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The structure of a covalent compound molecule based on its formula

The chemical formula of a covalent compound describes the structure of its molecules. Symbols of elements used in the formulae inform about types of atoms creating a molecule and numbers placed on at the bottom right of the symbol inform about their number. The numbers we use to express the number of atoms in a molecule are called stoichiometric indicesstoichiometric indexstoichiometric indices. If stoichiometric index is equal to one, number 1 is not written down. In a chemical formulae of water ${\text{H}}_{\text{2}}\text{O}$ stoichiometric index for hydrogen is 2 and the one for oxygen is – 1. On this basis, we can say that water molecule consists of two atoms of hydrogen and one atom of oxygen.

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Ilustracja przedstawia cząsteczkę wody ha dwa o oraz napisy objaśniające : The symbols of elements mean, that the water molecule is composed of hydrogen (H) and oxygen (O) atoms. The stoichiometric index placed after the hydrogen symbol in the water molecule formula indicates, that it contains two hydrogen atoms.

Description of the set of molecules

If we want to present a set consisting of more than one molecule, for example of three molecules of water, the number of these molecules is placed before its formula. Three molecules of water (${\text{H}}_{\text{2}}\text{O}$, ${\text{H}}_{\text{2}}\text{O}$, ${\text{H}}_{\text{2}}\text{O}$) will be described as $\text{3}{\text{H}}_{\text{2}}\text{O}$. The number placed before the compound’s formulae is called stoichiometric numberstoichiometric numberstoichiometric number.

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lustracja przedstawia 3 cząsteczki wody 3( współczynnik stechiometryczny) ha dwa o. Każda z cząsteczek wody przedstawiona jest jako czerwona kulka połączona w dwiema białymi kulkami. Ilustracja zawiera anglojęzyczną nazwę : stoichiometric coefficient

Let’s try to determine the number of individual atoms of elements in the set of three molecules of water: ${\text{3H}}_{\text{2}}\text{O}$. One molecule contains two atoms of hydrogen and one atoms of oxygen, while three molecules contain in total $\text{3 · 2}$, that is six atoms of hydrogen and $\text{3 · 1}$, that is three atoms of oxygen.

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lustracja przedstawia liczbę atomów poszczególnych pierwiastków w trzech cząsteczkach wody. 3 czerwone kulki z których każda połączona jest 2 białymi kulkami z atomami wodoru, razem 6.

Five molecules of ammonia, ${\text{5NH}}_{\text{3}}$, consist in total of ($\text{5 · 1 = 5}$) five atoms of nitrogen and ($\text{5 · 3 = 15}$) fifteen atoms of hydrogen.

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Ilustracja przedstawia liczbę atomów poszczególnych pierwiastków w pięciu cząsteczkach amoniaku. 5 atomów azotu - pięć niebieskich kulek, połączonych każda z 3 białymi kulkami (15) atomy wodoru. Ilustracja zawiera anglojęzyczne nazwy: nitrogen atoms, hydrogen atoms.

How to describe molecules

In OIndeks dolny 3₃ molecule a stoichiometric index with an element’s symbol equal to 3 indicates that ozone molecules are made up of three atoms of oxygen.

A stoichiometric number written before the symbol (for example $\text{6Cl}$ – 6 atoms of chlorine) or a formula (for example ${\text{6Cl}}_{\text{2}}$ – 6 molecules of chlorine) denotes the number of atoms or molecules. $\text{6Cl}$ indicates 6 free unbound atoms of chlorine, while ${\text{6Cl}}_{\text{2}}$ – indicates that there are 6 diatomic chlorine molecules, a total of 12 atoms.

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Ilustracja przedstawiająca six molecules of chlorine w postaci 6 par zielonych kulek.

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Ilustracja przedstawiająca six atoms of chlorine w formie sześciu zielonych kulek.

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Tabela opisu interpretacji chemicznych substancji.

Interpretation of formula of ionic compounds

Ionic compounds are formed when metals bind with non‑metals. Such compounds are made of ions that are arranged alternately and form structures known as crystals. In a crystal there are no isolated groups of atoms that could be called molecules. A molecular formula of an ionic compound thus represents the smallest set of repetitive ions in a crystal. Stoichiometric indices denote a number of ions of given element in this set.

In ionic compounds consisting of two elements, metal is always a cation and non‑metal is – an anion. Sodium chloride is an example of such a compound. Its formula, $\text{NaCl}$, indicates that a crystal of sodium chloride is made of sodium cations (${\text{Na}}^{\text{+}}$) and chloride anions (${\text{Cl}}^{-}$), and their quantitative ratio is 1:1.

Numbers placed before the formula of an ionic compound (stoichiometric numbers) denote a number of the smallest sets making up for a crystal. For example $\text{10NaCl}$ denotes ten sets which consist of one sodium cation (${\text{Na}}^{\text{+}}$) and one chloride anion (${\text{Cl}}^{-}$).

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Zbiór opisany jako 10 na ce el zawiera 10 jonów sodu i 10 jonów chlorkowych. Ce el minus przedstawiony został graficznie jako większe zielone kulki, na plus jako mniejsze szare kulki.

Interpretation of chemical formulae

Summary

• A certain number of molecules, atoms and ions can be represented using chemical formulae.

• The number of atoms in a molecule or ions in a unit is denoted using a stoichiometric index. In a formula this number is written down at the bottom right of the element’s symbol.

• The number of atoms, molecules or units of an ionic compound is denoted using a stoichiometric number – the number written before the molecular formula of a compound.

Keywords

chemical formula, stoichiometric index, stoichiometric number, molecule, atom

Glossary