DEFINITION
Phosphorus located in the third period of group V of the main (A) subgroup of the Periodic Table.
Phosphorus forms several allotropic changes: white, red and black phosphorus.
In its pure form, white phosphorus is completely colorless and transparent; technical white phosphorus is colored yellowish and appearance looks like wax. Density 1.83 g/cm 3 . In the cold, white phosphorus is brittle, but at temperatures above 15 o C it becomes soft and can be easily cut with a knife. In air, it is easily oxidized, as a result of which it glows in the dark. It has a molecular crystal lattice at the nodes of which are tetrahedral molecules P 4 . Poisonous.
Red phosphorus consists of several forms, which are polymeric substances, the composition of which is not fully understood. Slowly oxidizes in air, does not glow in the dark, non-toxic. Density 2.0-2.4g/cm 3 . Sublimates when heated. When red phosphorus vapor is cooled, white phosphorus is obtained.
Black phosphorus is formed from white by heating it under high pressure at 200-220 o C. It looks like graphite, greasy to the touch. Density - 2.7g / cm 3. Semiconductor.
Valency of phosphorus in compounds
Phosphorus is the fifteenth element in the Periodic Table of D.I. Mendeleev. He is in the third period in the VA group. The nucleus of a phosphorus atom contains 15 protons and 16 neutrons (the mass number is 31). There are three energy levels in the phosphorus atom, on which there are 15 electrons (Fig. 1).
Rice. 1. The structures of the phosphorus atom.
The electronic formula of the phosphorus atom in the ground state is as follows:
1s 2 2s 2 2p 6 3s 2 3p 3 .
And the energy diagram (built only for electrons of the outer energy level, which are otherwise called valence):
The presence of three unpaired electrons indicates that phosphorus is capable of exhibiting valency III (P III 2 O 3 , Ca 3 P III 2 , P III H 3, etc.).
Since, in addition to the 3s and 3p sublevels, there is also a 3d sublevel on the third energy layer, the phosphorus atom is characterized by the presence of an excited state: a pair of electrons of the 3s sublevel is depaired and one of them occupies a vacant orbital of the 3d sublevel.
The presence of five unpaired electrons indicates that valence V is also characteristic of phosphorus (P V 2 O 5, H 3 P V O 4, P V Cl 5, etc.).
Examples of problem solving
EXAMPLE 1
The properties of an atom are largely determined by the structure of its outer electronic layer. Electrons located on the outer, and sometimes on the penultimate, electronic layer of an atom can take part in the formation of chemical bonds. Such electrons are called valence. For example, in a phosphorus atom there are 5 valence electrons: (Fig. 1).
Rice. 1. Electronic formula of the phosphorus atom
The valence electrons of the atoms of the elements of the main subgroups are located on the s- and p-orbitals of the outer electronic layer. For elements of secondary subgroups, except for lanthanides and actinides, valence electrons are located on the s-orbitals of the outer and d-orbitals of the penultimate layers.
Valency is the ability of an atom to form chemical bonds. This definition and the very concept of valence is correct only in relation to substances with a covalent type of bond. For ionic compounds, this concept is not applicable; instead, the formal concept of "oxidation state" is used.
Valence is characterized by the number of electron pairs formed during the interaction of an atom with other atoms. For example, the valence of nitrogen in ammonia NH3 is three (Fig. 2).
Rice. 2. Electronic and graphical formulas of the ammonia molecule
The number of electron pairs that an atom can form with other atoms depends primarily on the number of its unpaired electrons. For example, in a carbon atom, two unpaired electrons are in 2p orbitals (Fig. 3). By the number of unpaired electrons, we can say that such a carbon atom can exhibit a valence equal to II.
Rice. 3. Electronic structure of the carbon atom in the ground state
In all organic matter and some inorganic compounds, carbon is tetravalent. Such a valence is possible only in the excited state of the carbon atom, into which it passes when additional energy is received.
In the excited state, 2s electrons are paired in the carbon atom, one of which passes to a free 2p orbital. Four unpaired electrons can participate in the formation of four covalent bonds. The excited state of an atom is usually denoted by an "asterisk" (Fig. 4).
