Ion bond. Method of formation and properties
The electrons from one atom can go completely to another. Such a redistribution of charges leads to the formation of positively and negatively charged ions (cations and anions). A special type of interaction arises between them - the ionic bond. Let us consider in more detail the method of its formation, the structure and properties of substances.
Atoms differ in electronegativity (EO) - the ability to attract electrons from the valence shells of other particles. For quantitative determination, the proposed scale of relative electronegativity proposed by L. Polling (dimensionless quantity) is used. Stronger than other elements, the ability to attract electrons from fluorine atoms is expressed, its EO is 4. In the Polling scale, oxygen, nitrogen, and chlorine immediately follow fluorine. The values of EO of hydrogen and other typical non-metals are equal to or close to 2. Most of the metals have electronegativity from 0.7 (Fr) to 1.7.There is a dependence of the ionicity of the bond on the difference between EO chemical elements. The larger it is, the higher the likelihood that an ionic bond will arise. This type of interaction is more common when the difference EO = 1.7 and above. If the value is less, then the compounds are polar covalent.
For the detachment of external electrons weakly bound to the nucleus, ionization energy (EI) is necessary. The unit of change of this physical quantity is 1 electron volt. There are regularities in the change of EI in the rows and columns of the periodic system, depending on the increase in the nuclear charge. In the periods from left to right, the ionization energy increases and acquires the greatest values for nonmetals. In groups, it decreases from top to bottom. The main reason is an increase in the radius of the atom and the distance from the nucleus to external electrons, which easily detach. A positively charged particle arises - the corresponding cation. The magnitude of the EI can be judged on whether an ionic bond occurs. Properties also depend on ionization energy. For example, alkali metals and alkaline earth metals have small EI values. They have pronounced restorative (metal) properties.Inert gases are chemically inactive due to their high ionization energy.
In chemical interactions, atoms can attach electrons to form a negative particle, the anion, the process is accompanied by the release of energy. The corresponding physical quantity is the affinity for the electron. The unit of measurement is the same as the ionization energy (1 electron-volt). But its exact values are not known for all elements. Halogens have the highest electron affinity. At the outer level of the atoms of the elements - 7 electrons, lacking only one to the octet. The electron affinity for halogens is high, they have strong oxidative (non-metallic) properties.
The interaction of atoms in the formation of ionic bonds
Atoms with an incomplete external level are in an unstable energy state. The desire to achieve a stable electronic configuration is the main reason that leads to the formation of chemical compounds. The process is usually accompanied by the release of energy and can lead to molecules and crystals that differ in structure and properties.Strong metals and nonmetals differ significantly among themselves in a number of indicators (EO, EI, and electron affinity). For them, a type of interaction, such as an ionic chemical bond, in which a unifying molecular orbital (a common electron pair) moves, is more suitable. It is believed that when ions are formed, metals completely transfer electrons to non-metals. The strength of the bond that has arisen depends on the work required to destroy the molecules that make up 1 mole of the test substance. This physical quantity is known as binding energy. For ionic compounds, its values range from several tens to hundreds of kJ / mol.
An atom that donates its electrons during chemical interactions turns into a cation (+). The host particle is the anion (-). To find out how atoms will behave, whether ions will arise, it is necessary to establish the difference between their EO. The easiest way to carry out such calculations for compounds of two elements, for example, sodium chloride.
- EO (Cl) = 3.16;
- EO (Na) = 0.99;
- 3,16 – 0,99 =2,17.
- 2.17 more than 1.7, an ionic bond arises.
Sodium has a total of 11 electrons, the configuration of the outer layer is 3s1. To complete it, an atom is easier to give away 1 electron than to attach 7. The structure of the valence layer of chlorine is described by the formula 3s23p5. The atom has a total of 17 electrons, 7 - external. One is not enough to achieve an octet and a stable structure. Chemical properties confirm the assumption that the sodium atom gives, and chlorine accepts electrons. There are ions: positive (sodium cation) and negative (chlorine anion).
Losing an electron, sodium acquires a positive charge and a stable shell of an inert gas atom of neon (1s22s22p6). Chlorine as a result of interaction with sodium receives an additional negative charge, and the ion repeats the structure of the atomic shell of a noble argon gas (1s22s22p63s23p6). Acquired electric charge is called ion charge. For example, Na+Ca2+Cl-F-. The composition of the ions may be atoms of several elements: NH4+SO42-. Inside such complex ions, the particles are bound by a donor – acceptor or covalent mechanism. Electrostatic attraction occurs between oppositely charged particles. In the case of an ionic bond, its magnitude is proportional to the charges, and with increasing distance between the atoms it weakens. Characteristic features of ionic bond:
- strong metals react with active non-metallic elements;
- electrons move from one atom to another;
- The resulting ions have a stable configuration of the outer shells;
- electrostatic attraction occurs between oppositely charged particles.
Crystal lattices of ionic compounds
In chemical reactions, metals of the 1st, 2nd, and 3rd groups of the periodic system usually lose electrons. The formation of one-, double-, and three-charged positive ions. Non-metals of the 6th and 7th groups usually attach electrons (except for the reaction with fluorine). Single and doubly charged negative ions appear. The energy expended on these processes, as a rule, are compensated for when a crystal of a substance is created. Ionic compounds are usually in a solid state, they form structures consisting of oppositely charged cations and anions. These particles attract and form giant crystal lattices in which positive ions are surrounded by negative particles (and vice versa). The total charge of matter is zero, because the total number of protons is balanced by the number of electrons of all atoms.
Properties of substances with ionic bonding
Ionic crystalline substances are characterized by high boiling points and melting points.Usually these compounds are heat resistant. The following feature can be detected by dissolving such substances in a polar solvent (water). The crystals are easily destroyed, and the ions go into a solution that has electrical conductivity. Ionic compounds are also destroyed by melting. Free charged particles appear, which means that the melt conducts an electric current. Substances with an ionic bond are electrolytes - conductors of the second kind.
Oxides and halides of alkali and alkaline-earth metals belong to the group of ionic compounds. Almost all of them are widely used in science, engineering, chemical production, metallurgy.