IB
Bonding

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
· Ionic bonding - compounds only
· Covalent 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.


Ionic bonding:

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


Ions have charges which attract in all directions and the bulk structure is built up by many repeating rows, files and columns of oppositely charged ions. There are many strong forces of electrostatic attraction acting in all directions. Each positive ion is surrounded by six negative ions and each negative ion is surrounded by six positive ions in sodium chloride for example, and this giant structure gives rise to its bulk properties.

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.

Covalent bonding:

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)
2. Dipole dipole interactions
3. Hydrogen bonds

These two different situations give rise to structures with very different bulk properties.

1. Giant molecular structures
2. Simple molecular structures


General bulk properties:

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



Summary

Type of structure Metallic Ionic Simple covalent Giant covalent
smallest particle metal ion opposite charged ions small molecules giant macromolecule
Interparticulate forces Electrostatic attractions depending on forces between the 'sea' of electrons and the metal ions. The magnitude of the force depends on the number of available electrons and the charge density of the metal ions (charge/volume ratio) Electrostatic attractions. The magnitude of the force depends on the charge densities of the positive and negative ions. High charge density = strong force. Weak forces dependent on the dipoles existent within the molecule. If the molecules are completely symmetrical or have no polar bonds then the only force is Van der Waals (induced dipole) attractions These are a special case as there is only one molecule. There cannot therefore be intermolecular forces. We normally talk about the bonding between the atons in the molecule and these are, of course, strong covalent bonds.
Melting point Usually high but as the charge densities decrease getting lower. ie trend in group 1 Li, Na, K, Rb, Cs High (in some cases the structures decompose before melting) High m.p. A function of the magnitue of electrostatic atrtraction which in turn is a function of the charge density (charge/volume ratio) Very low. In the case of small molecules the materials are gases at room temperature. Extremely high. Diamond and graphite have the highest melting points known.
Other bulk properties Metallic crystals, shiny, conductors of heat and electricity. Malleable and ductile Hard, brittle crystals or powders (microcrystals) Often soluble in water (if the balance of energy/entropy gives a negative value for Gibbs free energy). Sometimes crystals if the Mr is high enough Solids are soft and waxy. Non-conductors, solubility depends on polarity within the molecule or reaction between the molecules and water Crystals, hard non-conductors unless there are delocalised orbitals over the whole molecule (as in graphite)