PPT - Forming Ionic Compounds 26/04/2017 Electronegativity http://liakatas.org/chemblog/?page_id=17#Videos

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 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."

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

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?

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

Factors affecting electronegativity 1+ 9+ H – F

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.

Atomic radii – what’s the link?

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.

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

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

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

Formation of a covalent bond

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

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

                               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.

Non-polar bond Polar bond

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+

Covalent bonding Ionic bonding

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.

+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

+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

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

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

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

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

Summary of bonding