In complex compounds consisting of atoms of different elements, the electron density will always be shifted to one, the most “strong” neighbor. For example, in a water molecule (H 2 O), oxygen will be the winner, and in hydrochloric acid (HCl), the chlorine atom will win the duel. How to learn to determine this power? To do this, it is enough to disassemble what electronegativity is. Let's get started.
Atoms and elements
The first thing to be mastered is the difference between an atom and an element. Suppose there are as many as five atoms in the HNO 3 molecule and only three elements, which are hydrogen (H), nitrogen (N) and oxygen (O). If the name of some icon or symbol has been erased from memory, then Mendeleev's periodic system will come to the rescue.
It just lists all the elements that exist today. So, the first difficulty is overcome. Let's get closer to the question of what electronegativity is.
Pauling scale
In schools and universities, to identify the very strongest atom that will pull the electron density of weaker "neighbors" onto itself, the Pauling scale will be enough. You should not be afraid. Everything is extremely simple here. The relative electronegativity of chemical elements is arranged in ascending order and varies in the range of 0.7-4.0. The logic here is clear: whoever has this value is greater, he is stronger.
The value "0.7" belongs to the most active metal - France. Here he loses to absolutely everyone, that is, he is the least electronegative (the most electropositive). Fluorine boasts a maximum value of four. That is why he has no equal in strength.
Even without really understanding what electronegativity is, in any complex fluorine-containing compound, you can immediately determine the winner. Who will take over the electron density in lithium fluoride (LiF)? Of course, fluorine. Which element is more electronegative in silicon tetrafluoride (SiF 4)? Of course, again fluorine.
We consolidate the past
So, having analyzed what electronegativity is, let's support the theory with examples. Let's learn how to identify the strongest element present in the compound. Let's take a molecule of sulfuric acid (H 2 SO 4). Using the Pauling scale, we determine the relative electronegativity of all three required elements. For hydrogen, it will be 2.1. The value for sulfur is slightly higher - 2.6. But the clear leader will be oxygen, which has a maximum value of 3.5. This means that oxygen will be the most electronegative element in the H 2 SO 4 molecule. Thus, it is possible to determine the electronegativity value of any element.
Electronegativity (EO) is the ability of atoms to attract electrons when they bond with other atoms .
Electronegativity depends on the distance between the nucleus and valence electrons, and on how close the valence shell is to completion. The smaller the radius of an atom and the more valence electrons, the higher its ER.
Fluorine is the most electronegative element. Firstly, it has 7 electrons in the valence shell (only 1 electron is missing before an octet) and, secondly, this valence shell (…2s 2 2p 5) is located close to the nucleus.
The least electronegative atoms are alkali and alkaline earth metals. They have large radii and their outer electron shells are far from complete. It is much easier for them to give their valence electrons to another atom (then the pre-outer shell will become complete) than to “gain” electrons.
Electronegativity can be expressed quantitatively and line up the elements in ascending order. The electronegativity scale proposed by the American chemist L. Pauling is most often used.
The difference in the electronegativity of the elements in the compound ( ΔX) will allow us to judge the type of chemical bond. If the value ∆ X= 0 - connection covalent non-polar.
With an electronegativity difference of up to 2.0, the bond is called covalent polar, for example: the H-F bond in the HF hydrogen fluoride molecule: Δ X \u003d (3.98 - 2.20) \u003d 1.78
Bonds with an electronegativity difference greater than 2.0 are considered ionic. For example: the Na-Cl bond in the NaCl compound: Δ X \u003d (3.16 - 0.93) \u003d 2.23.
Oxidation state
Oxidation state (CO) is the conditional charge of an atom in a molecule, calculated on the assumption that the molecule consists of ions and is generally electrically neutral.
When an ionic bond is formed, an electron passes from a less electronegative atom to a more electronegative one, the atoms lose their electrical neutrality and turn into ions. there are integer charges. When a covalent polar bond is formed, the electron does not transfer completely, but partially, so partial charges arise (in the figure below, HCl). Let's imagine that the electron passed completely from the hydrogen atom to chlorine, and a whole positive charge +1 appeared on hydrogen, and -1 on chlorine. such conditional charges are called the oxidation state.
This figure shows the oxidation states characteristic of the first 20 elements.
Note. The highest SD is usually equal to the group number in the periodic table. Metals of the main subgroups have one characteristic CO, non-metals, as a rule, have a spread of CO. Therefore, non-metals form a large number of compounds and have more "diverse" properties compared to metals.
