CHEMICAL BONDING

Dr Sharipah Ruzaina Syed Aris

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3 main types of Bonding

• Ionic Bonding/Ionic bond• Covalent Bonding• Dative or coordinate bond

A chemical bond is a strong attractive force that exists between atoms in a substance

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• Lewis symbols provide a convenient way to keep track of valence electrons

• In this notation the symbol of the element is surrounded by dots (or similar symbols) that represent the atom’s valence electrons

• All the elements in a group have a similar Lewis symbol because they have the same number of valence electrons

::C and Na :Examples

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Ionic Bond

• An ionic bond is formed by the transfering of electron or electrons from a metal atom to a non-metal atom. Ionic bond also known as electrovalent bond.

• Lewis structure can be used to represent the formation of ionic bonds. Exp: NaCl

Na+ 2s22p6 Cl- 3s23p6

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Electron configurations

Li 1s22s1

Orbital diagrams

Lewis electron-dot symbols

+ F 1s22s22p5 Li+ 1s2 + F- 1s22s22p6

Three ways to represent the formation of Li+ and F- through electron transfer.

Li

1s 2s 2p

F

1s 2s 2p

+

Li+

1s 2s 2p

F-

1s 2s 2p+

.+ F: ::Li . Li+ + F -::

::

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Cl– Na+

Ionic bondingIonic bonding• Ionic bonding involves 3 steps (3 energies)• 1) loss of an electron(s) by one element, 2)

gain of electron(s) by a second element, 3) attraction between positive and negative

Cl–

Na Cl

e–1) 2)

3)

Na+

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[ O ]2–[Mg]2+OMg

Exercise:

aluminium fluoride AlF3

magnesium chloride MgCl2

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Physical Properties of Ionic Bond

1. Hard and brittle crystalline solids2. High melting and boiling points3. Soluble in water (polar solvent)4. Good electrical conductor in molten forms

and in aqueous solutions but insulators in solid forms.

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Covalent Bond

• A covalent bond is a chemical bond formed by the sharing of electrons between atoms. Covalent bonds are usually formed between non-metallic atoms.

• Exp: H2, O2, HCl, CO2

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Single covalent bond

• A single covalent bond is formed when a pair of electrons is shared between two atoms. A single covalent bond can be represented by two dots or by a short line.

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Double covalent bond• A double covalent bond is formed when two

bonding pairs of electrons are shared between two atoms.

• Atoms such as C, N, O and S can form double bond.

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Triple covalent bond

• A triple covalent bond is formed when three bonding pairs of electrons are shared between two atoms.

• Atoms such as C and N, can form triple bond.

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2 carbon and hydrogen atoms each         Acetylene (C2H2)

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Physical properties of covalent compounds

1. Low melting and boiling points: The covalent molecules in the crystal are held together by the weak van der Waals forces.

2. Insoluble in water but soluble in nonpolar solvents.

3. Electrical insulators: when melted, neutral molecules and atoms are released and they are not attracted to any electrode.

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Draw a Lewis structure for each of the following molecules:• H2 • N2 • O2 • F2

• H2O• NH3

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Covalent bonds of network covalent solids.

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Dative or Coordinate Bond

• It is formed by the sharing of a pair of electrons between two atoms, but only one atom donates the electrons for the bonding.

• The atom that donates the electrons is known as the donor and it must have at least one lone (nonbonding) pair of electrons.

• The other atom that shares the electrons is known as the acceptor and it must have an incomplete octet.

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• The reaction between ammonia and boron trifluoride, BF3

• The boron only has 3 pairs of electrons in its bonding level, whereas there would be room for 4 pairs. BF3 is described as being electron deficient.

• The lone pair on the nitrogen of an ammonia molecule can be used to overcome that deficiency, and a compound is formed involving a co-ordinate bond.

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Using lines to represent the bonds, this could be drawn more simply as:

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Metallic Bond

• A metal is a lattice of positive metal 'ions' in a 'sea' of delocalised electrons.

• Metallic bonding refers to the interaction between the delocalised electrons and the metal nuclei.

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Metallic bonding properties• The physical properties of metals are the result of the

delocalisation of the electrons involved in metallic bonding. • The physical properties of solid metals are:

– conduct heat – conduct electricity – generally high melting and boiling points – strong – malleable (can be hammered or pressed out of shape

without breaking) – ductile (able to be drawn into a wire) – metallic lustre – opaque (reflect light)

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