Co-ordinate (dative) covalent bonds Work out which molecules have dative covalent bonds and draw the structure

Specification reference Pages 52-53

Learning outcomes State what is meant by the term electronegativity State what makes one atom more electronegative than another State what the symbols δ + and δ - mean when placed above atoms in a covalent bond

Polar and non-polar bonds Covalent bond - ? Ionic bond - ?

Vote now... Which of the three molecules is polar? Why?

Non-polar Covalent bond – shared pair of electrons In hydrogen, the two bonding atoms are identical They have an equal share of the pair of electrons in the bond which results in a perfect 100% covalent bond Hydrogen is a non-polar molecule, the electrons are evenly distributed between the atoms that make up that bond What happens if the atoms are different?

Polar If the atoms bonding are different, one of the atoms is likely to attract the bonding electrons more The bonding atom with the greater attraction is more electronegative than the other atom Example, HCl The Cl atom is more electronegative than the H atom The Cl atom has a greater attraction to the bonding pair of electrons that the H atom The bonding electrons are closer to the Cl atom than the H atom

Trends in electronegativity LiBeBCNOF NaMgAlSiPSCl Br 2.8 Increasing electronegativity Increasing electronegativityIncreasing electronegativity Key definition – Electronegativity Electronegativity is the power of an atom to attract the electron density in a covalent bond towards itself.

Electronegativity Electron density is often used to describe the way the negative charge is distributed in a molecule The Pauling scale is used a measure of electronegativity (research opportunity…?) Why do the noble gases not have a number assigned? They do not (in general) form covalent bonds

Electronegativity Electronegativity depends on three things: The nuclear charge The distance between the nucleus and the outer shell electrons The shielding of the nuclear charge by electrons in inner shells Note – the smaller the atom, the closer to the nucleus the shared outer shell electrons ∴ the greater the electronegativity The larger the nuclear charge, the greater the electronegativity

Trends in electronegativity LiBeBCNOF NaMgAlSiPSCl Br 2.8 Increasing electronegativity Increasing electronegativityIncreasing electronegativity Small atoms Less shielding

Polar This leads to a charge difference across the H-Cl bond This charge difference leads to a permanent dipole and we represent it with the δ + or δ - symbol We now have a polar covalent bond Shows a small positive charge on the hydrogen Shows a small negative charge on the chlorine

Questions Identify which of these covalent bonds is/are polar, and explain your answer. H – H H – F F – F Add dipoles to the polar covalent bonds Arrange the following covalent bonds in order of increasing polarity: H – O, H – F, H – N Explain your answer

Polar molecules Poles many cancel each other out. E.g. HCl is linear – it has polar bonds and is a polar molecule. The molecule is non-symmetrical and so there is a charge across the whole molecule. Symmetrical molecules, the dipoles of any bond may cancel out. E.g. Tetrachloromethane

Non polar polar Each C-Cl bond is polar Symmetrical molecule (tetrahedral) Dipoles act in different directions and cancel each other out = a non-polar molecule with polar bonds!

Patterns

Trends in electronegativity Which atoms form the most electronegative atoms? Which atoms form the least electronegative atoms?

Electronegativity and bonding type What does this diagram describe to you?

Questions Complete the exam style questions

Homework Complete the relevant section of the webquest