Rice. 4. Electronic structure of the carbon atom in an excited state
Can nitrogen have a valence equal to five - according to the number of its valence electrons? Consider the valence possibilities of the nitrogen atom.
There are two electron layers in the nitrogen atom, on which only 7 electrons are located (Fig. 5).
Rice. 5. Electronic scheme of the structure of the outer layer of the nitrogen atom
Nitrogen can share three electron pairs with three other electrons. A pair of electrons in the 2s orbital can also participate in the formation of a bond, but according to a different mechanism - a donor-acceptor one, forming a fourth bond.
The depairing of 2s-electrons in the nitrogen atom is impossible, since there is no d-sublevel on the second electron layer. Therefore, the highest valency of nitrogen is IV.
Summing up the lesson
In the lesson, you learned to determine the valence possibilities of atoms of chemical elements. In the course of studying the material, you learned how many atoms of other chemical elements a particular atom can attach to itself, and also why the elements show different valence values.
Sources
http://www.youtube.com/watch?t=3&v=jSTB1X1mD0o
http://www.youtube.com/watch?t=7&v=6zwx_d-MIvQ
http://www.youtube.com/watch?t=1&v=qj1EKzUW16M
http://interneturok.ru/ru/school/chemistry/11-klass - abstract
The number of covalent bonds that an atom can form is called the valency of the element. The valence possibilities of atoms are due to the presence of valence electrons at the external energy level.
All elements of the planet are formed by atoms. These are the smallest particles, consisting of a positively charged nucleus and negatively charged electrons. The nucleus includes protons and neutrons. Electrons attracted by the nucleus are located and move in orbits at different distances from the center. The uneven position of electrons relative to the nucleus is called energy levels.
Rice. 1. The structure of the atom.
In the periodic table, the highest valency corresponds to the number of the group in which the element is located. The number of energy levels coincides with the number of the period, electrons - with the serial number.
Rice. 2. Periodic table.
Valence possibilities
To evaluate the valence possibilities of atoms of chemical elements, it is necessary to consider in detail the distribution of electrons at energy levels.
Valence corresponds to the number of unpaired electrons located in the s- and p-orbitals of the outer energy level. The valence electrons of the atoms of the elements included in the side groups of the periodic table are located on the s-orbitals of the outer level and d-orbitals that form the outer sublevel.
In the normal (stationary) state, electrons occupy a certain position in the atom. The stationary electronic configuration is fixed in the periodic table. In an excited state (reactions with other elements), the energy of an atom is redistributed, and the electrons change their position.
Consider an example. The stationary phosphorus atom has the electronic configuration 1s 2 2s 2 2p 6 3s 2 3p 3 .
This means that 15 electrons are distributed over three levels. The outer level, which includes the s- and p-orbitals, contains five valence electrons. In this case, three electrons in the p-orbital are unpaired, and two electrons in the s-orbital form a pair. Accordingly, three unpaired electrons can form covalent bonds, and the valency of phosphorus is three.
Phosphorus is in group V, the main subgroup. This means that there is an empty d-sublevel in the atom. In the excited state, the paired electrons of the s-level are depaired, and one electron passes to the d-sublevel. Five free, unpaired electrons are formed. Accordingly, the phosphorus atom acquires the fifth valence.
Rice. 3. Graphical electronic formula of phosphorus in the normal and excited state.
Steaming occurs with the expenditure of energy. Energy consumption is compensated by the formation of covalent bonds with the release of energy.
Depending on the ability to move into an excited state, elements are divided into two groups: with variable and constant valency. Alkali, alkaline earth metals, fluorine and aluminum have a constant valence (corresponds to the group number). Variable valency is inherent in all other elements. Inert gases do not react, therefore it is considered that they have no valency.
What have we learned?
Valency shows how many atoms an element can attach through covalent bonds. The valency value matches the number of electrons in the outer energy level and corresponds to the group number of the periodic table in which the element is located. Due to the ability to go into an excited state, most elements have a non-permanent valency. The same valence in any state is retained by active metals and fluorine.
The structure of the outer energy levels of atoms of chemical elements determines mainly the properties of their atoms. Therefore, these levels are called valence levels. The electrons of these levels, and sometimes of the pre-external levels, can take part in the formation of chemical bonds. Such electrons are also called valence electrons.