Examples of determining the degree of oxidation
Let's determine the oxidation states of chlorine in compounds:
The rules that we have considered do not always allow us to calculate the CO of all elements, as, for example, in a given aminopropane molecule.
Here it is convenient to use the following method:
1) We depict the structural formula of the molecule, the dash is a bond, a pair of electrons.
2) We turn the dash into an arrow directed to a more EO atom. This arrow symbolizes the transition of an electron to an atom. If two identical atoms are connected, we leave the line as it is - there is no transfer of electrons.
3) We count how many electrons "came" and "left".
For example, consider the charge on the first carbon atom. Three arrows are directed towards the atom, which means that 3 electrons have arrived, the charge is -3.
The second carbon atom: hydrogen gave it an electron, and nitrogen took one electron. The charge has not changed, it is equal to zero. Etc.
Valence
Valence(from Latin valēns "having force") - the ability of atoms to form a certain number of chemical bonds with atoms of other elements.
Basically, valency means the ability of atoms to form a certain number of covalent bonds. If an atom has n unpaired electrons and m lone electron pairs, then this atom can form n+m covalent bonds with other atoms, i.e. its valence will be n+m. When evaluating the maximum valency, one should proceed from the electronic configuration of the "excited" state. For example, the maximum valency of an atom of beryllium, boron and nitrogen is 4 (for example, in Be (OH) 4 2-, BF 4 - and NH 4 +), phosphorus - 5 (PCl 5), sulfur - 6 (H 2 SO 4) , chlorine - 7 (Cl 2 O 7).
In some cases, the valence may numerically coincide with the oxidation state, but in no way are they identical to each other. For example, a triple bond is realized in N 2 and CO molecules (that is, the valence of each atom is 3), but the oxidation state of nitrogen is 0, carbon +2, oxygen -2.
When elements interact, electron pairs are formed by accepting or giving off electrons. The ability of an atom to pull electrons was called by Linus Pauling the electronegativity of chemical elements. Pauling scaled the electronegativity of the elements from 0.7 to 4.
What is electronegativity?
Electronegativity (EO) is a quantitative characteristic of an element, showing the force with which electrons are attracted by the nucleus of an atom. EO also characterizes the ability to hold valence electrons in the outer energy level.
Rice. 1. The structure of the atom.
The ability to give or receive electrons determines whether the elements belong to metals or non-metals. Elements that easily donate electrons have pronounced metallic properties. Elements that accept electrons exhibit non-metallic properties.
Electronegativity is manifested in chemical compounds and shows the displacement of electrons towards one of the elements.
Electronegativity increases from left to right and decreases from top to bottom in Mendeleev's periodic table.
How to determine
You can determine the value using the electronegativity table of chemical elements or the Pauling scale. The electronegativity of lithium is taken as unity.
Oxidizing agents and halogens have the highest EO. The value of their electronegativity is greater than two. The record holder is fluorine with an electronegativity of 4.
Rice. 2. Table of electronegativity.
The smallest EC (less than two) have metals of the first group of the periodic table. Sodium, lithium, potassium are considered active metals, because. it is easier for them to part with a single valence electron than to accept the missing electrons.
Some elements are in between. Their electronegativity is close to two. Such elements (Si, B, As, Ge, Te) exhibit metallic and non-metallic properties.
For ease of comparison of EO, a series of electronegativity elements is used. On the left are metals, on the right are non-metals. The closer to the edges, the more active the element. Cesium is the strongest reducing agent, easily donating electrons and having the lowest electronegativity. Fluorine is an active oxidizing agent capable of attracting electrons.
Rice. 3. Series of electronegativity.
In non-metallic compounds, elements with a higher EC attract electrons. Oxygen with an electronegativity of 3.5 attracts carbon and sulfur atoms with an electronegativity of 2.5.
What have we learned?
Electronegativity indicates the degree to which the nucleus of an atom retains valence electrons. Depending on the value of EC, the elements are able to donate or accept electrons. Elements with greater electronegativity attract electrons and exhibit non-metallic properties. Elements whose atoms donate electrons easily have metallic properties. Some elements have a conditionally neutral EO (about two) and can exhibit metallic and non-metallic properties. The degree of EO increases from left to right and from bottom to top in the periodic table.
The term is widely used in chemistry. electronegativity (EO) - the property of the atoms of a given element to pull electrons from the atoms of other elements in compounds is called electronegativity. The electronegativity of lithium is conventionally taken as unity, the EC of other elements is calculated accordingly. There is a scale of values of EO elements.
