Metals

The metallic elements are found on the left-hand side of the Periodic Table. Metals are lustrous, malleable, and ductile. They conduct heat and electricity quite well. Metals tend to have high melting points with several notable exceptions of which mercury(Hg) is the most well known. Metals are not soluble in water.

The proposed structure for samples of metals is that of the "band theory" of metallic bonding. Metal samples are considered to be composed of metal atoms that are held together by delocalized bonds formed by all of the atoms in the sample. It is proposed that electrons can move easily within the sample. An earlier theory was sometimes called the "electron sea model" which pictured the sharing of valence electrons by all atoms in the sample.

Properties of Metals:

1. Solid and liquid samples of metals can conduct an electric current due to the mobility of the valence electrons of the atoms in the sample.
2. Most metals will have high melting points.
3. Metals are lustrous(shiny), malleable(able to flattened into sheets) and ductile(able to be drawn into wires).

Ionic Compounds

Ionic compounds are formed when metallic elements from the left-hand side of the Periodic Table react with nonmetallic elements from the right-hand side of the Table. Ionic compounds have high melting and boiling points. Many ionic compounds are soluble in water. Ionic compounds are brittle and do not conduct electricity when in the solid form. The molten (liquid) form of ionic compounds will conduct an electric current. In a molten sample of an ionic compound the ions are free to move. The cations can move to the negative electrode and acquire electrons. Similarly, the anions can move to the positive electrode and release electrons, thereby causing a flow of electricity in the outer circuit.

The proposed structure for ionic compounds is that positive cations ( formed from the elements on the left-hand side of the Periodic Table) attract negative anions(formed from the elements on the right-hand side of the Periodic Table) to form a network or lattice of oppositely charged ions arranged in three-dimensional patterns that depend on the size and charge of the ions.

Properties of Ionic Compounds

1. Solid ionic compounds do not conduct an electric current.
2. Molten samples of ionic compounds can conduct an electric current due to the mobility of the ions which are free to move to the electrodes and react.
3. Ionic compounds have high melting and boiling points.
4. Many ionic compounds can dissolve in water.
5. Dissolved ionic compounds separate into cations and anions in solution.
6. The mobile ions move to the electrodes and react to accept and release electrons creating a flow of electricity in the outer circuit.
7. Ionic compounds that are water soluble are strong electrolytes.

Molecular Compounds

Molecular compounds form when two or more nonmetal atoms form units that are called molecules. Molecular compounds generally have low melting and boiling points. Molecular compounds do not conduct electricity in the solid form or in the liquid form. Some molecular compounds dissolve in water and some do not.

The proposed model for molecular compounds is that atoms of nonmetallic elements can bond by sharing electrons to form units called molecules. A very common example is the water molecule that forms when an oxygen atom shares electrons with two hydrogen atoms to form one unit that is called a water molecule. A sample of water is thought to be a set of these molecular units.

Molecules are attracted to other molecules by attractions that are called Intermolecular Attractions (attractions between molecules). In a physical change such as melting or evaporation these intermolecular attractions must be broken. Each molecule is still a molecular unit but it is separated from the other molecular units. Intermolecular attractions are generally weaker attractions and do not require as much energy to break as do metallic, ionic or covalent bonds. Therefore, molecular compounds generally have low melting points and low boiling points.

Molecules may be polar molecules which have a charge separation due to the shape of the molecule and the polarity of the bonds between the atoms forming the molecule. These molecules are pictured as having a positive end and a negative end and are said to be dipoles or polar molecules. Other molecules are non-polar molecules. These are molecules that have no charge separation again due to the shape of the molecule and the polarity of the bonds between the atoms forming the molecule. These non-polar molecules are pictured as having no dipole character.

LIKE DISSOLVES LIKE. Polar molecules tend to dissolve materials with charged nature such as ionic compounds and other polar molecules. Likewise, non-polar molecules tend to dissolve other non-polar molecules.

Properties of Molecular Compounds

1. Solid molecular compounds do not conduct an electric current.
2. Molten samples of molecular compounds do NOT conduct electricity.
3. Molecular compounds have low melting and boiling points.
4. Polar molecular compounds can dissolve in water. Some very polar molecular compounds can be "ripped" into ions by the water molecules. These are said to be "dissociated" into ions by the dipole nature of the water molecules.
5. Polar molecular compounds are usually not soluble in non-polar solvents.
6. Non-polar molecular compounds are usually not soluble in water. They are soluble in other non-polar solvents such as toluene and other organic solvents.
7. Polar molecular compounds that are dissociate into ions by water electrolytes.