Summary

An atom is the smallest particle of matter that can combine chemically with another atom or molecule.  It has a nucleus, composed of protons and neutrons, around which electrons orbit.
These particles have mass and another property called electric charge:

• The proton carries a positive one charge noted by +e
• The electron carries a charge equal to the proton but opposite in charge (-e).
• The neutron carries no charge and is therefore neutral.

The number of protons in an atom signifies its name:

• 1 proton: Hydrogen
• 2 protons: Helium
• 17 protons: Chlorine
• …

An atom has as many protons as electrons. The total charge is zero which is why matter is usually neutral.

In some cases, with the help of an external energy input, an atom can lose or gain one or more electrons. This is the phenomenon of ionization.

Example:  An atom of sodium (Na) has 11 protons and 11 electrons. If it loses an electron, it  then has 11 protons but only 10 electrons. A positive charge +e is no longer balanced and the resulting sodium ion is positively charged. The sodium ion, Na+, indicates that it carries a charge +e.

A molecule is an assembly of atoms that share electrons to achieve greater stability.

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Learning Goals

• To know the structure of an atom and a molecule.
• To define an electrical charge.
• To understand the phenomenon of the ionization of an atom.
• To know the difference between a valence electron and a free electron.
• To recognize two types of atomic bonds.

Learn More

Atoms are often depicted with electrons orbiting around its nucleus in strict orbits called energy levels or layers.  The orbit, or layer, closest to the nucleus has the highest energy associated with it. Those further away have less energy holding electrons in the orbit or layer. The outer most layer is called the valence layer. 

Some atoms are ionized more easily than others. A chemical law states that for every atom of more than 4 protons (Z> 4), the electrons combine for eight in the outer most layer (valence layer). This law is called the Octet Rule. 

Therefore, for chlorine (Z=17), 7 electrons are found in the valence layer and it is missing an eighth to satisfy the Octet Rule.  That is why the chlorine atom ionizes easily by capturing a free electron. The chlorine atom then becomes a chloride ion Cl-.

Inversely, sodium (Z=11) has only one electron in the valence layer.  It needs only to loose that electron to satisfy the Octet Rule.

Because of this, chlorine and sodium combine very easily to form the molecule NaCl (sodium chloride).  This is the combination of two ions Na+ and Cl-. The molecule formed is electrically neutral.