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Introduction to Chemical Formulae

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Introduction to Chemical Formulae
Organic Chemistry | Alkanes | Alkenes | Alkynes | Alkanols
Alkanoic acids | Esters | Alkanals | Alkanones | Further information


Carbon (organic) compounds are mostly bonded together by covalent bonds - bonds in which the atoms share electrons. These are strong chemical bonds that cannot easily be broken.

The number of electrons being shared is double the number of bonds that there are between the two atoms. Therefore, when

  • two electrons are shared, the bond is a single bond
  • four electrons are shared, the bond is a double bond
  • six electrons are shared, the bond is a triple bond

The most common atoms found in organic compounds are carbon atoms (C), hydrogen atoms (H) and oxygen atoms (O). In most organic compounds, carbon forms four bonds, hydrogen one bond and oxygen two bonds. The number of bonds that an atom must form is known as its valency. Therefore, for example, carbon has a valency of four.

A single bond is shown as X - Y where X and Y are the two atoms bonded together.

Similarly. a double bond is shown as X = Y and a triple bond as X ≡ Y


In chemistry, there are four main types of formulae:
  • The empirical formula
  • The molecular formula
  • The displayed (structural) formula
  • The general formula

Empirical Formula

This is the simplest whole-number ratio of atoms in a substance. For example, all hexose sugars1 have the empirical formula CH2O. This shows that, for every carbon atom, there are two hydrogen atoms and one oxygen atom.

Molecular Formula

This shows the exact number of atoms in every molecule of the substance. It is possible, but not essential, for the molecular formula to be the same as the empirical formula. For example, sucrose has the molecular formula C12H22O11. This shows that in every sucrose molecule, there are twelve carbon atoms, twenty-two hydrogen atoms and eleven oxygen atoms.

The Displayed Formula

This is also known as the structural formula of a molecule. This shows both the number of each element2 in the compound and also which atoms of each element are bonded together. For example, methane (CH4)3, has the displayed formula:

H | H - C - H | H

This shows that the carbon atom is bonded by single bonds to all four hydrogen atoms.

The General Formula

This is a formula that shows the ratio of atoms in a group of similar organic compounds (a homologous series). For example, all alkanols4 have the general formula CnH2n+1OH where n is the number of carbon atoms in the molecule. Therefore, ethanol5 (where n = 2) has the molecular formula C2H5OH.

1Sugars that contain six carbon atoms. For example, glucose and fructose.2An element is a type of atom. For example, hydrogen is an element and carbon is an element.3Methane is natural gas.4Alkanols are often referred to as alcohols.5Ethanol is the alkanol referred to when people say 'alcohol'.

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