Numerical values of EO elements have approximate values: it is a dimensionless quantity. The higher the EC of an element, the more pronounced its non-metallic properties. According to the EO, the elements can be written as follows:
F > O > Cl > Br > S > P > C > H > Si > Al > Mg > Ca > Na > K > Cs
Fluorine has the highest EO value. Comparing the EO values of the elements from francium (0.86) to fluorine (4.1), it is easy to see that the EO obeys the Periodic Law. In the Periodic system of elements, EO in a period increases with an increase in the element number (from left to right), and in the main subgroups it decreases (from top to bottom). In periods, as the charges of the nuclei of atoms increase, the number of electrons on the outer layer increases, the radius of the atoms decreases, therefore, the ease of giving off electrons decreases, the EO increases, therefore, the non-metallic properties increase.
The difference in the electronegativity of the elements in the compound (ΔX) will make it possible to judge the type of chemical bond.
If the value Δ X \u003d 0 - non-polar covalent bond.
With the difference in electronegativity up to 2.0 bond is called covalent polar, for example: the H-F bond in the HF hydrogen fluoride molecule: Δ X \u003d (3.98 - 2.20) \u003d 1.78
Connections with the difference in electronegativity greater than 2.0 are considered ionic. For example: the Na-Cl bond in the NaCl compound: Δ X \u003d (3.16 - 0.93) \u003d 2.23.
Electronegativity depends on the distance between the nucleus and valence electrons, and on how close the valence shell is to being completed. The smaller the radius of an atom and the more valence electrons, the higher its ER.
Fluorine is most electronegative element. Firstly, it has 7 electrons on the valence shell (only 1 electron is missing before an octet) and, secondly, this valence shell is located close to the nucleus.
The least electronegative atoms are alkali and alkaline earth metals. They have large radii and their outer electron shells are far from complete. It is much easier for them to give their valence electrons to another atom (then the pre-outer shell will become complete) than to “gain” electrons.
Electronegativity can be expressed quantitatively and line up the elements in ascending order. Most commonly used the electronegativity scale proposed by the American chemist L. Pauling.
Oxidation state
Compounds made up of two chemical elements are called binary(from lat. bi - two), or two-element (NaCl, HCl). In the case of an ionic bond in the NaCl molecule, the sodium atom transfers its outer electron to the chlorine atom and turns into an ion with a charge of +1, while the chlorine atom accepts an electron and turns into an ion with a charge of -1. Schematically, the process of transformation of atoms into ions can be depicted as follows:
During chemical interaction in the HCl molecule, the common electron pair is shifted towards the more electronegative atom. For example, 
, i.e., the electron will not completely transfer from the hydrogen atom to the chlorine atom, but partially, thereby causing a partial charge of the atoms δ:
H +0.18 Сl -0.18. If we imagine that in the HCl molecule, as well as in NaCl chloride, the electron completely passed from the hydrogen atom to the chlorine atom, then they would receive charges +1 and -1:
Such conditional charges are called oxidation state. When defining this concept, it is conditionally assumed that in covalent polar compounds, the binding electrons have completely transferred to a more electronegative atom, and therefore the compounds consist only of positively and negatively charged atoms.
The oxidation state is the conditional charge of the atoms of a chemical element in a compound, calculated on the basis of the assumption that all compounds (both ionic and covalently polar) consist only of ions. The oxidation state can have a negative, positive, or zero value, which is usually placed above the element symbol at the top, for example:
Those atoms that have received electrons from other atoms or to which common electron pairs are displaced have a negative value for the oxidation state, i.e. atoms of more electronegative elements. Those atoms that donate their electrons to other atoms or from which common electron pairs are drawn have a positive oxidation state, i.e., atoms of less electronegative elements. The zero value of the oxidation state has atoms in the molecules of simple substances and atoms in the free state, for example:
In compounds, the total oxidation state is always zero.
Valence
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 possibilities of atoms are determined by:
The number of unpaired electrons (one-electron orbitals);
The presence of free orbitals;
The presence of lone pairs of electrons.
In organic chemistry, the concept of "valence" replaces the concept of "oxidation state", which is customary to work with in inorganic chemistry. However, they are not the same. The valence has no sign and cannot be zero, while the oxidation state is necessarily characterized by a sign and can have a value equal to zero.
Basically, valency refers to the ability of atoms to form a certain number of covalent bonds. If an atom has n unpaired electrons and m unshared electron pairs, then this atom can form n + m covalent bonds with other atoms, i.e. its valence will be equal to n + m. When evaluating the maximum valency, one should proceed from the electronic configuration of the "excited" state. For example, the maximum valence of an atom of beryllium, boron and nitrogen is 4.
