Bonding in chemical compounds
Bonding means the joining of atoms together to make compounds. There are three fundamentally different methods by which atoms do this. These are:
· Metallic bonding - may be an element or a compound
These types of bonding must not be confused with the intermolecular forces which exist between molecules ie one molecule to the next.
The type of bonding within a substance gives rise to a bulk structure with corresponding properties. In other words an ionically bonded substance will have bulk properties dependent on and due to the nature of the bonds. This is common sense, after all a house built of straw will have different properties to a house built of bricks or wood.
(If you don't believe me, just ask the three little piggies. A straw house can be blown down very easily, whereas a brick house is very solid and resistant to the biggest huff or puff available to even the biggest baddest wolf).
The smallest particles of a straw house are pieces of straw. The forces of attraction between the straws is very weak and, as such, the whole structure lacks strength. The brick house has strong cement bonding the bricks together and this makes the structure difficult to break down. The key here is not the nature of the particles themselves, rather the strength of the bonds (or attractions) holding them together.
OK, having said all that, to understand the the nature of the bonds that are created between different atoms we need to appreciate how and why they are formed.
Metallic bonding exists between metal atoms. All metal atoms easily lose their outer (valence electons). When an atom does this it makes a positively charged particle called a positive ion (cation). In the metallic structure there are many ions (made from atoms which have lost control of their valence electrons) surrounded by a sea of these delocalised electrons. The ions have positive charges and these are held together by the negatively charged sea of electrons.
Bulk properties: malleable, hard, strong, high m.p., shiny, sonorous, ductile, electrical conductor, heat conductor.
Write down an explanation of each of the bulk properties by reference to the structure and bonding within a metal.
When metals react with non-metals, the metals always wish to lose electrons to form positive ions and in this case the non-metals wish to gain electrons to form negative ions. In this way both species can attain full outer shells. They react together in stoichiometries depending upon the ionic charges (valencies) of the ions to be formed.
2Na + Cl2 -->2NaCl
2Ca + O2 -->2CaO
4Na + O2 -->2Na2O
Bulk properties: High m.p., brittle, hard, soluble in water (many exceptions), electrolyte in solution, electrolyte when molten.
Explain each of the bulk properties by reference to the particles and structure of the compound.
This takes place between non-metals. Non-metals cannot lose electrons to attain full outer shells and so they have to share electrons to create bonds consisting of shared pairs of electrons between atoms.
There are two distinct possibilities:
Giant covalent structure
· Many, many atoms are held together by shared pairs of electrons (covalent bonds) in a giant molecular covalent structure (macromolecule). The whole structure is effectively one molecule.
Simple molecular structure
· Has only a couple of atoms bonded together at a time held by strong covalent bonds to make small discrete molecules. These molecules in turn are then held together by only weak forces of attraction which may be:
1. Van der Waals forces (induced dipole forces)
These two different situations give rise to structures with very different bulk properties.
1. Giant molecular structures
Giant molecular structure: Hard, very high m.p., brittle, insoluble, non-conductors (exception graphite).
Simple molecular structures: Soft, low m.p.(often gases at room temp.) insoluble in water (many exceptions), soluble in non-polar solvents, non-conductors.
Explain each of the bulk properties by reference to the particles and structure of the substance