Electronegativity

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AQA AS SpecificationLessons Topics

1-2 Bonding•understand ionic bonding, covalent bonding, co-ordinate bonding and metallic bonding in terms of electrons and forces.

3Electronegativity•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 ElectronegativityHow desperate are you for this lesson to end? (tick one) Very

Slightly

Not at all

N/A

What group do you belong to? Group I & VII

Group II & VI

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

Why are the halogens missing?

Describe the trends of electronegativity across the PT

Increases

Dec

reas

es

What does electronegativity depend upon?

1. Nuclear charge2. Distance between the nucleus

and the outer electrons3. The shielding by inner electrons

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

1.5

Si

1.8

P

2.1

S

2.5

Cl

3.0

Ar

K

0.8

Ca

1.0

Sc

1.3

Ti

1.5

V

1.6

Cr

1.6

Mn

1.5

Fe

1.8

Co

1.8

Ni

1.8

Cu

1.9

Zn

1.6

Ga

1.6

Ge

1.8

As

2.0

Se

2.4

Br

2.8

Kr

Rb

0.8

Sr

1.0

Y

1.2

Zr

1.4

Nb

1.6

Mo

1.8

Tc

1.9

Ru

2.2

Rh

2.2

Pd

2.2

Ag

1.9

Cd

1.7

In

1.7

Sn

1.8

Sb

1.9

Te

2.1

I

2.5

Xe

Cs

0.7

Ba

0.9

La

1.1

Hf

1.3

Ta

1.5

W

1.7

Re

1.9

Os

2.2

Ir

2.2

Pt

2.2

Au

2.4

Hg

1.9

Tl

1.8

Pb

1.8

Bi

1.9

Po

2.0

At

2.2

Rn

Factors affecting electronegativity

9+ 1+

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.

Rules1. The smaller the atomic radius, the closer the nucleus is to the shared

electrons, the larger the electronegativity2. 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

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

polarIs it polar, or non polar?

threeHow many lone pairs are there?

What other bonding possibilities are there

Electron probability (density) map

                              

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

X Y X Y Y+X- Y+X-

Electronegativity Difference0 4

Pure covalent Polar covalentElectrons not equally shared

Polar ionicDistorted ions

Pure ionic

Polarisation of covalent bonds

Polarisation of ions

Favoured by small, highly charged +ve ions, e.g. Li+, Be2+

-+

0

10

20

30

40

50

60

70

80

90

100

0 0.5 1 1.5 2 2.5 3 3.5 4

difference in electronegativity

% io

nic

ch

arac

ater

Ionic bonding

Covalent bonding

Question:Which of the following molecules will be polar?

HCl, H2O, CCl4, CH2 OAll 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.

Difference in electronegativity decreases

NaCl MgCl2 AlCl3 SiCl4

Mpt 801ºC 714ºC 190ºC -70ºC

Structure IonicPolar ionic

Polar covalent

Covalent

+ve ion gets smaller and more highly charged, so –ve polarised

more

Difference in electronegativity decreases

BeCl2 MgCl2 CaCl2 SrCl2 BaCl2

Mpt 401ºC 714ºC 782ºC 870ºC 963ºC

Structure

Polar covale

nt Ionic Ionic Ionic Ionic

+ve ion gets smaller and more highly charged, so –ve polarised

more

OHH

+ +

-

H2O

Bonds: polarMolecule: polar

NHH

+ +

-

NH3

Bonds: polarMolecule: polar

H+

CO+-

CO2

Bonds: polarMolecule: non-polar

O-

CCl4

Bonds: polarMolecule: non-polar

C

- -

+

Cl-

ClCl

Cl-

Summary of bonding