Polarity, Electronegativity; Lewis Structures for Molecules and Ions 1

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1 1 Class 5: Polarity, Electronegativity, and Lewis Structures for Molecules and Polyatomic Ions Sec 9.6 Electronegativty and Electronegativity, Dipole Moment and Percent Ionic Character Sec 9.7 Lewis Structures of Molecular Compounds and Polyatomic Ions Writing Lewis Structures for Molecular Compounds Writing Lewis Structures for Polyatomic Ions 2 One of the limitations of Lewis theory is that it assumes that in covalent bonding, electrons are evenly shared and in ionic bonding, electrons are completely transferred. Most chemical bonds have character consistent with both bonding types 3 Lewis theory represents bonding electrons as equally shared between two centers. Covalent bonding between different atoms actually results in unequal sharing of the electrons. The shared electrons lie closer to one atom than the other Otherwise stated, when two different atoms share electrons, one atom will exhibit a greater electron density. Less electron density surrounds the hydrogen centre is more electronegative therefore, more electron density is located around that centre. Structures for Molecules and Ions 1

2 4 Polar covalent bond Positive and Negative poles are created due to the unequal sharing of the electrons This type bond is intermediate between pure covalent and ionic bonds. Note: A non-polar covalent bond exists between two non-metal atoms of the same type, where the electrons are equally shared between both atoms. 5 Partial charges the unequal sharing of electrons in a bond leads to a partial negative charge (d ) on the most electronegative atom (more non-metallic ), and a partial positive charge (d + ) on the least electronegative atom (more metallic ). δ + ( delta plus ) indicates a partial positive charge δ + δ Cl δ ( delta minus ) indicates a partial negative charge The bond dipole always points towards the most electronegative atom Start with a + sign under the positive atom and extend outwards 6 Electronegativity A measure of the electron-attracting power of a bonded atom. Different atoms have different abilities to attract electrons Related to electron affinity and ionization energy Atoms with high ionization energy and large, negative electron affinities will have high electronegativity relative to atoms with low ionization energy and small electron affinity. What does that mean? metals have low electronegativities non-metals have high electronegativities. Example: (high EN) vs. Na (low EN) Structures for Molecules and Ions 2

3 7 Electronegativity Linus Pauling devised an electronegativity scale that allows comparisons to be made between various elements. Pauling s values range from 0.7 (less EN) to 4 (highly EN). 8 Electronegativity On the periodic table: Electronegativity increases from left to right Electronegativity decreases from top to bottom This means: Cesium and francium have the lowest electronegativities. They also have low ionization energies. It is therefore relatively easy to transfer an electron from cesium or francium to another element. luorine has the highest electronegativity. luorine also has a high ionization energy; meaning it takes a large amount of energy to remove an electron from fluorine. 9 Electronegativity Structures for Molecules and Ions 3

4 10 By assessing the difference in electronegativty between two atoms bonded together, we can determine the bond polarity Describes how electrons involved in the bond are shared The electronegativity difference is simply the difference between the electronegativities of both atoms involved Given as EN, always taken as a positive value 11 EN is small (0 0.4) electrons are shared equally; bond is pure covalent (non-polar) EN is intermediate ( ) electrons are unequally shared; bond is polar covalent EN is large (2.0 +) electrons are completely transferred from one atom to the other; bond is ionic 12 Structures for Molecules and Ions 4

5 13 As a general rule: Two atoms of the same type bonded to each other will always be non-polar Two different non-metals will usually be polar (but pay attention to EN) A metal bonded to a non-metal will be ionic 14 Dipole Moment and Percent Ionic Character Bond polarity can also be quantified by calculating a dipole moment and/or the percent ionic character. Dipole moment (m) measured in debye (D); results from the separation of the centres of positive and negative charge. Percent Ionic Character this compares the experimentally measured dipole moment of a bond to the calculated dipole moment that would result from completely transferring an electron from one atom to another. 15 Percent Ionic Character In general, as electronegativity difference increases, percent ionic character increases In general, bonds with a percent ionic character greater than 50% are referred to as ionic bonds. Structures for Molecules and Ions 5

