1. PPT - Forming Ionic Compounds
26/04/2017
Electronegativity

2. AQA AS Specification Lessons Topics 1-2 Bonding
understand ionic bonding, covalent bonding, co-ordinate bonding and metallic bonding in terms of electrons and forces. 3
Electronegativity
understand electronegativity and that the electron distribution in a covalent bond may not be symmetrical
• know that covalent bonds between different elements will be polar to different extents
3-5
• understand qualitatively how molecules may interact by dipole forces and hydrogen bonding
• understand the importance of hydrogen bonding in determining the boiling points of compounds and the structures of some solids (e.g. ice) and to understand changes of state
5-7
Bonding and structure
recognise the four types of crystal and know the structures of the following crystals: sodium chloride, magnesium, diamond, graphite, iodine and ice
be able, in terms of electron pair repulsion, to predict the shapes of, and bond angles in, simple molecules and ions.

3. 3.3 Electronegativity
How desperate are you for this lesson to end? (tick one)
Very
Slightly
What group do you belong to?
Group I & VII
Not at all
Group II & VI
N/A
Group III & V
Group 4
If you answered “very”, then you are like elements in Groups I and VII. They are very desperate to lose or gain an electron!
This demonstrates the concept of electronegativity:
"The power of an atom in a molecule to attract electrons to itself."

4. Electronegativity = the power of an atom to attract the electrons in a covalent bond

5. The Pauling scale Increases Why are the halogens missing? Decreases
Describe the trends of electronegativity across the PT
Nuclear charge
Distance between the nucleus and the outer electrons
The shielding by inner electrons
What does electronegativity depend upon?

7. H
2.1 He Li
1.0 Be
1.5 B
2.0 C
2.5 N
3.0 O
3.5 F
4.0 Ne Na
0.9 Mg
1.2 Al Si
1.8 P S Cl Ar K
0.8 Ca Sc
1.3 Ti V
1.6 Cr Mn Fe Co Ni Cu
1.9 Zn Ga Ge As Se
2.4 Br
2.8 Kr Rb Sr Y Zr
1.4 Nb Mo Tc Ru
2.2 Rh Pd Ag Cd
1.7 In Sn Sb Te I Xe Cs
0.7 Ba La
1.1 Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

8. Factors affecting electronegativity1+ 9+
H – F

9. Factors affecting electronegativity
1) Nuclear charge – the more protons, the stronger the attraction from the nucleus to the bonding pair of electrons.
2) Atomic radius – the closer the bonding electrons to the nucleus, the stronger the attraction from the nucleus to the bonding pair of electrons.
3) Shielding – the less shells of electrons shielding (repelling) the bonding electrons, the stronger the attraction from the nucleus to the bonding pair of electrons.

10. Atomic radii – what’s the link?

11. Factors affecting electronegativity
1) Nuclear charge – more protons, stronger attraction between nucleus and bonding pair of electrons.
2) Atomic radius – closer to the nucleus, stronger attraction between nucleus and bonding pair of electrons.
3) Shielding – less shells of electrons between the nucleus and the electrons, less shielding (less repulsion), stronger attraction between nucleus and bonding pair of electrons.

12. Rules
The smaller the atomic radius, the closer the nucleus is to the shared electrons, the larger the electronegativity
The larger the nuclear charge (for a given shielding effect), the greater the electronegativity

13. Trend down a group Electronegativity decreases Atomic radius increases
More shielding
 Less attraction between nucleus and bonding pair of electrons

15. Trend across a period Electronegativity increases
Atomic radius decreases
More nuclear charge
 Stronger attraction between nucleus and bonding pair of electrons

16. Formation of a covalent bond

18. Inequality The molecule becomes polar
What happens if the partner of oxygen doesn’t want to give away it’s electron??
The molecule becomes polar

23. Polar molecules What molecule is this? HCl Is it polar, or non polar?
How many lone pairs are there?
three
Electron probability (density) map
What other bonding possibilities are there

24.                               
HCl polar bond showing the unequal sharing of a cloud of electron density.
HCl polar bond showing the partial (+) change on hydrogen and the partial ( -) change on chlorine.
HCl polar bond showing the direction of the dipole with an arrow pointing toward the more negative atom. The + on the opposite end also reminds us which atom is more positive.

26. Non-polar bond
Polar bond

27. Favoured by small, highly charged +ve ions, e.g. Li+, Be2+
Electronegativity Difference 4 - + X Y X Y X- Y+ X- Y+
Pure covalent
Polar ionic
Distorted ions
Pure ionic
Polar covalent
Electrons not
equally shared
Polarisation of covalent bonds
Polarisation of ions
Favoured by small, highly charged +ve ions, e.g. Li+, Be2+

28. Covalent
bonding
Ionic
bonding

29. Question: HCl, H2O, CCl4, CH2 O
Which of the following molecules will be polar?
HCl, H2O, CCl4, CH2 O
All of them, except CCl4
How do we know if a molecule is going to be polar?
Step 1: Draw a reasonable Lewis structure for the substance
Step 1: If the difference in electronegativity for the atoms in a bond is greater than 0.4, we consider the bond polar.
Step 3: If there are no lone pairs on the central atom, and if all the bonds to the central atom are the same, the molecule is nonpolar.
If the central atom has at least one polar bond and if the groups bonded to the central atom are not all identical, the molecule is probably polar.
Step 4: Describe the polar bonds with arrows pointing toward the more electronegative element. Use the length of the arrow to show the relative polarities of the different bonds. Check for symmetry.

30. +ve ion gets smaller and more highly charged, so –ve polarised more
NaCl
MgCl2
AlCl3
SiCl4
Mpt
801ºC
714ºC
190ºC
-70ºC
Structure
Ionic
Polar ionic
Polar covalent
Covalent
Difference in electronegativity decreases
+ve ion gets smaller and more highly charged, so –ve polarised more

31. +ve ion gets smaller and more highly charged, so –ve polarised more
BeCl2
MgCl2
CaCl2
SrCl2
BaCl2
Mpt
401ºC
714ºC
782ºC
870ºC
963ºC
Structure
Polar covalent
Ionic
Difference in electronegativity decreases
+ve ion gets smaller and more highly charged, so –ve polarised more

32. H2O - O H H + +
Bonds: polar
Molecule: polar

33. NH3 - N H H + H + +
Bonds: polar
Molecule: polar

34. CO2 - + - O C O
Bonds: polar
Molecule: non-polar

35. CCl4 C - + Cl
Bonds: polar
Molecule: non-polar

37. Summary of bonding