Starter: Spot the errors

Answers

What is electronegativity?

Electronegativity A measure of the ability of an atom in a covalent bond to attract the electron bonding pair.

Pauling Scale A scale for measuring electronegativity. What do you notice about the electronegativity across the periods? What do you notice about the electronegativity down the groups? Which element has the highest electronegativity?

Pauling Scale Values increase across periods Values decrease down groups. Fluorine has the highest value.

Why do we have this trend? Electronegativity increases across a period as the nuclear charge on the atoms increases but the shielding stays the same, so the electrons are more strongly attracted to the atom. Electronegativity decreases down a group as the number of shells increases, so shielding increases and the electrons are less strongly attracted to the atom.

Values An atom which has a high electronegativity is said to be electronegative; an atom which does not have a high electronegativity is said to be electropositive. Electronegativities are relative; electronegativity has no units and is measured on a scale from 0.7 to 4.0 Noble gases cannot be ascribed an electronegativity since they do not form bonds

Electronegativity and bond polarity

Bonds between identical atoms In a covalent bond, electrons are shared. When the 2 atoms are identical, the electrons are shared equally. H-H Cl-Cl

Bonds between different atoms What do you think happens when different atoms form bonds?

Bonds between different atoms Different atoms have different electronegativities. One will pull the electrons closer to its end it will have a slight negative charge δ-. The other will have a slight positive charge δ+ . δ+ δ- H-Cl

Bonds between different atoms This leads to a charge difference called a dipole. Dipoles can be permanent. The greater the difference in electronegativity the greater the polarity of the bond. δ+ δ- H-Cl

Differences in electronegativity If the difference is greater what type of character do you think will form? δ+ δ- H-Cl

Differences in electronegativity Large difference between the electronegativities of the bonding atoms, the greater the ionic character of the bond. The greater the similarity between the electronegativities of the bonding atoms, the greater the covalent character. δ+ δ- H-Cl

Differences in electronegativity Predict the polarity in the following bonds; where applicable, draw in the δ+ and δ− a) S—Cl b) S—O c) N—O d) C—O e) F—Cl f) C—Cl Element Electronegativity Cl 3.16 O 3.44 N 3.04 F 3.98 C 2.55 S 2.58

POLAR and NON-POLAR molecules. For molecules that are symmetrical , the dipoles of any bonds within the molecule can cancel out. Tetrachloromethane is a non-polar molecule despite having bonds with polar bonds. Each C-Cl bond is polar but as CCl4 is a symmetrical molecule and the dipoles act in different directions they cancel each other out.

Which of the following molecules are polar? CH3Cl H2S C2H5OH NH3

Van der Waals forces Weak intermolecular forces made up of dipole-dipole and induced dipole-induced dipole forces of attraction.

What causes van der Waals’ forces? The movement of electrons in shells unbalances the distribution of charge within the electron shells. At any moment there will be an instantaneous dipole across the molecules. The instantaneous dipole induces a dipole in neighbouring molecules which in turn induce further dipoles on their neighbouring molecules. Induced dipole δδ We use 1 δ symbol to represent permanent dipoles and 2 δδ to represent fluctuating ones. Random dipole δδ

Strengths of different types of bond. Bond type Relative strength Ionic and covalent bonds 1000 Hydrogen bonds 50 Dipole-dipole forces 10 Van der Waals’ forces 1

A bit of biology! A gecko’s toes are covered with microscopic hairs that create van der Waal’s forces. It is the combined attraction from the great number of hairs that allows the gecko to stick to virtually any surface , even polished glass.

Boiling points and van der Waal’s Noble gas Boiling point (˚C) Number of electrons He -269 2 Ne -246 10 Ar -186 18 Kr -153 36 Xe -108 54 Rn -62 86 Electrons increase Van der Waals’ forces increase Boiling point increases

Intramolecular bonding effects chemical properties e. g. reactivity Intramolecular bonding effects chemical properties e.g. reactivity. Intermolecular bonding effects physical properties e.g. boiling points.

Plenary: The boiling point of the Group 7 elements are shown below. Each element exists as diatomic molecules. F2 -188˚C Cl2 -35˚C Br2 59˚C I2 184˚C Explain this trend, in terms of intermolecular forces.