Atomic and Molecular Bonding - electrical theory - (science -campus)

1.07 Atomic and molecular bonds

How atoms bond to form molecules
There are three ways in which atoms bond to form molecules
Covalent bonds The atoms share their outer electrons, this helps to stabilise the outer shell of each atom. The electrons orbit around each atom holding them together
Polar covalent The atoms share their outer electrons as above but one of the atoms has a more dominant effect and the electrons are offset towards the more dominant atom. This causes a negative charge at the dominant atom and a positive charge at the other. Hence the molecule is called a dipole.
Ionic bonds One atom has a very loosely bound outer electron and the other needs electrons to fill the outer shell. As a result the electron is detached from one atom and is added to the other. This leaves a positive and a negative ion which then bond together due to electrostatic attraction.
How molecules bond to form materials
Covalent bonds The electrons of neighbouring atoms orbit around each other. These are the strongest bonds, the material is effectively a giant molecule.
Molecular bonding The dipoles formed from polar covalent bonded molecules bind together due to electrostatic attraction.
Ionic bonds The positive and negative ions bond together due to their electrostatic attraction.
Metallic bonds The outer electrons become dislocated from their parent atoms creating positive ions and free electrons. The material can be thought of as positive ions in a sea of electrons. The electrostatic attraction between the positive ions and the negative electrons holds the material together.

From above it can be seen that materials having a metallic structure have more free electrons than other types of structures. In materials with a structure made of covalent bonds or molecular bonds the electrons are "held" in the covalent bonds (for molecular bonding the covalent bonds are between the atoms in the molecules) In ionic bonds the electrons are held by the ions.