Topic 2 - Structure and Bonding

Define Ionic bonding

the electrostatic attraction between positive and negative ions

what determines the strength of an ionic bond?

• ionic radius

• ionic charge

• stronger when ions are smaller/higher charge

Explain the trend in ionic radius down a group

ionic radii increases going down the group. Ions have more shells of electrons and so the outermost electron experience less pull from positive nucleus

explain the trend in ionic radius for set of isoelectronic ions

• proton number increases

• same number of electrons

• so the nucleus attraction between the outermost electrons and nucleus increases and ions get smaller

define covalent bonding

electrostatic attraction between a shared pair of electrons and two nuclei

define metallic bonding

electrostatic attraction between the positive metal ions and the sea of delocalised electrons

in what type of solvents do ionic lattice dissolve?

polar solvents e.g. water

Why are ionic compounds soluble in water?

• water has a polar bond

• hydrogen atoms have a delta positive charge

• oxygen have a delta negative

• these charges are able to attract charged ions

what is the effect of multiple covalent bonds on bond length and strength?

double/triple bonds exert greater electron density and so the attraction between nucleus and electron is greater resulting in a shorter and stronger bond.

what is a lone pair?

Electrons in the outer shell that are not involved in the bonding.

What is a dative bond?

A bond where both of the shared electrons are supplied by one atom

Draw the bonding structure diagram for Al₂Cl₆

what does expansion of the octet mean?

When a bonded atom has more than 8 electrons in the outer shell.

Describe bonding in simple molecular structure

• Atoms within the same molecule are held by strong covalent bonds and different molecules are held by weak intermolecular forces

• dissolve in non polar solvent

How does graphite conduct electricity?

Delocalised electrons present between the layers are able to move freely carrying the charge

name shapes of molecules

• linear

• trigonal planar

• tetrahedral

• trigonal bipyramid (5 bonded pairs)

• Octahedral (6 bonded pairs)

• pyramidal (1 lone pair, base tetrahedral)

• bent (2 lone pairs)

shape of molecules/ions determined by the repulsion between the electron pairs that surround a central atom

define electronegativity

the ability of an atom to attract the pair of electrons in a covalent bond

measured on Pauling scale

what does it mean when the bond is non-polar?

the electrons in the bond are evenly distributed

How is a polar bond formed? Why some molecules are non-polar

Bonding atoms have different electronegativities

They are symmetrical, which means dipoles cancel out, there is no overall dipole. Or have a lone pair of electrons

define intermolecular force

Attractive force between neighbouring molecules

describe permanent dipole- induced dipole interactions

describe permanent dipole-permanent dipole interactions

• when a molecule with a permanent dipole is close to other non polar molecules it causes the non polar molecule to become slightly polar leading to attraction

• some molecules with polar bonds have permanent dipoles - forces of attraction between those dipoles and those of neighbouring molecules

describe london forces/ how its formed

• caused by fluctuations in electron density around the molecule

• leads to instantaneous dipoles

• instantaneous dipole induces a dipole in nearby molecules

• induced dipoles attract one another, weak force of attraction between dipoles

• greater force in larger molecules due to more electrons

• affect by number of protons/electrons and whether the molecule is branched

why is ice less dense than water?

In ice, the water molecules are arranged in a orderly pattern. It has open lattice with hydrogen bonds.

In water, the lattice is collapsed and molecules are closer together.

Why does solubility depend on chain length?

Insoluble when long chain - non-polarity of C-H bond takes priority

Expanding the Octet

• from P onwards (period 3)

• able to hold extra electrons in the empty d subshell as it is easily accessibly

• close in energy to p subshell

• endothermic - compensated by making more bonds

Factors affecting bond length

• atomic radii

• nuclear charge

• single/double bonds - larger number of electrons shared so stronger force of attraction

why do lone pairs repel more than bonding pairs

lone pairs bigger and shorter, more concentrated distribution of negative charge held closer to the nucleus

Ione pairs pushed electrons by 2.5 degree, caused by repulsion of electrons

Why shapes of molecules?

• number of bonding pairs/bonding regions/lone pairs

• electron pairs repel to be as far apart as possible to maximise separation to minimise repulsion

• lone pairs repel more strongly than bonding pairs, bonding pairs repel each other equally

• the repulsion between lone pair and bonding pair is stronger than bonding pair and bonding pair

• so the molecule adopts the shape

difference in electronegativity

• small difference (0-0.4)- covalent bonding

• medium difference (0.4-1.7) - polar covalent bonding

• large difference(>=1.7) - ionic bonding - electrons are completely transferred to the more electronegative atom

Explain the trend in boiling temperatures of hydrogen halides

• as group goes down, atomic radii increases(further distance between valence electrons and nucleus)

• electron number of halides increases

• stronger London force

• so higher boiling point, more energy required to weaken London force

Why magnesium chloride has a higher solubility in water than lithium fluoride

• hydrogen bonding in water must be overcome

• attraction between ions must be overcome

• water molecules hydrate the ions (i.e. O2- to cations and H+ to anions)

• the energy released when ions are hydrates compensates for the energy needed to overcome the lattice energy in ammonium nitrate

• magnesium ions have a 2+ charge whereas lithium ions have a + charge

• ions in magesium chloride are more strongly hydrated

• this compensates for the strong forces that must be overcome

Why is fluorine more electronegative than boron

• greater nuclear charge/protons

• smaller atomic radius

• so greater pull from nucleus on bonding electrons

which bond is stronger, B-Cl or B-F

• B-F because this has a shorter bond length

• because F has a smaller atomic radius/less shielding

What happens when an ionic compounds have a huge difference in electronegativity

• less covalent character

• more ionic character

Suggest how a chemical process could have a higher yield but a low atom economy

Has a higher yield if there is a good conversion of reactants to product, but has a lower economy if waste produce may be produced

How to form hydrogen bonds

• caused by water having a permanent dipole on the OH bond

• oxygen is more electronegative than hydrogen

• hydrogen bond is formed between molecules

Why is PCl5 possible but not NCl5

Phosphorus can expand its octet, but Nitrogen does not have a (2)d orbitals, so can only accommodate 8 electrons in valence shell