Chapter 7: Covalent Substances

Nota:

• When atoms of non metallic elements combine together they often share electrons between them.  If an atom of H and Cl combine to form HCl, the atoms come close enough together for their outer electron shells overlap.

1. More

   Nota:

   • The shared region of electron shells bond the two nuclei together.
2. Hydrogen Chloride

   Nota:

   • In HCl the shared region of electron shells contain two electrons, one electron was supplied by the H atom and the other by the Cl atom. A bond like this is called a covalent bond. HCl is a covalent compound.  Since the new particle formed is a molecule, HCl is described as being a simple molecular compound because it consists of individual molecules.

Nota:

• Both elements and compounds can exist as simple molecular substances

1. Diatomic

   Nota:

   • The elements hydrogen, fluorine, chlorine, bromine, iodine, oxygen, nitrogen all exist as diatomic molecules under normal conditions
2. Properties

   Nota:

   • Simple molecular substances usually have low melting and boiling points. This is because the forces of attraction between the molecules are weak compared to the electrostatic forces of attraction between oppositely charged ions and therefore very littlle energy is needed to break them
3. Gas'

   Nota:

   • Simple molecular substances are often gas' (H, O, N, and Methane) or low boiling points liquids ( water and bromine), or low melting point solids (iodine) at room temperature. When simple molecular substances change state, the covalent bonds between the atoms are not usually broken. Covalent bonds are strong compared to the forces of attraction between the molecules

Nota:

• Some substances are made up of millions of atoms covalently bonded together to form a giant structure. Like diamond, graphite and silicon dioxide.

1. Graphite

   Nota:

   • Each carbon atom forms three single covalent bonds to other carbon atoms. A layered is formed. Only 3 of the electrons are used to form covalent bonds. The fourth electron from each atom exsists between layers and is delocalised.
   • The covalent bonds are strong. The forces of attraction between the layers are weak.
   • Very high melting point (many strong covalent bonds have to be broken which requires a lot of heat energy) Soft and slippery (forces of attraction between the layers are weak so the layers easily slide over one another and can easily be seperated.Conducts electricity (only 3 valence electrons are used in forming covalent bonds, the 4th is delocalised between the layers and free to move parellel to the layers
   • Used as lubricant Electrodes for electrolysis
2. Diamond

   Nota:

   • Each carbon atom forms four single covalent bonds to other carbon atoms. A three dimensional strucure is formed  All four of the outer shell electrons are used to form covalent bonds.
   • The covalent bonds are strong. There are no weak forces in the structure. Very high melting point  Very hard and abrasive Does not conduct electricity- no electrons are delocalised Used: Cutting tools Jewellry