Valence is a kind ofelementOne ofatomAtomization with other elements {constituteschemical compound}The nature of time.Generally, the valence number of the valence is equal to the number of electrons gained or lost by each atom during the combination, that is, the number of electrons gained or lost when the element can reach a stable structure, which is often determined by the electronic arrangement of the element, mainly the outermost electron arrangement, and of course, it may also involve the sub stable structure composed of sub layers that can be reached by the secondary outer layer.
Chinese name
Valence
Foreign name
Valence
Composition
Sign and Valence
Definition
Properties of atoms when they combine with other atoms
It's physicalatomNumber of electrons gained or lost orShared electron pairThe number of offsets.
ValenceRepresents the number of electrons gained or lost by atoms when they combine with each other.
ValenceIt is also a property of elements when they form compounds.
elementWhen combining with each other, the ratio of the number of atoms in the reactant is not certain, but based on the atomicNumber of outermost electronsDetermined.For example, aSodium ion(The valence is+1, and an electron is lost) It must beChloride ion(The valence is - 1 to get an electron).One magnesium ion (valence+2, loss of two electrons) must beChloride ioncombination.If the sum of the valence algebra of the ions of the formed compound is not zero, it is impossible to makeCovalent compoundThe outermost electronic layer of the atom of a molecule becomes a stable structure.It will not form stable compounds.
The concept of valence comes from this. The number of extranuclear electrons of an element combining with each other determines the valence of this element. The valence is set to facilitate the expression of the number of atoms combining with each other.When learning valence, you should understand the regulations of valence of elements in compounds.
In addition, the chemical valence of the element in the simple substance molecule is defined asFatal Frame, regardless ofIonic compoundIt is also a covalent compound, and the algebraic sum of the valence of the positive and negative ions of its composition is zero.Ionic compounds, such as NaOH (sodium valence is positive 1,Hydroxyl radicalIonsThe combined price is negative 1, which offset each other asZero priceSuch compoundsWritingIs established)
Valence - atomic formationChemical bondAbility.It is the chemical property of each other's elements in the formation of stable compounds.(i.e., a kind of ability to form a stable structure for each element)
be careful:The "valence" of an element is an important property of the element, which can only be shown when it is combined with other elements.That is, when the elementFree stateWhen it exists, that is, when it does not combine with other elements to form a compound, the valence of the element is "0".Such as iron and other metal elements, carbon and other non-metallic elements, helium, etcNoble gas。
university
<The definition of valence in university is different from that in middle school. The valence in middle school is defined asOxidation number>
Definition: The apparent charge number of an atom of an element, which is calculated on the assumption that the electrons in each chemical bond are assigned to atoms with greater electronegativity.
international
In 1970, the International Union of Pure and Applied Chemistry (IUPAC) strictly defined the concept of element oxidation number: the oxidation number (also called oxidation value) of an element is the charge number of an atom of an element, which can be obtained by assuming that the electrons in each bond are assigned to atoms with greater electronegativity.[1]
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1. In the compound, calculate the valence of the specified element according to the sum of positive and negative valence algebras.
2. The chemical valence of the elements shall be clearly marked on the top of the elements, with positive and negative signs.
3. When the oxygen element appears, the oxygen element is written at the back.
4. In the compound composed of metal and non-metal, when writing the chemical formula, the element symbol of metal element is written in the front, and the element symbol of non-metal element is written in the back.
Since most of the outermost electrons of the atoms of metal elements are less than 4chemical reactionIt is easy to lose the outermost electrons and show positive valence, that is, the valence of metal elements is generally positive (very few metals can show negative valence, such as antimony, which is - 3 in InSb).When nonmetallic elements combine with metallic elements, they usually get electrons with negative valence.However, when several non-metallic elements are combined,ElectronegativityThe lower one will show the positive valence.For example, oxygen is the element with the second highest electronegativity, which usually shows - 2 valence.But when it encounters the most electronegativefluorineElement, the+2 valence will be displayed, formingOxygen difluorideOF2。
Calculation rule
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After the formula is given, if you know the valence of an element, you can multiply its valence by the number of atoms of the element in its molecule.Since the electric property of the valence is zero, subtract zero from the previous valence and the elementAtomic numberThe product of is divided by the number of atoms of another element in the molecule to get the valence of another element.
Given the valence of two elements, the least common multiple of the absolute value of the valence is obtained.ReuseLeast common multipleDivide by the absolute value of the valence to find the number of atoms in the molecule.
The expression method of valence:+1,+2,+3, - 1, - 2,... 0, etc. shall be marked on the positive and negative valenceElement symbolDirectly above.(e.g
）
To determine the valence of elements in the compound, attention should be paid to
(1) The combination price includes positive price and negative price
(2) Oxygen usually shows - 2 valence.
(3) Hydrogen usually has+1 valence.
(4) When metal elements are combined with non-metallic elements, metal elements show positive valence and non-metallic elements show negative valence (generally, positive valence is written at the front and negative valence is written at the back).
(5) Some elements may have different valence in the same substance.
(6) The algebraic sum of positive and negative valence in the compound is 0
(7) The valence of an element is a property that the atoms of an element show when forming a compound. Therefore, in a simple molecule, the valence of an element is 0
Calculation of valence
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Valence is a property of elements. It is only shown when elements combine with each other.In a compound, the algebraic sum of positive and negative valence is equal to zero, which is the criterion for finding valence.Generally speaking, there are the following solutions:[2]
Chemical formula or radical formula
1. Formula for calculating the valence of element A in AmBn compound:
(valence of element B × number of atoms of element B)/number of atoms of element A
2. Formula for calculating the valence of elements with unknown valence in multi-component compounds:
(algebraic sum of valence numbers of elements with known valence)/number of atoms of elements with unknown valence
3. Determine the valence of elements (or atomic clusters) according to the number of positive and negative charges.
In the radical, the algebraic sum of the total number of positive and negative valence is equal to the number of positive and negative charges carried by the radical.
