electrons should be around it. They occupy certain positions around the core. Each electron is "smeared" around the nucleus on its orbitals. Its shape, position and distance to the nucleus are determined by four quantum numbers.

Why polarity arises?

In a neutral atom located far from other particles (for example, in deep space, outside the galaxy), all orbitals are symmetric with respect to the center. Despite the rather complicated form of some of them, the orbitals of any two electrons do not intersect in one atom. But if our separately taken atom in vacuum meets another (for example, it enters a gas cloud), then it will want to interact with it: the orbitals of the valence external electrons will stretch towards the neighboring atom and merge with it. There will be a common electronic cloud, a new chemical compound and, consequently, a polarity of the bond. How to determine which atom will take a large part of the total electronic cloud, we will tell further.

What are the chemical bonds

Depending on the type of interacting molecules, the difference in the charges of their nuclei and the force of the emerging attraction, there are the following types of chemical bonds:

• One-electron;
• Metal;
• Covalent;
• Ionic;
• Van der Waals;
• Hydrogen;
• Two-electron three-center.

In order to ask how to determine the polarity of a bond in a compound, it must be covalent or ionic (as, for example, NaCl salt). In general, these two types of connection differ only in how much the electronic cloud moves 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 ionic bonding, the shift 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 fact, there are no fully polar compounds: just one ion attracts a general electronic cloud to itself. So strong that the remaining piece of balance can be neglected. So, we hope it became clear that the polarity of the covalent bond can be determined, and the polarity of the ionic bond does not make sense. Although in this case the difference between these two types of connection is an approximation, a model, and not a true physical phenomenon.

Determining the polarity of the connection

We hope that the reader has already understood that the polarity of the chemical bond is the deviation of the distribution in the space of the general electron cloud from the equilibrium one. And the equilibrium distribution exists in an isolated atom.

Methods for measuring polarity

How to determine the polarity of the connection? This question is far from straightforward. To begin with, we must say that since the symmetry of the electron cloud of a polarized atom differs from a similar neutral one, the X-ray spectrum will also change. Thus, the displacement of the lines in the spectrum will give an idea of the polarity of the bond. And if you want 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. It is required to find out:

• The dimensions of the atoms participating in the bond;
• Charges of their nuclei;
• What ties were created at the atom before the emergence 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 polarity of the connection is denoted as the upper sign of the following form: 0.17+ or 0.3-. It is also worth remembering that the same kind of atoms will have a different polarity of connection in connection with different substances. For example, in Oxide BeO, oxygen has a polarity of 0.35, and in MgO it has a polarity of 0.42.

The polarity of the atom

The reader can also ask such a question: "How to determine the polarity of the chemical bond, if there are so many factors?" The answer is both simple and complex. Quantitative measures of polarity are defined as effective charges of the 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 sufficiently well shows some asymmetry of the electron cloud, which arises when a chemical bond is formed. The difficulty is that it is almost impossible to determine exactly which area of the electron's location belongs precisely to this bond (especially in complex molecules). So, as in the case of the separation of chemical bonds into ionic and covalent bonds, scientists resort to simplifications and models. In this case, those factors and values that affect the result are negligible.

The physical meaning of the polarity of the compound

What is the physical meaning of the polarity of the connection? Let's consider one example. The hydrogen atom H is included both in hydrofluoric acid (HF) and in hydrochloric acid (HCl). Its polarity in HF is 0.40 +, in HCl it is 0.18+. This means that the total electronic cloud is much more deflected towards fluorine than towards chlorine. This means that the electronegativity of the fluorine atom is much stronger than the electronegativity of the chlorine atom.

The polarity of an atom in a molecule

But the thoughtful reader will remember that, in addition to simple compounds in which two atoms are present, there are also more complex ones. For example, to form one sulfuric acid molecule (H ₂ SO ₄ ), two hydrogen atoms are required, one is sulfur, and as many as four oxygen. Then another question arises: how to determine the greatest polarity of the bond in the molecule? To begin with, we must remember that any connection has some structure. That is, sulfuric acid is not the accumulation of all atoms in one large heap, but a certain structure. The central atom of sulfur is joined by four oxygen atoms, forming a semblance of a cross. On two opposite sides, oxygen atoms join to sulfur by double bonds. On the other two sides, the oxygen atoms are attached to the sulfur by single bonds and "held" on the other side by hydrogen. Thus, in the molecule of sulfuric acid there are the following links:

• OH;
• SO;
• S = O.

Having determined by the directory the polarity of each of these links, 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" on itself electronic clouds of neighboring bonds, increasing their polarity. In a structure more complex than the chain, other effects are quite possible.

Why 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 uncovered. But these words are also found in other phrases that relate to this section of chemistry. Probably readers are interested in how chemical bonds and the polarity of molecules interact. We answer: these concepts mutually complement each other and are impossible separately. This we demonstrate on the classic example of water.

In the molecule H ₂ O, two identical HO bonds. Between them an angle of 104.45 degrees. So the structure of the water molecule is something like a two-pronged plug with hydrogen at the ends. Oxygen is a more electronegative atom, it draws on itself electron clouds of two hydrogen. Thus, with a general electroneutrality, the fork teeth are slightly more positive, and the ground 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 molecules to slightly pull on themselves electronic clouds of other substances, separating the crystals into molecules, and molecules to atoms.

To understand why there is a polarity in the absence of charge, one must remember: it is important not only the chemical formula of matter, 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 into it.

Induced or forced polarity

In addition to its own polarity, there is also an 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 is able to change the configuration of the electron clouds. That is, if the oxygen molecule pulls the clouds of hydrogen in H ₂ O, and the external field is co-directed with this action, the polarization is amplified. If the field seems to interfere with oxygen, then the polarity of the connection decreases slightly. It should be noted that it takes a lot of effort 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 in space processes. An ordinary microwave oven only amplifies the amplitude of vibrations of water and fat atoms. But this does not affect the polarity of the connection in any way.

In which case is the direction of the polarity

In connection with the term that we are considering, we can not fail to mention what is the direct and reverse polarity. If we are talking about molecules, then the polarity has a plus sign or a minus sign. This means that the atom either gives up its electronic cloud and thus becomes a little more positive, or, on the contrary, pulls the cloud towards itself and acquires a negative charge. And the direction of polarity makes sense only when the charge moves, that is, when a current flows through the conductor. As is known, electrons move from their source (negatively charged) to the place 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 one. 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 attached. Therefore, it is important to know how the polarity is connected: directly or in the opposite direction. In some appliances, even household appliances, this also matters.