The structure of matter

The building blocks of matter are atoms. They are constituted of protons, neutrons and electrons. They can be classified into elements, which are classified in the periodic table. They can combine to form molecules. Atoms and molecules can be electrically charged, and form ions.

Protons, electrons and neutrons

Protons are positively charged particles. The number of protons of an atom determines which element it belongs to. (See the example in the "Elements" paragraph.)

Nitrogen atoms have 7 protons.

Neutrons are non-charged particles. The atoms of a given element can have different numbers of neutrons, and they are called isotopes. Different isotopes of an element have the same chemical properties.

Carbon atoms all have 6 protons, but can have 6, 7 or 8 neutrons. They are the different isotopes of carbon.

Together, protons and neutrons form the nucleus of the atoms, located at its center. The nucleus is very small compared to the size of the atom, but represents the biggest part of its mass.

Electrons are negatively charged particles. They are located around the nucleus. They represent a small part of the mass of an atom, but a big part of its size. They are responsible of the chemical bonds in the molecules.

Elements and the periodic table

The elements are all the different kinds of atoms that exist. Each element has its symbol.

Name Symbol
Oxygen O
Magnesium Mg
Chlorine Cl
Copper Cu
Uranium U

All the known elements are classified in the periodic table. They are classified by number of protons. Elements which are in the same column of the table have similar chemical properties.

The periodic table

You can download the periodic table as a PDF.


Molecules are made of atoms linked together. These links, called chemical bonds, can be simple, double or triple.

Molecules can be symbolized by their molecular formula. A molecular formula contains the symbols of the elements composing the molecule, followed by the number of atoms of these elements written in subscript, if this number is superior to 1.

The molecular formulas of some common molecules :

Name Molecular formula
Water H2O
Carbon dioxide CO2
Aspirin C9H8O4

A molecular formula tells us which elements (and how many) are present in a molecule, but it doesn't tell us how the atoms are linked together. This information is provided by the structural formula of the molecule. A structural formula contains the atoms (represented by their symbols) and the chemical bonds (represented by simple, double or triple lines, respectively for simple, double or triple bonds) of the molecule.

Name Structural formula
Carbon dioxide


Ions are atoms or molecules that are electrically charged. This charge can be either positive or negative. Ions can have one or multiple charges. Ions are represented by the symbol of the element (for an atomic ion) or the molecular formula (for a molecular ion) followed by the charge written in superscript.

Here are some common ions :

Name Symbol
Sodium ion Na+
Calcium ion Ca2+
Hydroxide OH-
Chloride Cl-
Sulfate SO42-
Phosphate PO43-

Ions are always associated to other ions, to form electrically neutral ionic compounds. There must be as much positive charges as negative charges in the compound in order to be neutral.

In sodium chloride, each sodium ion (charge: +1) is associated to a chloride ion (charge: -1). In iron hydroxide, each ferric ion (charge: +3) is associated to three hydroxide ions (charge: -1).

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