What is the value of H2O



The Valence, also valence, of an atom of a chemical element indicates how many atoms it can bond to itself in the case of a chemical bond, or how many single bonds it would have to form with other atoms in order to achieve the octet state. It is used to calculate chemical formulas (sum formulas) of simple chemical compounds.

In the case of more complex chemical compounds, it must be noted that the atom in question does not have to form a bond with all of the atoms in the molecule of the chemical compound in order to be part of the compound. This has the consequence that z. B. a trivalent atom can also form a molecule with nine other atoms, whereby it connects with a maximum of three of the nine atoms via a chemical bond.

The value is usually noted with a Roman number.

history

In 1852 the English chemist Sir Edward Frankland determined that every atom has a certain ability to bond to other atoms. The concept of value arose from this determination.

In modern chemistry, the concept of valence only plays a subordinate role and is now often used as a synonym for the oxidation number, although both numbers are only the same in ionic compounds. In the modern atomic picture, bonds are created through orbital interactions, so that the number and type of orbitals available for chemical bonds determines the "bondability". For example, for oxygen, which according to the old definition has a valence of II, covalently structured compounds with one, two, three or even four bonds are known. In addition, so-called multi-center bonds can occur and the transition from covalent to ionic interactions is often fluid, so that higher coordination numbers occur than corresponds to the number of valence orbitals of the atom in question.

The value of chemical elements

The list can be sorted by column by clicking on the symbol

Valence of oxygen and hydrogen:

The value of main group elements compared to oxygen

The valence of a chemical element from one of the main groups of the periodic table compared to oxygen always corresponds to the main group number of the chemical element. The elements of main group VIII are not included, as these noble gases usually do not form chemical bonds.

The valence of main group elements compared to hydrogen

Each main group element of groups I to VII has two valences.

  1. The value that corresponds to its main group number
  2. The value that corresponds to the difference between 8 and the main group number

The lower valuation is the more frequent. The elements of main group VIII are not included, as these are noble gases and generally do not form chemical compounds.

The value of subgroup elements

Subgroups usually have a value of 2 or 3, but can also have other values. There are two ways of specifying the value of the respective subgroup element.

A Greek numeral

For example:

  • sulfurtrioxide → S.O3
  • carbondioxide → C.O2

The Greek numeral indicates the index of the element below in the name.

A German numeral

For example:

  • Copper(I.) oxide (pronounced "copper-one oxide") → C.u2O
  • Iron(III) -oxide (pronounced "iron-three-oxide") → F.e2O3

The German numeral for the Roman numeral indicates the value of the element in the name. But not in

Calculation of chemical compounds

From atoms of two different chemical elements

To calculate chemical compounds from atoms of two chemical elements, the smallest common multiple (short: KGV) of the valences of the two chemical elements involved must be formed. In order to determine the number of atoms of the respective element in the compound, the P / E ratio must be divided by the valence.

Example: Determining the chemical formula of water:

  • Valence of hydrogen
    I (or 1) because hydrogen is always monovalent
  • Valence of oxygen
    II (or 2) because oxygen is always bivalent
  • P / E ratio
    2 * 1 = 2
  • Number of oxygen atoms involved
  • Number of hydrogen atoms involved
  • Finished chemical formula:
    H2O1
    and since the number one is not noted in the index of O:
    H2O

Example: Finding the chemical formula of aluminum oxide:

  • Value of aluminum
    III (or 3) because aluminum in III. Main group stands
  • Valence of oxygen
    II (or 2) because oxygen is always bivalent
  • P / E ratio
    3 * 2 = 6
  • Number of oxygen atoms involved
  • Number of aluminum atoms involved
  • Finished chemical formula:
    A.l2O3

Example: Determining the chemical formula of ammonia:

  • Valence of nitrogen
    VIII - V = III (or 8 - 5 = 3) because nitrogen is in main group V
  • Valence of hydrogen
    I (or 1) because hydrogen is always monovalent
  • P / E ratio
    3 * 1 = 3
  • Number of hydrogen atoms involved
  • Number of nitrogen atoms involved
  • Finished chemical formula:
    N1H3
    and since the one in the index of nitrogen is not noted:
    NH3

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Chemical element

Category: Chemical quantity