Symbol notation

from Wikipedia, the free encyclopedia

In the periodic table of the elements (PSE) all elements are indicated with abbreviations, the element symbols; for example Na stands for sodium , C for carbon (from the Latin word carboneum ) or O for oxygen (Latin oxygenium ). The symbols are derived either from the old Latin or Greek or, with the newer elements, from the English names. Some of them are also named after the places of discovery and famous personalities, mostly chemists or physicists .

If a symbol consists of two letters, the first letter is always capitalized and the second letter is capitalized; therefore you should pay close attention to upper and lower case letters.

Examples of the upper and lower case of element symbols

Element symbols with more than two letters do not exist for finally named elements.

Connections in symbol notation

These element symbols are used to express connections of atoms . For this purpose, the symbols of all elements contained in the connection and their number are given.

The number of individual atoms is indicated by an index (small, subscript number after the symbol). The number “1” as an index is almost always left out.

Examples of the symbol notation of important connections:

water = H 2 O ... consists of 2 hydrogen atoms and 1 oxygen atom
Carbon monoxide = CO ... consists of 1 carbon atom and 1 oxygen atom
carbon dioxide = CO 2 ... consists of 1 carbon atom and 2 oxygen atoms
ammonia = NH 3 ... consists of 1 nitrogen atom and 3 hydrogen atoms
Hydrogen peroxide = H 2 O 2 ... consists of 2 hydrogen atoms and 2 oxygen atoms
Sodium chloride = NaCl ... is an ion lattice in which there is 1 chloride anion for every sodium cation

You read out the symbol notation by pronouncing the individual letters and numbers one after the other:

  • H 2 O = H-2-O ( Hah-Two-Oh )
  • CO 2 = CO-2 ( Ceh-Oh-Zwei )
  • Fe 2 O 3 = Fe-2-O-3 ( Ef-Eh-Two-Oh-Three )

The sequence of the element symbols in the formulas of the compounds depends on their position in the periodic table: The element symbol that is further to the left is the first mentioned, as soon as element symbols are one below the other, the lower one is mentioned first.

There are exceptions with hydrogen when it combines with nitrogen or carbon .

Examples with hydrogen at the back :

  • NH 3 = ammonia
  • CH 4 = methane

Naming of connections

In addition to the so-called trivial names , by which many compounds are known (see examples above), there is also a systematic naming, the nomenclature of compounds, which results from the symbol spelling.

The systematic naming of connections results from the names of the elements involved, using numerals. They are named in turn: mono- , di- , tri- , tetra , penta- , hexa- , hepta- , octa- , nona- , deca- , and so on. For example, As 2 O 5 is called diarsenpent (a) oxide in words .

The element in the back is often given with the ending -id - especially in the case of connections that consist of only two elements ; partly using the Latin / Greek. Called.

Examples of endings :

  • -oxid = connection with oxygen (from Latin oxygenium)
  • -sulfid = compound with sulfur (from Latin sulfur)
  • -carbide = compound with carbon (from Latin carboneum)
  • -hydride = connection with hydrogen (from Latin hydrogenium)
  • -fluoride = compound with fluorine
  • -chloride = compound with chlorine
  • -iodide = compound with iodine
  • -nitride = compound with nitrogen

Connections with three or more elements usually have trivial names.

Examples of systematic names:

Fe 2 O 3 Diiron trioxide
CO (Mono) carbon mon (o) oxide
AlCl 3 Aluminum trichloride
H 2 O Dihydrogen monoxide (chemical name for water)

Multiple particles

If you have several particles of a compound, you can also write this using the symbol notation by adding a number (the so-called coefficient ) in front of the formula .

For example, 4H 2 O means that the "4" refers to the whole compound; that is, 4 water molecules contain 8 hydrogen atoms (4 x 2) and 4 (4 x 1) oxygen atoms.