Permanent valencies:
- H, Na, Li, K, Rb, Cs - Oxidation state I
- O, Be, Mg, Ca, Sr, Ba, Ra, Zn, Cd - Oxidation state II
- B, Al, Ga, In — Oxidation state III
Valence Variables:
- Cu - I and II
- Fe, Co, Ni - II and III
- C, Sn, Pb - II and IV
- P- III and V
- Cr- II, III and VI
- S- II, IV and VI
- Mn- II, III, IV, VI and VII
- N- II, III, IV and V
- Cl- I, IV, VIandVII
Using valencies, you can compose the formula of the compound.
A chemical formula is a conditional record of the composition of a substance by means of chemical signs and indices.
For example: H 2 O is the formula of water, where H and O are the chemical signs of the elements, 2 is an index that shows the number of atoms of this element that make up the water molecule.
When naming substances with variable valence, its valency must be indicated, which is placed in brackets. For example, P 2 0 5 - phosphorus oxide (V)
I. Oxidation state free atoms and atoms in molecules simple substances is equal to zero— Na 0 , R 4 0 , O 2 0
II. AT complex substance the algebraic sum of CO of all atoms, taking into account their indices, is equal to zero = 0. and in complex ion its charge.
For example:
For example, let's analyze several compounds and find out the valence chlorine:
Reference material for passing the test:
periodic table
Solubility table
You can find out the activity of simple substances using the table of electronegativity of chemical elements. Denoted as χ. Read more about the concept of activity in our article.
What is electronegativity
The property of an atom of a chemical element to attract the electrons of other atoms to itself is called electronegativity. For the first time the concept was introduced by Linus Pauling in the first half of the twentieth century.
All active simple substances can be divided into two groups according to physical and chemical properties:
- metals;
- non-metals.
All metals are reducing agents. In reactions, they donate electrons and have a positive oxidation state. Non-metals can exhibit the properties of reducing and oxidizing agents depending on the value of electronegativity. The higher the electronegativity, the stronger the properties of the oxidizing agent.
Rice. 1. Actions of an oxidizing agent and a reducing agent in reactions.
Pauling created the electronegativity scale. In accordance with the Pauling scale, fluorine (4) has the highest electronegativity, and francium (0.7) has the lowest. This means that fluorine is the strongest oxidizing agent and is able to attract electrons from most elements. On the contrary, francium, like other metals, is a reducing agent. He seeks to give, not accept electrons.
Electronegativity is one of the main factors that determine the type and properties of a chemical bond formed between atoms.
How to determine
The properties of elements to attract or donate electrons can be determined from the electronegativity series of chemical elements. According to the scale, elements with a value of more than two are oxidizers and exhibit the properties of a typical non-metal.
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Item number |
Element |
Symbol |
Electronegativity |
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Strontium |
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Ytterbium |
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Praseodymium |
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Prometheus |
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Americium |
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Gadolinium |
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Dysprosium |
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Plutonium |
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Californium |
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Einsteinium |
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Mendelevium |
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Zirconium |
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Neptunium |
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Protactinium |
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Manganese |
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Beryllium |
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Aluminum |
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Technetium |
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Molybdenum |
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Palladium |
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Tungsten |
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Oxygen |
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Substances with an electronegativity of two or less are reducing agents and exhibit metallic properties. Transition metals, which have a variable degree of oxidation and belong to the side subgroups of the periodic table, have electronegativity values in the range of 1.5-2. Elements with an electronegativity equal to or less than one have pronounced properties of a reducing agent. These are typical metals.
In the electronegativity series, metallic and reducing properties increase from right to left, while oxidizing and non-metallic properties increase from left to right.
Rice. 2. Series of electronegativity.
In addition to the Pauling scale, you can find out how pronounced the oxidizing or reducing properties of an element are using the periodic table of Mendeleev. Electronegativity increases in periods from left to right as the atomic number increases. In groups, the value of electronegativity decreases from top to bottom.
Rice. 3. Periodic table.
What have we learned?
Electronegativity refers to the ability of elements to donate or accept electrons. This characteristic helps to understand how pronounced the properties of an oxidizing agent (non-metal) or reducing agent (metal) are for a particular element. For convenience, Pauling developed the electronegativity scale. According to the scale, fluorine has the maximum oxidizing properties, and francium has the minimum. In the periodic table, the properties of metals increase from right to left and from top to bottom.
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