Bonding and Structure

Nature of forces holding atoms together

- chemical bonding: A result of rearrangement of electrons between 2 atoms, leading to a state of lower

  energy (acquire a noble gas electronic structure by sharing or transfer of electrons)

- electrostatic interactions between electrons and nuclei due to different types of bonding

Ionic bond

- by the transfer of electrons from metal atoms to non-metal atoms (in group VI and VII)

- Factor affecting ionic radius:

  (1) Number of protons: Increase in the number of protons results an increase in nuclear charge, thus

        decreasing the ionic radius, and vice versa.

  (2) Number of electron shells: The presence of more electron shells increase the shielding effect and

        therefore an increase in ionic radius.

Covalent bond

- by sharing of electrons (or overlapping of atomic orbital) between non-metal atoms

- Factors affecting strength of covalent bond:

  (1) Size of atoms: The smaller the atom, the smaller distance between bonding electrons and nucleus.

        There is more attraction and hence stronger bond.

  (2) Number of bonding electrons: The more the bonding electrons, the stronger the bonds.

Shape of Simple Molecules

As the number of lone pair on central atom increase, lone pair repels bond pair strongly.

Polar Molecules

A molecules is polar if polar bonds are present AND effects of polar bonds do not cancel out

Extended Covalent Structure

- Diamond Structure: Each carbon atoms is surrounded tetrahedrally by 4 other carbon atoms. Carbon

  atoms are jointed together by strong covalent bond.

- Graphite structure: The structure consists of layers of hexagonal rings of carbon atoms. Each carbon

  atom within the layers is jointed to 3 other carbon atoms by strong covalent bond. 1 outer electron

  from each carbon atom is delocalized. These delocalized free electrons can move within a layer and

  they are responsible for electrical conductivity in graphite. The layers are held by weak van der Waal's

  forces.

Metallic bond

- metallic structure consists of positive cores surrounded by a sea of free electrons. These mobile

   electrons come from the outermost shells of metal atoms. Metallic bond is strong and electrostatic in

   nature.

- Factors affecting metallic bond strength include number of valence electrons and charge density of

  positive cores.

- Physical properties of metal:

   (1) conduct electricity: Free electrons are present.

   (2) malleable and ductile: When a force is applied to a metal crystal, the layers of atoms can slide over

        one another.

   (3) Shiny appearance: Energy levels due to the delocalized electrons are very close together, the metal

         can absorb and then radiate almost all frequencies of radiation.

   (4) High melting point and boiling point: Metallic bond is strong and a lot of energy is required to melt

        or vapourize a metal.

Dipole-Dipole attractions (dipolar force)

- Polar molecules acts as permanent dipoles: the positive end of a dipole attracts the negative end of

  adjacent one.

Van der Waal's forces

- Origin: Electrons in a molecule are moving randomly. At one time there would be more electrons on

  one side than other. The instantaneous dipole induces dipole in the neighbouring molecules to have

  attraction. Diagram illustration. (99/8(a)(i))

- Factors affecting Van der Waal's force:

  (1) Molecular mass: As number of electrons increases, possibility that the instantaneous dipole will be

        larger increases and hence larger Van der Waal's force.

  (2) Surface area of molecules: The larger the surface area of the molecules is, the arger Van der Waal's

       force is.

Hydrogen bond

- Evidences: Graphical Sketch. (99/8(a)(iv)) The abnormally high boiling point of H2O, HF and NH3 are

  due to the presence of intermolecular Hydrogen bond.

- Origin: Hydrogen bonds are possible in molecules with electronegative atoms (N, O or F) bonded to

  Hydrogen atoms AND there is/are lone pair(s) on electronegative atom. Electrons on Hydrogen atoms

  is shifted to the electronegative atom, forming a partial positive charge on Hydrogen and this positive

  charge interacts with lone pairs of other atoms in another molecule.

Structures and Properties

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