The valence of an atom of a chemical element is determined primarily by the number of unpaired electrons that take part in the formation of a chemical bond.
The valence electrons of the atoms of the elements of the main subgroups are located on the s- and p-orbitals of the outer electronic layer. In the elements of secondary subgroups, except for lanthanides and actinides, valence electrons are located on the s-orbitals of the outer and d-orbitals of the pre-outer layers.
In order to correctly assess the valence capabilities of atoms of chemical elements, it is necessary to consider the distribution of electrons in them by energy levels and sublevels and determine the number of unpaired electrons in accordance with the Pauli principle and Hund's rule for the unexcited (ground, or stationary) state of the atom and for the excited (then there is one that has received additional energy, as a result of which the electrons of the outer layer are depaired and transferred to free orbitals). An atom in an excited state is denoted by the corresponding element symbol with an asterisk. For example, consider the valence possibilities of phosphorus atoms in the stationary and excited states:
In the unexcited state, the phosphorus atom has three unpaired electrons in the p sublevel. During the transition of an atom to an excited state, one of the pair of electrons of the d-sublevel can pass to a free orbital of the d-sublevel. The valency of phosphorus changes from three (in the ground state) to five (in the excited state).
The separation of paired electrons requires energy, since the pairing of electrons is accompanied by a decrease in the potential energy of atoms. At the same time, the energy consumption for the transfer of an atom to an excited state is compensated by the energy released during the formation of chemical bonds by unpaired electrons.
Thus, a carbon atom in a stationary state has two unpaired electrons. Consequently, with their participation, two common electron pairs can be formed, carrying out two covalent bonds. However, you are well aware that tetravalent carbon atoms are present in many inorganic and all organic compounds. Obviously, its atoms formed four covalent bonds in these compounds while in an excited state. 
The energy expended on the excitation of carbon atoms is more than offset by the energy released during the formation of two additional covalent bonds. So, for the transfer of carbon atoms from the stationary state 2s 2 2p 2 to the excited state - 2s 1 2p 3, about 400 kJ / mol of energy is required. But during the formation of a C-H bond in saturated hydrocarbons, 360 kJ / mol is released. Consequently, upon the formation of two moles of C–H bonds, 720 kJ will be released, which exceeds the energy of transferring carbon atoms to an excited state by 320 kJ/mol.
In conclusion, it should be noted that the valence possibilities of atoms of chemical elements are far from exhausted by the number of unpaired electrons in the stationary and excited states of atoms. If you remember the donor-acceptor mechanism for the formation of covalent bonds, then you will understand the other two valence possibilities of atoms of chemical elements, which are determined by the presence of free orbitals and the presence of unshared electron pairs that can give a covalent chemical bond according to the donor-acceptor mechanism. Recall the formation of the ammonium ion NH4+. (We will consider in more detail the realization of these valence possibilities by atoms of chemical elements when studying the chemical bond.) Let us draw a general conclusion.
In chemistry lessons, you have already got acquainted with the concept of the valency of chemical elements. We have collected in one place all the useful information on this issue. Use it when preparing for the GIA and the Unified State Examination.
Valency and chemical analysis
Valence- the ability of atoms of chemical elements to enter into chemical compounds with atoms of other elements. In other words, it is the ability of an atom to form a certain number of chemical bonds with other atoms.
From Latin, the word "valence" is translated as "strength, ability." Very true name, right?
The concept of "valence" is one of the main ones in chemistry. It was introduced even before the structure of the atom became known to scientists (back in 1853). Therefore, as the structure of the atom was studied, it underwent some changes.
So, from the point of view of electronic theory, valency is directly related to the number of external electrons of an atom of an element. This means that by "valency" is meant the number of electron pairs by which an atom is bonded to other atoms.
Knowing this, scientists were able to describe the nature of the chemical bond. It lies in the fact that a pair of atoms of a substance shares a pair of valence electrons.
You may ask, how could chemists of the 19th century be able to describe valency even when they believed that there were no particles smaller than an atom? It cannot be said that it was so simple - they relied on chemical analysis.
