A Level Chemistry: More on electronegativity, bond polarity, polarisation, atomic/ionic radii
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Extra Notes on chemical bonding for advanced A level chemistry students
Extra notes on electronegativity, bond polarity, polarisation by cations, polarisability of anions, atomic and ionic radii and the influence of these factors on the type of chemical bonding - bond character
(also notes on dipoles)
Electronegativity is defined as the power of an atom to attract the pair of electrons in a covalent bond situation.
The electron distribution in a covalent bond between elements with different electronegativities will be unsymmetrical.
This produces a polar covalent bond, and may cause a molecule to have a permanent dipole which is indicated by partial charges (δ+ and δ–) to show that a bond is polar.
You should be able to recognise when a molecule is likely to be polar and explain why some molecules with polar bonds do not have a permanent dipole.
Electronegativity is the power of an atom to attract electron charge from another atom it is covalently bonded to. Some Pauling values of electronegativity are quoted below.
Generally speaking electronegativity increases from left to right across a period of the periodic table and decreases down a group of the periodic table.
2. Bond polarity and type of chemical bonding
Three situations are considered
(a) Non-polar covalent bond
(b) Polar covalent bond
(c) Ionic bond
3. Bond character complications If only it was a simple as a, b, c above!
Even before considering other factors, there are going to be cases where the electronegativity difference will be somewhere between that of a very covalent compound and a very ionic compound.
This will inevitably lead to compounds which are:
I'll illustrate some cases of (i) ionic compound with some covalent character, via the data table below, but first the various terms used are defined and explained below.
In an exam question, you might be provided with data, in the absence of data you need to be able to explain the situation on a descriptive conceptual basis!
(i) Cases of ionic compounds with varying covalent character
Sodium chloride: An appreciable electronegativity difference leads to a well defined giant ionic lattice which has a relatively high melting point and enthalpy of fusion.
Lithium chloride: Now for the complications! On face value you might expect lithium chloride to be quite similar in properties to sodium chloride. Since the lithium ion has a smaller ionic radius than sodium, the attractive force between the 'closer' ions should be greater, giving LiCl a greater melting point and greater enthalpy of fusion than NaCl. BUT this is not the case. In fact lithium chloride has a significantly lower melting point and enthalpy of fusion compared to sodium chloride. So, what's going on!
Lithium iodide: Following on from the LiCl discussion, in lithium iodide we have an even more polarisable anion in iodide.
The curious case of aluminium chloride
Aluminium chloride is very ambiguous! X-ray diffraction shows that solid, AlCl3, 'appears' to consist of an ionic lattice of Al3+ ions, each surrounded by six Cl– ions, BUT on heating, at about 180oC, the thermal kinetic energy of vibration of the ions in the lattice is sufficient to cause it break down and sublimation takes place (s ==> g) to form the readily vapourised covalent dimer molecule, Al2Cl6.
(ii) Cases of covalent compounds showing some ionic character
The O-H bond in water is a highly polar molecule (δ+H–Oδ2––Hδ+). The relatively large difference in electronegativity between hydrogen and oxygen (H 2.1, O 3.5, Δ elec. = 1.4) introducing some ionic character into the bond, and a tiny fraction of molecules can undergo self-ionisation giving water a tiny, but real and measurable electrical conductivity.
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