The Chemistry of Titanium 1c – Bonding Types 2 Compounds

Learning Intentions To gain an understanding of how electronegativity affects polarity of bonds To be able to identify Pure Co-valent, Polar Co- valent and Ionic bonding from electronegativities To be able to place molecules within the bonding continuum To be able to relate molecular shape/symmetry to polarity of molecules

Covalent Bonding A covalent bond is a shared pair of electrons electrostatically attracted to the positive nuclei of two atoms. The atoms achieve a stable outer electron arrangement (a noble gas arrangement) by sharing electrons Both nuclei try to pull the electrons towards themselves This is like a tug-of-war where both sides are pulling on the same object. It creates a strong bond between the two atoms.

Covalent Bonding Picture a tug-of-war: If both teams pull with the same force the mid-point of the rope will not move.

Pure Covalent Bond This even sharing of the rope can be compared to a pure covalent bond, where the bonding pair of electrons are held at the mid-point between the nuclei of the bonding atoms. HH e e

Covalent Bonding What if it was an uneven tug-of-war? The team on the right are far stronger, so will pull the rope harder and the mid-point of the rope will move to the right.

Polar Covalent Bonding A polar covalent bond has some ionic character. e H H e Increasing ionic character There is a small difference between the electronegativities of both atoms and the bonding electrons are pulled more closely to the more electronegative atom. e e PCl δ-δ- δ+δ+ e P e δ-δ- δ+δ Electronegativities Increasing ionic character

Polar Covalent Bond A polar covalent bond is a bond formed when the shared pair of electrons in a covalent bond are not shared equally. This is due to different elements having different electronegativities.

Polar Covalent Bond e.g. Hydrogen Iodide If hydrogen iodide contained a pure covalent bond, the electrons would be shared equally as shown above. However, iodine has a higher electronegativity and pulls the bonding electrons towards itself (winning the tug-of-war) This makes iodine slightly negative and hydrogen slightly positive. This is known as a dipole. H I e e δ-δ- δ+δ+

Polar Covalent Bond In general, the electrons in a covalent bond are not equally shared. e.g. δ- indicates where the bonding electrons are most likely to be found C Cl Electronegativities δ-δ- δ+δ+

Polar Covalent Bond Consider the polarities of the following bonds: Bond Increasing Polarity Electronegativities Difference O H C Cl P H P H C Cl δ-δ- δ+δ+ O H δ-δ- δ+δ+ Complete a similar table for C-N, C-O and P-F bonds. 2.2

Polar-Polar Attractions The differing electronegativities of different atoms in a molecule and the spatial arrangement of polar covalent bonds can cause a molecule to form a permanent dipole. ++ -- -- -- -- 4 polar covalent C-Cl bonds in CCl 4 tetrahedral shape -- -- -- ++ 3 polar covalent C–Cl bonds and 1 polar covalent C-H bond in CHCl 3 No permanent dipole Symmetrical molecule Permanent dipole Asymmetrical molecule NON-POLAR molecule POLAR molecule e.g. also CO 2 e.g. also H 2 O

Polar molecules and permanent dipoles Both propanone and butane have the same formula mass of 58 however, butane boils at – 1 o C while propanone boils at 56 o C Butane has no permanent dipoles, so only Van der Waals forces between molecules. So has a lower boiling point. Propanone is a polar molecule as it has a permanent dipole, so has polar-polar attraction as well as Van der Waals’ forces between molecules. ++ -- b.p. 56 o C b.p. -1 o C

Polar Molecules Water is a polar molecule so it is a polar solvent. A liquid that substances dissolves in is called a SOLVENT. Solvents can be either polar or non-polar molecules. Immiscible liquids do not mix, e.g. oil and water, however, non-polar liquids are miscible with each other. Polar solvents will usually dissolve polar molecules. Non-polar solvents will usually dissolve non-polar molecules. ++ ++  --

Ionic Bonds Ionic bonds are formed between atoms with a large difference in electronegativities. They are often (though not always) between metals and non-metals. For example, in potassium bromide, the difference in electronegativities is so large that potassium will lose an electron and form a positive ion. Bromine gains this electron and forms a negative ion. The ionic bond is the electrostatic force of attraction between a positive and negative ion.

Reacting Elements: Electron Arrangement: During Reaction: New Electron Arrangement: Ions Formed: K Br 2,8,8,1 2,8,7 loses 1e - gains 1e - 2,8,8 transfer of an electron e-e- K Br -+

The electrostatic force of attraction between the oppositely charged ions is called the ionic bond Ionic compounds form a LATTICE STRUCTURE. Millions of oppositely charged ions are held together in a very stable arrangement. Br - K +

Ionic Bond e e e H H e Increasing ionic character e P Cl e δ-δ- δ+δ+ Increasing ionic character An ionic bond exists when the difference in electronegativities is so great that the movement of the bonding electrons between the two atoms is complete. LiF Electronegativities + - Li + F - F + - There is no sharing of the electrons and oppositely charged ions are formed.

Bonding Continuum e H H e e P Cl e δ-δ- δ+δ+ Increasing ionic character Li F + - Pure Covalent Bond Polar Covalent Bond Ionic Bond To judge the type of bonding in any particular compound it is more important to look at the properties it exhibits rather than simply the names of the elements involved.

Electronegativity is useful at predicting how electrons will be shared. The Pauling scale is used for electronegativity values Increasing difference in electronegativity Equal sharing of electrons Increasing unequal sharing of electrons Transfer of electrons Non-polarslightly polar covalentvery polar covalentionic 4.0 FF Li F OH 2.1 The greater the difference in electronegativity the greater the polarity between two bonding atoms and the more ionic in character.