6 16 Problems Determine whether the bond formed between each of the following pairs of atoms is pure covalent (non polar), polar covalent or ionic: (a) Br-Br (b) Sr- (c) P-Cl Which is the more polar bond: -Cl or -O? 17 Lewis Structures for Molecular Compounds and Polyatomic Ions undamental requirements of Lewis structures: 1. All the valence electrons of the atoms in a Lewis structure must appear in the structure. 2. Usually, all the electrons in a Lewis structure are paired. 3. Usually each atom acquires an outer-shell octet of electrons, although more is possible (discussed later!) Remember: ydrogen has only 2 outer-shell electrons! 4. Multiple covalent bonds may be needed. Multiple covalent bonds are most readily formed by C, N, O, P and S atoms. 18 Lewis Structure Terminology Skeletal structure an arrangement of atoms in a Lewis structure this is starting point in a Lewis Structure before adding all electrons Central atom any atom that is bonded to two or more other atoms and is found on the inside of the molecule Terminal atom any atom that is bonded to only one other and is found on the outside of the molecule / polyatomic ion Structures for Molecules and Ions 6

7 19 Lewis Structure Terminology Skeletal Structure for Ethanol (C 2 5 O) C C O All others are Terminal Atoms Central Atoms 20 All steps will pertain to drawing the structure for carbon disulfide, CS 2 1. Determine the total number of valence electrons that must be included in the structure Consider the valence electrons for each free atom Can be found using the last digit from the element s group number on the periodic table ere: (valence e in C) + 2 (valence e in S) = (6) = 16 e total in this Lewis Structure Identify central and terminal atoms elpful ints: Central atoms are normally those with the lowest electronegativity, however: Carbon atoms are always central atoms ydrogen atoms are always terminal atoms Molecules and polyatomic ions generally have compact, symmetrical structures This typically means the odd one out is the central atom Large, chain-like organic molecules are an exception to this generality. Structures for Molecules and Ions 7

8 22 2. This means that in CS 2 ; C is the central atom S are the terminal atoms 3. Connect terminal atoms to central atoms using a single bond pair of electrons S C S OR S C S ow many e are left? or each bond added, subtract 2 e from the starting amount 2 bonds formed means 4 electrons used 16 total e in the molecule 4 e used in bonds = 12 e remaining Using the remaining electrons, complete the octets of the atoms in the following order: Terminal Atoms Central Atoms S C S Structures for Molecules and Ions 8

9 25 6. If a central atom is left with an incomplete octet, convert a lone pair from a neighbouring terminal atoms into a bonding pair between the two S C S Only sees 4 e instead of 8! S C All atoms now have a full octet; S This is the correct Lewis Structure for CS 2 26 Problems Draw Lewis Structures for the following molecules: CCl 4 CN COCl 2 27 Lewis Structures for Polyatomic Ions All steps will pertain to drawing the structure the polyatomic ion B 4 1. Determine the total number of valence electrons that must be included in the structure Consider the valence electrons for each free atom as before Consider the charge on the ion Negative charge Add e ; Positive charge Remove e ere: (valence e in B) + 4 (valence e in ) + 1 (extra e for 1 charge) = (7) + 1 = 32 e total in this Lewis Structure Structures for Molecules and Ions 9

10 28 Lewis Structures for Polyatomic Ions 2. Identify central and terminal atoms Remember from before: Central atoms are normally those with the lowest electronegativity Molecules and polyatomic ions generally have compact, symmetrical structures This typically means the odd one out is the central atom So, here: B is the Central Atom atoms are Terminal 29 Lewis Structures for Polyatomic Ions 3. Connect terminal atoms to central atoms using a single bond pair of electrons B 4. ow many e are left? or each bond added, subtract 2 e from the starting amount 4 bonds formed means 8 electrons used 32 total e in the ion 8 e used in bonds = 24 e remaining 30 Lewis Structures for Polyatomic Ions 5. Using the remaining electrons, complete the octets of the atoms in the following order: Terminal Atoms Central Atoms B Now that all e have been added, remember that this is a negatively charged ion! Structures for Molecules and Ions 10

11 31 Lewis Structures for Polyatomic Ions 6. If a central atom is left with an incomplete octet, convert a lone pair from a neighbouring terminal atoms into a bonding pair between the two B Not necessary for this ion 32 Problems Draw Lewis structures for the following polyatomic ions: (a) NO + (b) O Coming up next! Sec 9.7 Lewis Structures of Molecular Compounds and Polyatomic Ions (con t) Writing Lewis Structures for Molecular Compounds Writing Lewis Structures for Polyatomic Ions Sec 9.8 Resonance, ormal Charge Resonance ormal Charge Structures for Molecules and Ions 11