Elements, Mixtures and Compounds and Atoms and Molecules

This page is about the composition of elements, mixtures and compounds.
For more general information see the main page of definitions of elements, mixtures and compounds.

A common question concerns the smallest identifiable units within elements, mixtures and compounds whose structure and properties explains the properties of the bulk material (element, mixture or compound).
Are they atoms or molecules, or something else ?

This can be answered in both words and diagrams. It often helps to include both.

More Advanced Note: There are also even smaller units called 'sub-atomic particles' which, together, form atoms. It is not necessary to know about those in order to understand the differences between elements, mixtures and compounds so they are not included on this page.

See the images below and the following text.

In the following diagrams shown above ('Elements') and below ('Mixtures', followed by 'Compounds'):

Atoms are represented by single spheres.
Spheres of the same size and colour represent atoms of the same element.

Molecules are represented by two or more spheres joined together.

Hence molecules of elements are reresented by two or more spheres of the same size and colour joined together.

In the following diagrams ('Mixtures', followed by 'Compounds'):

Molecules of Compounds are represented by two or more spheres of different sizes and colours joined together.

Do elements, mixtures and compounds consist of atoms or molecules ?

Answer in words:

  • Elements can consist of either atoms or molecules but if molecules then those molecules are formed only from atoms of the same type (that is, atoms of the same element).
    For example, a molecule of oxygen consists of two atoms of oxygen and has the chemical formula O2 where "O" is the chemical symbol of the element oxygen.
  • Mixtures can consist of either atoms or molecules - but must include at least two different atoms or molecules.
    For example, a mixture of neon and argon gases would consist of atoms only because both neon and argon exist as atoms rather than as molecules. However, a mixture of oxygen and nitrogen gases would consist only of molecules because oxygen gas exists as oxygen molecules (O2) and nitrogen gas exists as nitrogen molecules (N2). A mixture of neon and nitrogen gases would consist of atoms of neon and molecules of nitrogen.
  • Compounds consist only of molecules (not individual atoms) and all the molecules of any one compound are the same. For example, methane gas has the chemical formula CH4 because each molecule of methane is formed from one atom of carbon, whose chemical symbol is 'C', and four atoms of hydrogen, whose chemical symbol is 'H'. Pure methane gas does not include any other atoms or molecules apart from the methane molecules described by the formula CH4.

This can be summarized in a table:

Atoms

Molecules

Elements

Either atoms or molecules, but only of one type of atom.

How do you know which it is ?
Solid elements, e.g. metals, consist of many atoms of the element packed very densely together, e.g. copper, whose symbol is 'Cu'. Only a few elements exist naturally as a liquid at standard temperature and pressure. An example is the metal mercury, whose symbol is 'Hg', which takes the form of many atoms that can move around each other easily but not away from the other atoms into the surrounding space, as in the case of gases.

Many elements exist as a gas at standard temperature and pressure. In some cases, e.g. the inert or 'noble' gases, the gas exists in the form of individual atoms, whereas in other cases the gas exists in the form of molecules (e.g. diatomic molecules - which means that each molecule is formed from two atoms of the the element, attached together), e.g. oxygen gas, whose formula is O2, nitrogen gas whose formula is N2 and hydrogen gas whose formula is H2.

Elemental gases exist as either atoms or molecules according to the size and structure of the atoms of that element - which is usually explained later in school chemistry lessons.

Mixtures

Any combination of (different) : atoms + atoms

or molecules + molecules

or atoms + molecules

That is, in the case of mixtures there must be at least two different types of atoms,
or at least two different types of molecules,
or at least one type of atom plus at least one type of molecule present.

So, a mixture of gases may consist of molecules of the element oxygen (O2) plus molecules of the element nitrogen (N2) plus molecules of the compound carbon dioxide (CO2).
Alternatively, a mixture of gases may consist of atoms of neon (Ne) and atoms of argon (Ar), and so on ...

Compounds
 

Compounds consist of only one type of molecule: A molecule is the smallest part of a compound whose properties are those of the compound.

More advanced: What else could there be ?

Atoms and molecules are usually the first chemical units to be introduced in school chemistry lessons.
Ions are also important but are slightly more complicated so are usually introduced later. The first thing to know about ions is that they have electrical charge and so are either positive (+) or negative (-). It is important to know about ions in order to explain certain crystalline structures (solids) and also the behaviour and reactions of certain chemical solutions.

For example, at an introductory (basic chemistry) level, water may be described as the molecule H2O.
If you continue to study chemistry to a more advanced level you may also need to be able to understand water as consisting of both hydrogen ions (H+) and hydroxide ions (OH-).

As descriptions of elements, mixtures and compounds are among the first topics taught in chemistry lessons in school, they are usually in terms of atoms and molecules only. Complete homework using material taught in class.


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