A chemical bond is a strong force of attraction holding atoms together in a molecule or crystal.
Atoms can form 1, 2 or 3 bonds between them.
One bond between two atoms is called a single bond and an example is the bond between Na and Cl.
Two bonds between two atoms is called a double bond and an example is the bond between Ca and O.
Three bonds between two atoms is called a triple bond and an example is the bond between Al and N.
Atoms tend to gain, lose or share electrons until they are surrounded by eight valence electrons.
An ionic bond is a chemical bond that results from the electrostatic attraction between positive metal ions and negative non-metal ions. As a metal and a non-metal approach one another, the valence electrons interact and the metal (indicated by the red sphere) transfers its valence electrons to the non-metal (indicated by the blue sphere). The metal becomes positively charged through the loss of electrons and the non-metal becomes negatively charged through the gaining of electrons. In this way, both the metal and the non-metal complete their valence shell to obtain a stable electron configuration.
Example
Sodium has its 11 electrons arranged in three energy levels. Closest to the nucleus will be the first level containing 2 electrons, then the second level containing 8 electrons, leaving 1 electron in the outermost energy level. The easiest way for sodium to gain a stable electron configuration is to donate the single outermost electron to another atom. This will result in a +1 charge for this new entity - a sodium ion (Na+).
Chlorine has 17 electrons, 7 of which are in its outermost energy level. It can readily gain a stable electron configuration by gaining 1 electron from another atom. This means it will have a net charge of -1, and becomes Cl-.
Therefore, one sodium atom will combine with one chlorine atom to form sodium chloride.
Ionic bonds are the most polar type of bond.
An ionic bond does not form a molecule, but rather a crystal and it has no set size. This crystal lattice structure has cations surrounded by anions and the anions are surrounded by cations.
As two non-metals approach one another, the valence electrons interact. A covalent bond is formed between the two non-metals, which share a pair of electrons so that each obtains a filled valence shell.
Covalent bonds are the most important type of bond occurring in organic molecules. Depending on the involved partners, simple, double or triple bonds can be formed.
There are two types of covalent bonds. In a non-polar covalent bond, the electrons are shared equally between the two atoms. The electrons move around the nuclei with the electrons generating temporary positive and negative charges within the molecule.
In a polar covalent bond the electrons are shared unequally between the two atoms. In this situation, one atom has a greater ability to pull the bonding electrons towards it and is said to be more electronegative. The electrons move around the nuclei with the electrons spending the majority of the time near the more electronegative element. This generates a partial negative charge near the more electronegative element and a partial positive charge near the less electronegative element.
Since covalent compounds do not break up into ions when they melt or boil, or when they are added to a solution, they are also called molecular compounds.
Molecular polarity is dependent upon bond polarity and molecular geometry. For small molecules, if all the regions surrounding an atom are similar in their electronegativities, the molecule will be non-polar. If the regions are different, then the molecule will be polar. It is possible to have a molecule with 4 polar bonds, each pulling in their own direction, giving the molecule as a whole a change of 0. This is similar to two teams pulling with equal strength in opposite directions at tug-o-war; the result is no movement.
The type of bond that occurs between two atoms can be determined by calculating the difference between the electronegativities of the two atoms involved. It should be noted however, that when a metal and a non-metal are involved in the bond, it will be an ionic bond.
Ionic compounds have the following properties:
Contain a metal and a non-metal
Have strong bonds
Have high melting points
Are soluble in water
Dissociate into ions in solution
Are hard and brittle
Most are soluble in polar solvents
Usually form crystalline compounds in a solid state
All ionic compounds are solids at room temperature
Are poor electricity conductors as solids
Can conduct electricity when dissolved in a solution or when in a liquid form
Covalent compounds have the following properties:
Contain non-metals
Weak bonds
Valence electrons are shared between atoms
Polar molecules dissolve in polar solvents
Volatile
Low melting and boiling points
Insoluble in water
Non polar compounds do not conduct electricity; polar compounds can conduct small amounts of electricity
As two non-metals approach one another, the valence electrons interact. A covalent bond is formed between the two non-metals, which share a pair of electrons so that each obtains a filled valence shell. Covalent bonds are the most important type of bond occurring in organic molecules. Depending on the involved partners, simple, double or triple bonds can be formed.
Intermolecular attractions are attractions between one molecule and a neighbouring molecule. All intermolecular attractions are known collectively as Van der Waals forces.
Electrostatic forces occur between charged species and are responsible for the extremely high melting and boiling points of ionic compounds and metals.
All molecules have the capability to form London forces (also known as dispersion forces). They result from the movement of the electrons in the molecule which generates temporary positive and negative regions in the molecule. These are the only types of forces that non-polar covalent molecules can form.
Dipole-dipole forces are caused by positive and negative ends of a molecule which attract each other like little magnets. Only polar covalent molecules have the ability to form dipole-dipole attractions between molecules.
Hydrogen bonding occurs between polar covalent molecules that possess hydrogen bonded to an extremely electronegative element, specifically nitrogen, oxygen and fluorine. These often occur in organic molecules. It is much weaker than a covalent bond, but in large molecules with many hydrogen bond interactions, the sum of the interactions can be an important determinant in retaining the shape and structure of a molecule, such as protein and DNA structure.