Here, too, you read out the symbol spelling by pronouncing the individual letters and numbers one after the other:

4 H 2 O = 4 H-2-O ( Four-Hah-Two-Oh )

Types of connections

Depending on the type of connection, the symbol notation of a connection can have different meanings:

  1. The symbol notation H 2 O of water describes the composition of the smallest particles in water. These smallest particles are also known as molecules . The notation of molecules with the help of the element symbols is also known as the sum formula .
  2. In the case of many compounds, the so-called salts , however, the formula does not specify the composition of a molecule. The formula NaCl describes the ratio with which the sodium cations and chlorine anions are present in sodium chloride ( table salt ). The ratio here is 1 to 1, which means that there is one chlorine anion for every sodium cation. The formula of salts is therefore a ratio formula . Curly brackets can also be used to delimit molecules, ie {NaCl}.
Another example of relationship formulas : FeS 2 or {FeS 2 }… there are 2 sulfur atoms for every iron atom.

Reaction equations in symbol notation

Symbols are also used to set up reaction equations. Reaction equations describe chemical processes that are observed in nature , for example , or that are intentionally brought about in the laboratory .

The following example describes the combustion of ethanol ( alcohol ) with oxygen to produce carbon dioxide and water:

Instead of the word equation:

Ethanol + oxygen → carbon dioxide + water

one writes in symbol notation:

However, a reaction equation does not only provide qualitative information about the reaction. It also includes quantitative ( how much? ) Information. Since the law of the conservation of mass applies, the mass of the starting materials must be equal to the mass of the end products.

This equality does not yet exist here, which can be seen from the fact that the number of atoms of each element involved does not match on both sides of the reaction equation.

C: 2, H: 5 + 1, O: 1 + 2 C: 1; H: 2, O: 2 + 1

After balancing the reaction equation with the help of stoichiometric factors (or coefficients), it then reads:

The numbers that appear in subscripts in the formulas (for example H 2 O) are called index numbers . These relate to the chemical element in front of it and indicate how many atoms of it are in the molecule. If the formula of a substance is known or has been established, these numbers may no longer be changed, because other numbers would indicate new substances.

The numbers that appear in front of the chemical formulas in the formula scheme are called coefficients . These indicate the number of molecules of the respective substance and may be changed for the compensation calculation.

Principles of the stoichiometric equalization calculation (mathematical)

Complex compensation calculations can be solved with the help of systems of equations.

For example using this equation:

A different variable is assigned for each starting material and product, so that the reaction equation looks like this.

Now you set up a balance equation (also called sum or gross equation) for each element. To do this, the individual coefficients for each element are multiplied by the number of elements in the respective molecule and the terms are summed up. The sum of the starting materials and products is equated. Applied to each element, the following system of equations results:

Four unknowns and three equations cannot be solved clearly. However, there are two constraints that allow the system of equations to be solved uniquely: 1. The solution set is the set of natural integers for all the coefficients sought. This comes from the fact that the world is made up of atoms. 2. The smallest set of these coefficients is sought. I.e. If all the coefficients are divisible by an integer, this division must also be made. Due to these constraints, it is possible to set a variable temporarily. Whereby it is generally advantageous to set it with 1 and if fractional numbers result, then to multiply the whole set of all coefficients with the denominator. If a larger number is used, then it must be checked whether the resulting solution is the set of smallest numbers. The result is the following solution:

The result is now:

What you also need to know

In addition to the formulas of important compounds (such as water), you also need to know the following:
Many gases (apart from the noble gases ) in their elemental state (i.e. as elements) do not consist of individual atoms, but of molecules with two atoms, such as B. O 2 , N 2 , Cl 2 , F 2 . As a rule, one can say that all gaseous elements under normal conditions are present as diatomic molecules with the exception of the noble gases. The halogens are also diatomic, i.e. the elements of the 7th main group (fluorine, chlorine, bromine, iodine).

It is also important whether a number is used in front of it ( stoichiometric coefficient) or below (as an index):

2 cl = 2 single chlorine atoms
Cl 2 = 1 particle that is composed of 2 chlorine atoms
CO 2 = 1 particle ( molecule ) that consists of 1 carbon atom and 2 oxygen atoms
2 CO = 2 particles, each consisting of 1 carbon atom and 1 oxygen atom
3 CO 2 = 3 particles each consisting of 1 carbon atom and 2 oxygen atoms; So it contains 3 carbon atoms and 6 oxygen atoms.

history

The symbols for the chemical elements were introduced in 1813/1814 by the Swedish chemist Jöns Jakob Berzelius (1779–1848).

Individual evidence

  1. ^ Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, p. 1369.