Exercise 1 Saltchemical formula(Molecular formula) is Rm (SO4) n, then what is the valence of R?
From the mass ratio of elements
1. (Relative atomic mass of element A × valence of element B)/(Relative atomic mass of element B × valence of element)=mass ratio of element A/mass ratio of element B
2. The mass ratio (or percentage composition) of element A × the valence of element A/A's relative atomic mass=the mass ratio (or percentage composition) of element B × the valence of element B/B's relative atomic mass
Exercise 2 What is the valence of nitrogen in the oxide combined with nitrogen and oxygen at a mass ratio of 7 ∶ 4?
By mass ratio
(valence of B × relative of AatomMass ratio)/(valence of A × relative atomic mass ratio of B)=mass ratio of element A/mass ratio of element B
Exercise The relative atomic mass ratio of element 3A to element B is 2:1 In a compound composed of only these two elements, the mass ratio of element A to element B is 2:3, where B is negative n valence, what is the valence of A in this compound?
Seeking chemical valence
1. If B is - 2 price and C is - 1 price, when A is an odd price, then price A:
(2AC type quantity - AB type quantity)/(C type quantity × B price - B type quantity × A price)
2. If B is - 2 and C is - 1, when A is even, then A is:
2 (AC type quantity - AB type quantity)/(C type quantity × B price - B type quantity × A price)
3. If the price of B and C are the same, the price of A is: (AC type quantity - AB type quantity)/(C type quantity - B type quantity)[3]
Determination method
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Chemically, valence is used to express the number of phase combinations between atoms.It is an important property of elements. Determining the chemical valence of elements is a basic skill that junior high school students should master. Now, we will summarize it in junior high school in order to help students learn.[4]
Other element method
[Example 1] Try to determine the valence of Mn element in compound K2MnO4.
Analysis: assuming the valence of Mn element in the compound is+x, according to the principle that the positive and negative algebraic sum of the valence of each element in the compound is zero, the solution of 2 × (+1)+1 × (+x)+4 × (- 2)=0 gives x=6
Therefore, the valence of Mn element in K2MnO4 is+6.
Electron layer structure method
[Example 2] If there is one electron on the outermost layer of the atom of element X and six electrons on the outermost layer of the atom of element Y, then the chemical formula of the compound that can be formed by the two elements X and Y is []
A．XY B．X2Y C．XY2 D．X3Y
Resolution: The key to this problem can be said to be to first determine the valence of X and Y when forming compounds.Since there is only one electron on the outermost layer of X, the highest positive valence is+1, and there are six electrons on the outermost layer of Y, which are still two from the stable structure of eight electrons, the lowest negative valence is - 2, so the molecular formula of the compound formed by X and Y is X2Y, and B should be selected.
Mass fraction method
[Example 3]Relative atomic massIs 59, and the mass fraction of this element in its oxide is 71%, then its valence is []
A．+1 B．+2 C．+3 D．+4
Analysis: suppose the chemical formula of the oxide of this element is RxOy
According to the meaning of the question, 59x/(59x+16y) * 100%=71%
The solution is x/y=2:3
Therefore, the chemical formula is R2O3, the valence of R is+3, and C is selected.
Law of conservation of mass
[Example 4] Somemetallic oxideReact with sufficient hydrochloric acid, and the ratio of the generated chloride to the molecular number of water is 2:3, then the valence of the metal is []
A．+1B．+2C．+3D．+4
Analysis: assuming the chemical formula of generated chloride is RClx, the ratio of molecular number RClx ∶ H2O=2:3 according to the meaning of the questionLaw of conservation of massIt can be seen that the atomic type and number of each element remain unchanged before and after the reaction, and the atomic number ratio of H and Cl in the product should also be 1:1, so the x value is 3, then the valence of R is+3, and C is selected.
relative molecular mass
[Example 5] Oxide of a metal elementrelative molecular massM, the relative molecular weight of the homovalent chloride is N, and the valence value of this element is []
Analysis: Let the valence of the element be+x, and the relative atomic mass be MR
(1) If x is an odd number, the chemical formula of oxide is R2Ox, and the chemical formula of chloride is RClx
2MR+16x=M (1)
MR+35.5x=N (2)
(2) * 2 - (1), the value of x is
(2) When x is an even number, the chemical formula of oxide is ROx/2, and the chemical formula of chloride is RClx
MR+35.5x=N ( 4 )
In conclusion, A and D should be selected.
Quality relation method
[Example 6] The metal element ag with relative atomic mass M and sufficientDilute sulfuric acid reaction, generate Bg hydrogen, then the valence of the metal element in the reaction is []
Analysis: assuming that the valence of metal in the reaction is+x, then the metal element will generate HtwoIt has the following relations:
2R～xHtwo
2M 2x
a b
Therefore, B should be selected.
Discussion on relevant factors
[Example 7] M atom of an elementNumber of outermost electronsLess than 5, the chemical formula of the oxide is MxOy, and the chemical formula of the chloride is MClz. When y ∶ z=1 ∶ 2, the valence of M may be []
A．+1 B．+2 C．+3 D．+4
Analysis: The valence of M is numerically equal to the value of z
If y=1z=2 (reasonable)
Y=2z=4 (reasonable)
Y=3z=6 (inconsistent with the number of outermost electrons less than 5)