By chemical analysis, scientists of the past determined the composition of a chemical compound: how many atoms of various elements are contained in the molecule of the substance in question. To do this, it was necessary to determine what is the exact mass of each element in a sample of a pure (without impurities) substance.
Admittedly, this method is not without flaws. Because the valency of an element can be determined in this way only in its simple combination with always monovalent hydrogen (hydride) or always divalent oxygen (oxide). For example, the valency of nitrogen in NH 3 - III, since one hydrogen atom is bonded to three nitrogen atoms. And the valency of carbon in methane (CH 4), according to the same principle, is IV.
This method for determining valency is only suitable for simple substances. But in acids in this way we can only determine the valency of compounds like acid residues, but not all elements (except for the known hydrogen valence) separately.
As you have already noticed, valency is indicated by Roman numerals.
Valency and acids
Since the valence of hydrogen remains unchanged and is well known to you, you can easily determine the valency of the acid residue. So, for example, in H 2 SO 3 the valency of SO 3 is I, in HClO 3 the valency of ClO 3 is I.
In a similar way, if the valency of the acid residue is known, it is easy to write down the correct formula of the acid: NO 2 (I) - HNO 2, S 4 O 6 (II) - H 2 S 4 O 6.
Valency and formulas
The concept of valence makes sense only for substances of a molecular nature and is not very suitable for describing chemical bonds in compounds of a cluster, ionic, crystalline nature, etc.
Indices in the molecular formulas of substances reflect the number of atoms of the elements that make up their composition. Knowing the valency of the elements helps to correctly arrange the indices. In the same way, by looking at the molecular formula and indices, you can name the valences of the constituent elements.
You perform such tasks in chemistry lessons at school. For example, having chemical formula a substance in which the valency of one of the elements is known, the valence of another element can be easily determined.
To do this, you just need to remember that in a substance of molecular nature, the number of valencies of both elements are equal. Therefore, use the least common multiple (corresponding to the number of free valences required for the connection) to determine the valence of the element that you do not know.
To make it clear, let's take the formula of iron oxide Fe 2 O 3. Here, two iron atoms with valence III and 3 oxygen atoms with valence II participate in the formation of a chemical bond. Their least common multiple is 6.
- Example: you have formulas Mn 2 O 7 . You know the valence of oxygen, it is easy to calculate that the least common multiple is 14, hence the valency of Mn is VII.
Similarly, you can do the opposite: write down the correct chemical formula of a substance, knowing the valencies of its constituent elements.
- Example: in order to correctly write down the formula of phosphorus oxide, we take into account the valency of oxygen (II) and phosphorus (V). Hence, the least common multiple for P and O is 10. Therefore, the formula has the following form: P 2 O 5.
Knowing well the properties of the elements that they exhibit in various compounds, one can determine their valence even by the appearance of such compounds.
For example: copper oxides are red (Cu 2 O) and black (CuO) in color. Copper hydroxides are colored yellow (CuOH) and blue (Cu(OH) 2).
And to make covalent bonds in substances more clear and understandable for you, write their structural formulas. The dashes between the elements depict the bonds (valencies) that arise between their atoms:
Valency characteristics
Today, the determination of the valency of elements is based on knowledge about the structure of the outer electron shells of their atoms.
Valence can be:
- constant (metals of the main subgroups);
- variable (non-metals and metals of side groups):
- highest valence;
- lower valence.
The constant in various chemical compounds remains:
- valency of hydrogen, sodium, potassium, fluorine (I);
- valency of oxygen, magnesium, calcium, zinc (II);
- valency of aluminum (III).
But the valency of iron and copper, bromine and chlorine, as well as many other elements, changes when they form various chemical compounds.
Valence and electronic theory
Within the framework of the electronic theory, the valence of an atom is determined on the basis of the number of unpaired electrons that participate in the formation of electron pairs with the electrons of other atoms.
Only electrons located on the outer shell of the atom participate in the formation of chemical bonds. Therefore, the maximum valence of a chemical element is the number of electrons in the outer electron shell of its atom.
The concept of valence is closely related to Periodic Law, discovered by D. I. Mendeleev. If you look closely at the periodic table, you can easily notice: the position of an element in the periodic table and its valency are inextricably linked. The highest valency of elements that belong to the same group corresponds to the ordinal number of the group in the periodic system.
