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CHE3: ATOMIC AND MOLECULAR STRUCTURE: CHEMICAL BONDING

Under this Unit; Describe the role of outermost electrons in chemical bonding, Explain qualitative formation of covalent and ionic bonds and Illustrate using diagrams the covalent and ionic bonds in simple compounds.

ATOMIC AND MOLECULAR STRUCTURE: CHEMICAL BONDING

BONDING AND STRUCTURE

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:

Ionic/electrovalent bonding

This involves transfer of electrons from a metal atom to a non metal atom. It occurs between metals and non.

Covalent bonding

This involves sharing of electrons between two or more non metal atoms/elements. The atoms/elements involved contribute to the bonding electrons.

Dative/co-ordinate bonding

This involves sharing of the bonding electrons which are donated by one molecule or atom

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.

Formation of some ionic compounds

Sodium chloride

When sodium burns in chlorine to form sodium chloride, the sodium atom loses an electron and forms a positively charged sodium ion(Na+) while the chlorine atom gains the electron and forms a negatively charged chloride ion (Cl- ).

The positively charged sodium ion and negatively charged chlorine ion are attracted to each other by a strong force called ionic bond.

Magnesium oxide

Magnesium atom loses two electrons to the oxygen atom and forms a positively charged magnesium ion    while  the  oxygen  atom forms  a  negatively  charged oxide ion . These two oppositely charged ions are then attracted to each other.

Calcium chloride

During the formation of calcium chloride, calcium atom loses two electrons which are gained by two chlorine atoms. This leads to the formation of   and two    which are then attracted to each other.

Illustration

Covalent bonding

This type of bonding occurs between non metal elements/ atoms to form compounds. Covalent bonding involves mutual sharing of electrons between elements in which each of the atoms involved gives equal number of electrons to be shared. On sharing the electrons, the atoms attain stable electronic structure of noble gases. The compound formed are called covalent compounds. Covalent bonds may by classified  as single, double, triple or quadruple depending on the number of electrons shared.

Single covalent bond:

This is formed when one pair of electron is shared. Each of the atoms involved contribute an electron for the bond

Example

Hydrogen molecule (H2)

Water molecule (H2O)

Ammonia (NH3)

Methane (CH4)

N.B. The pair of electron on the outer most energy level that does not take part in bonding is the lone pair or non bonded pair.

Double covalent bond

This is formed when two pairs of electrons are shared between two bonded atoms.

Example

Carbon dioxide (CO2)

Triple covalent bond

This involves sharing of three pairs of electrons between atoms.

Example

Nitrogen molecule (N2)

Hydrogen cyanide (HCN)

Quadruple covalent bond

This is formed by sharing four pairs of electrons. It occurs between carbon atoms.

Illustration

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.

Example

Formation of ammonium ion ()

  • During the formation of ammonium ion, ammonia reacts with an acid. The nitrogen in ammonia has a lone pair of electron which it donates to be shared with the hydrogen ion (proton) from the acid.Formation of ammonium ion ( )Formation of ammonium ion.

Illustration

Formation of phosphorus (V) trichloride

This is formed between phosphorus (III) chloride which donates the lone pair of electron to be shared and oxygen atom.

Illustration using outer most shell

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

END OF UNIT