Bonding

Bonding is the chemical combination of atoms or elements to form compounds. The force of attraction holding atoms or elements together in a molecule/crystal is referred to as a chemical bond. Chemical bonding /combination occurs mainly in four forms as:

  1. Ionic/electrovalent bonding-this involves transfer of electrons from a metal atom to a non metal atom. It occurs between metals and non metals.
  1. Covalent bonding-this involves sharing of electrons between two or more non metal atoms/elements. The atoms/elements involved contribute to the bonding electrons.
  2. Dative/co-ordinate bonding-this involves sharing of the bonding electrons which are donated by one molecule or atom involved.
  3. Metallic bonding-this occurs between atoms of metal elements.

Electrovalent/ ionic bonding
This involves transfer of electrons from a metal to a non metal. The number of electrons lost by the metal atom or gained by non metal atom is equivalent to its valency. The loss of electrons from a metal atom leads to formation of a positively charged ion (cation) and the gain of electrons by a non metal atom leads to the formation of a negatively charged ion (anion). The positively charged ion and the negatively charged ion are attracted to each other and the force of attraction holding them together is known as electrovalent/ionic bond. The compounds formed are referred to as ionic/ electrovalent compounds. Metal atoms lose electrons so as to gain stable electronic configuration of noble gases and non metal atoms also gain electrons to become stable.

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ii) Water molecule (H2O)

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Dative/ co-ordinate bond
This involves sharing of electrons but the shared pair of electrons is donated by one atom/ molecule. Here, one molecule/ atom donates the pair of electrons to be shared with an ion or another atom. Normally it is atoms/ molecules with lone pair of electrons that form this bond-by donating the lone pair of electrons to be shared with another atom/ion. The bond may be represented by an arrow originating from the donor atom (atom that donates electrons to be shared) to the atom accepting the electrons.

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Metallic bonding
This is the type of bonding in metals due to the attraction between metal ions and the valency electrons within the structure of the metal (metal lattice). In the structure of a metal, the loosely held valency electrons (electrons in the outer mos energy level) are released in to a general pool and the atoms become positively charged. These electrons move freely around the ions formed and are termed as mobile/delocalized electrons.


The ions formed and the electrons attract each other forming metallic bond.
The strength of metallic bond increases with increase in the number of electrons released in to the electron cloud. Thus the bond is very strong in metals like iron and aluminum that release up to three electrons each to the electron pool and weak in metals like sodium and potassium that only release one electron each to the electron pool.


Illustration

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Each sodium ion is surrounded by six chloride ions and each chloride ion is also surrounded by six sodium ions. The co-ordination number is there fore 6:6.

Properties of ionic compounds

  1. They are solids with a regular shape. This is because of the strong electrostatic forces of attraction keeping the ions closely packed.
  2. They have high melting points. This is due to the strong electrostatic force af attraction between oppositely charged ions.
  3. They do not conduct electricity in solid states but do conduct in their molten states or solution form. This is because in solid states, the ions are localized and the electrons are not mobile but in molten form, the ions and electrons are free to move (electrons become delocalized) and thus conduct electricity.
  4. They have high density as the ions are closely packed.
  5. Ionic compounds are soluble in water and other polar solvents but they a re insoluble in organic solvents like benzene. Ionic compounds dissolve in polar solvents like water as the ions are attracted by the polar molecule.
    A polar compound is a covalent compound in which charge separation exist between atoms. This is due to the differences in electro negativity between the atoms.
    Electro negativity is the tendency of an atom to attract bonding electron towards itself. Electro positivity is the tendency of an atom to push away bonding electrons from itself.
    For example, in a water molecule oxygen is more electro negative and attracts the bonding electrons towards itself giving it a partial negative charge. Hydrogen attains a partial positive charge.
    Structure of a water molecule
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NB Most metals are malleable and ductile. This is because, since the bonding agent in a metal is a moving electron cloud, the ions of the metal; usually slide relative to one another under stress (without shattering the lattice and produce a new position of stability).

Simple molecular structures
The structure consists of simple independent molecules joined together by weak intermolecular forces of attraction. E.g. Vander Waal‘s force of attraction. The atoms in the molecule are held together by strong covalent bond. Compounds with this structure exist as gases, liquids or solids with low melting points. Examples include iodine, carbon dioxide , ammonia and water.

Structure of iodine

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Properties of simple molecular structures

  • They have low melting and boiling points because the molecules are held by weak intermolecular forces of attraction.
  • They are non conductors of electricity because they do not have mobile electrons/ ions.
  • Most of them are gases and liquids. Very few are solids.
  • They have low densities as the molecules are not closely packed.
  • They dissolve in organic solvents.
    Giant atomic/molecular structure
    Compounds with such structures consist of molecules /atoms linked together by strong covalent bonds resulting into a giant three dimensional or three dimensional structures like in graphite and diamond respectively (see details under Carbon and its compounds)
    Properties of giant molecular structures
  • They have high melting and boiling pints because of great energy needed to break the bonds.
  • They do not conduct electricity except graphite which has delocalized electrons.
  • They are insoluble in water.
    Giant metallic structure
    Giant metallic structure consists of very many metal ions surrounded by a sea /cloud of electrons. These electrons are released by metal atoms.
    Properties of giant metallic structures
  • They are solids with high melting and boiling points except mercury which is a liquid.
  • They conduct electricity in both liquid and solid states due to the presence of mobile electrons.

Sample questions on bonding and structure
Bonding and structure

  1. Describe briefly what is meant by the following types of bonding: electrovalent, covalent, coordinate and metallic. Use examples to illustrate how the above bonds are formed between any two named atoms or molecules.
  2. Explain what is meant by metallic bond. Explain why a typical metal is (a) a good conductor of electricity (b) a good conductor of heat (c) in some conditions at leas malleable and ductile.
  3. The compounds named below are all covalent compounds. With the aid of the table of electron structures, give a diagram for a molecule of each of these compounds showing the outer most electron shells only: (i) tetra chloromethane CCl4, phosphorus(III) chloride PCl3 (iii) silane SiH4 (iv) tri chloromethane CHCl3 (v) phosphine PH3 and (vi) dichloromethane CH2Cl2
  4. Explain briefly why ammonia and oxygen molecules participate readily in coordinate bonding. Give an example for the formation of an ion by ammonia by this means. Show by means of electronic diagram the formation of a coordinate linkage between phosphorus tri chloride and oxygen.
  5. By means of electronic diagrams, show the structure of (a) calcium atom (b) chlorine atom (c) calcium chloride. State the differences between electrovalent and covalent compounds.
  6. Describe how structures of the following compounds account for their property in term of electrical conduction (a) copper (b) graphite (c) diamond
  7. With the aid of well labeled drawings explain the structure of the following: Graphite, diamond, sodium chloride and chlorine molecule.