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Structure of the Atom
An atom consists of a tiny central core called the nucleus. Orbiting the nucleus are electrons. Most of the mass of the atom is concentrated in the nucleus.

Parts of the Atom
The Protons are positively charged particles found in the nucleus.
The Electrons are negatively charged particles that orbit the nucleus.
The Neutrons have no charge and are found in the nucleus.

The following table illustrates the differences between the different sub-atomic particles.

ParticleChargeSizeWhere found
Electron-1Approx 1/2000Orbiting Nucleus
Proton+11In Nucleus
Neutron01In Nuclear

Electron is green Proton is blue Neutron is Grey

Electron Energy Levels
The electrons are arranged in orbitals or energy levels.These orbitals are sometimes referred to also
as shells. The first energy level is closest to the atom and holds a maximum of 2 electrons.The second
level holds a maximum of 8 electrons. Any other electrons left over are put in the third energy level.
Electrons fill the lowest energy level firstly and then progressively fill outwards.

3rd Energy Level
2nd Energy Level

1″ Energy Level

An atom is electrically neutral when the number of the protons equals the number of the electrons.
The difference between atoms is simply the number of protons. For example Hydrogen has one
proton and Helium has two protons.
To help us understand the structure of an atom we use the following ideas:

Mass number: this is the number of protons plus the number of the neutrons.
Atomic number: this is the number of protons.

Number neutrons = Mass number – atomic number

This means to work out the number of the neutrons.

These definitions are illustrated in the following example:

The largest number is the mass number, The smaller number is the atomic number

The number of neutrons = 12-6
= 6

Therefore C has 6 neutrons, 6 protons and, when electrically neutral, has 6 electrons.

An atom can also exist with a different number of neutrons. An atom which has a varying number of
neutrons is called an isotope.


All these atoms are hydrogen. they have specific names to identify them.

Periodic table of the Elements

Periodic Table of the Elements
The periodic table is a table of the elements placed in ascending order of atomic number.
The periodic table is arranged in periods and groups.
• A period is a row running from left to right.
• A group is a column. In these groups elements will react in similar ways, e.g. the halogens are
group 17 and have, for example, Fluorine, Chlorine, Bromine, etc.

s3Valence Electrons 

A substance of one type of atom is called an element. Everything we see, touch and smell is normally many elements joined together. Before we can determine how they are joined together we must first determine the number of valence

Procedure to determine the valence electrons:

1. Determine the atom’s atomic number. e.g. Al (aluminium) is 13. This will be 3rd Energy Level equal to the number of electrons in remaining electron, an electronically neutral atom. Therefore the no. of electrons = 13.

2. Fill the shells innermost first, then 2nd Energy Level moving outwards.

3. The electron(s) in the outermost shell regardless of what orbital it is are called the valence elections. Aluminium has three valence electrons. 

Lithium Fluoride crystal lattice. 

The ionic bonding that occurs between metals and non metals.

Fluoride ion is green and the lithium ion is purple.

Covalent Bonding

Covalent bonding is when electrons are shared between atoms. The sharing of these electrons binds the atoms together, as it fills the orbitals of both atoms and makes them stable.

Hydrogen Atom

Hydrogen Molecule – shared electrons

Hydrogen Atom

Both atoms share the two electrons and fill the first orbital, This makes the new molecules stable.  The electrons can orbit both atoms.


Metallic Bonding
Metal atoms combine together in a lattice network. This means the metal atoms line up alongside one another. The outermost electrons are free to move through the lattice. When they all move in one direction they form an electric current.  

The following example of a copper (Cu) wire illustrates and describes how this occurs.


Electrons (black dots) flow from left to right

Electrons moving Copper wire are 
an electric current.

Another way to describe metallic bonding is metal atoms in a ‘sea of electrons’. Metals generally have high melting and boiling points due to a strong attraction in the metallic lattice.