Why are significant numbers important to the chemist



In the periodic table of the elements (PSE) all substances are indicated with abbreviations, with atomic symbols, such as sodium = Na, carbon = C (from the Latin word carboneum) or oxygen = O (Latin oxygenium). The symbols are derived either from the old Latin or Greek names of the elements or, in the case of the newer elements, from the English names.

If a symbol consists of two letters, the first letter is always capitalized and the second letter is capitalized.

Examples for the upper and lower case of element symbols

Connections in symbol notation

These atomic symbols are also used to express connections between 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 = H2O ... consists of 2 hydrogen atoms and 1 oxygen atom
Carbon monoxide = CO ... consists of 1 carbon atom and 1 oxygen atom
carbon dioxide = CO2 ... consists of 1 carbon atom and 2 oxygen atoms
ammonia = NH3 ... consists of 1 nitrogen atom and 3 hydrogen atoms
Hydrogen peroxide = H2O2 ... consists of 2 hydrogen atoms and 2 oxygen atoms
Table salt = NaCl (sodium chloride) ... consists of 1 sodium atom and 1 chlorine atom

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

  • H2O = "H-2-0" (Hah-two-oh)
  • CO2 = "C-O-2" (Ceh-oh-two)
  • Fe2O3 = "Fe-2-O-3" (Ef-Eh-Two-Oh-Three)

The order of the atomic symbols in the formulas of the compounds depends on the position in the periodic table: The atomic symbol, which is further to the left, is the first named, as soon as atomic symbols are one below the other, the lower one is named first.

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

Examples where hydrogen is at the back:

  • NH3 = Ammonia
  • CH4 = Methane

Naming of connections

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

The systematic naming of connections results from the names of the elements involved, using numeral words. They are named in turn: mono-, Di-, Tri-, Tetra, Penta, Hexa-, Hepta-, Octa-, Non-, Deca-, and so on. That's the name of As2O5 for example in words Diarsenpent (a) oxide.

The element in the end is often used with the ending -id specified; partly using the lat./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

Connections with three or more elements usually have trivial names.

Examples of systematic names:

Fe2O3Diiron trioxide
CO (Mono) carbon mon (o) oxide
AlCl3Aluminum trichloride

Multiple particles

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

For example, 4 means H2O: 4 water particles = 8 hydrogen atoms and 4 oxygen atoms (because the 4 refers to the whole compound).

Types of connections

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

  1. With the symbol notation H2The composition of the smallest particles of water is described by water. These smallest particles are also known as molecules. The spelling of molecules with the help of the atomic 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 of table salt describes the ratio with which the sodium atoms and chlorine atoms are present in the table salt. Here the ratio is 1 to 1, i.e. for every sodium atom there is one chlorine atom. The formula of salts is therefore a ratio formula. To differentiate between molecules, you can also use curly brackets, ie {NaCl}.
Another example of ratio formulas: FeS2 or {FeS2} ... for every iron atom there are 2 sulfur atoms.

Reaction equations in symbol notation

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

The following example describes the combustion of alcohol (ethanol) with oxygen to 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 (What?) Information about the reaction. Quantitative (how much?) Information is included. 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 + 2C: 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 the formulas (for example, H2O) subscript occur is called Index numbers. These relate to the chemical element in front of it and indicate how many atoms of it are present 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 in the formula scheme in front the chemical formulas are called Coefficients. These indicate the number of molecules of the respective substance and may be changed for the offset calculation.

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) do not consist of individual atoms in their elemental state (i.e. as elements) but of molecules with two atoms, such as O2, N2, Cl2, F2. As a rule, one can say that all gaseous elements under normal conditions except the noble gases are present as diatomic molecules. 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
Cl2 = 1 particle composed of 2 chlorine atoms
H2O = 1 particle (molecule), each consisting of 2 hydrogen atoms and 1 oxygen atom
2 HO = 2 particles, each consisting of 1 hydrogen atom and 1 oxygen atom
3 H.2O = 3 particles each consisting of 2 hydrogen atoms and 1 oxygen atom; it contains 6 H atoms and 3 O atoms.

Category: Chemistry