You will find out the lowest valency when you subtract the group number of the element that interests you from the number of groups in the periodic table (there are eight of them).
For example, the valency of many metals matches the group numbers in the table periodic elements to which they belong.
Table of valency of chemical elements
|
Serial number chem. element (atomic number) |
Name |
chemical symbol |
Valence |
| 1 | Hydrogen Helium / Helium Lithium / Lithium Beryllium / Beryllium Carbon / Carbon Nitrogen / Nitrogen Oxygen / Oxygen Fluorine / Fluorine Neon / Neon Sodium Magnesium / Magnesium Aluminum Silicon / Silicon Phosphorus / Phosphorus Sulfur Chlorine / Chlorine Argon / Argon Potassium / Potassium Calcium / Calcium Scandium / Scandium Titanium / Titanium Vanadium / Vanadium Chromium / Chromium Manganese / Manganese Iron / Iron Cobalt / Cobalt Nickel / Nickel Copper Zinc / Zinc Gallium / Gallium Germanium /Germanium Arsenic / Arsenic Selenium / Selenium Bromine / Bromine Krypton / Krypton Rubidium / Rubidium Strontium / Strontium Yttrium / Yttrium Zirconium / Zirconium Niobium / Niobium Molybdenum / Molybdenum Technetium / Technetium Ruthenium / Ruthenium Rhodium Palladium / Palladium Silver / Silver Cadmium / Cadmium Indium / Indium Tin / Tin Antimony / Antimony Tellurium / Tellurium Iodine / Iodine Xenon / Xenon Cesium / Cesium Barium / Barium Lanthanum / Lanthanum Cerium / Cerium Praseodymium / Praseodymium Neodymium / Neodymium Promethium / Promethium Samaria / Samarium Europium / Europium Gadolinium / Gadolinium Terbium / Terbium Dysprosium / Dysprosium Holmium / Holmium Erbium / Erbium Thulium / Thulium Ytterbium / Ytterbium Lutetium / Lutetium Hafnium / Hafnium Tantalum / Tantalum Tungsten / Tungsten Rhenium / Rhenium Osmium / Osmium Iridium / Iridium Platinum / Platinum Gold / Gold Mercury / Mercury Waist / Thallium Lead / Lead Bismuth / Bismuth Polonium / Polonium Astatine / Astatine Radon / Radon Francium / Francium Radium / Radium Actinium / Actinium Thorium / Thorium Proactinium / Protactinium Uranus / Uranium |
H | I (I), II, III, IV, V I, (II), III, (IV), V, VII II, (III), IV, VI, VII II, III, (IV), VI (I), II, (III), (IV) I, (III), (IV), V (II), (III), IV (II), III, (IV), V (II), III, (IV), (V), VI (II), III, IV, (VI), (VII), VIII (II), (III), IV, (VI) I, (III), (IV), V, VII (II), (III), (IV), (V), VI (I), II, (III), IV, (V), VI, VII (II), III, IV, VI, VIII (I), (II), III, IV, VI (I), II, (III), IV, VI (II), III, (IV), (V) There is no data There is no data (II), III, IV, (V), VI |
In brackets are given those valences that the elements possessing them rarely show.
Valency and oxidation state
So, speaking of the degree of oxidation, they mean that an atom in a substance of an ionic (which is important) nature has a certain conditional charge. And if valence is a neutral characteristic, then the oxidation state can be negative, positive or equal to zero.
It is interesting that for an atom of the same element, depending on the elements with which it forms a chemical compound, the valency and oxidation state can be the same (H 2 O, CH 4, etc.) and differ (H 2 O 2, HNO 3 ).
Conclusion
Deepening your knowledge of the structure of atoms, you will learn more deeply and in more detail about valency. This characterization of chemical elements is not exhaustive. But it has great applied value. What you yourself have seen more than once, solving problems and conducting chemical experiments in the classroom.
This article is designed to help you organize your knowledge of valency. And also to recall how it can be determined and where valence is used.
We hope that this material will be useful for you in preparing homework and self-preparation for tests and exams.
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