3) Chemical bonding

Question 1

Electronegativity

Answer 1

• the ability of an atom to attract a pair of electrons towards itself in a covalent bond
• fluorine is the most electronegative with a value of 4.0 as it is the best at attracting electron density towards itself when covalently bonded to another atom

Question 2

Factors influencing electronegativity

Answer 2

1) Nuclear charge- an increase in the number of protons increases electronegativity

2) Atomic radius- increased atomic radius results in a decreased electronegativity

3) Shielding by inner shells and sub-shells- an increased number of shells will result in a decreased electronegativity

Question 3

Across the period electronegativity increases, because…

Answer 3

• nuclear charge increases with the addition of protons to the nucleus
• shielding remains relatively constant as no new shells are being added to the atoms
• nucleus has an increasing strong attraction for the bonding pair of electrons of atoms across the period, which results in a smaller atomic radii

Question 4

Down the group electronegativity decreases, because…

Answer 4

• nuclear charge increases as more protons are being added to the nucleus
• however each element has an extra filled shell which increases shielding
• the addition of extra shells increases the distance between the nucleus and the outer electrons resulting in a larger atomic radii
• decrease in attraction between the nucleus and outer bonding electrons
• increased shielding and atomic radius outweighs the affects of the increased nuclear charge

Question 5

Ionic bonds

Answer 5

• when atoms of different electronegativities form a molecule, the shared electrons are not equally distributed in the bond
• the more electronegative atom will draw the bonding pair of electrons towards itself and a molecule with partial charges form
• the more electronegative atom will have a delta negative charge and the other a delta positive charge
• leads to a polar covalent molecule
• if there is a large difference in electronegativity the least electronegative atoms electrons will transfer to the other atom which leads to an ionic bond
• cation is + and lost electrons and anion is - and has gained electrons

Question 6

Covalent bond

Answer 6

• single covalent bonds are formed by the sharing of electrons between the 2 atoms
• in diatomic molecules the electron density is shared equally between the 2 atoms

Question 7

Difference in electronegativity and bond type

Answer 7

Covalent= <1.0
Polar covalent= 1.0 - 2.0
Ionic= > 2.0

Question 8

Ionic bonding

Answer 8

• the electrostatic attraction between oppositely charged ions
• very strong and requires a lot of energy to overcome therefore high m.p in ionic compounds
• ions form a lattice structure which is an evenly distributed crystalline structure arranged in a regular repeating pattern

Question 9

Metals and non-metals

Answer 9

Metals- lose electrons from their valence shell to form positively charged cations

Non-metals- gain electrons to form negatively charged anions

Question 10

Ionic bonding examples you need to know

Answer 10

• sodium chloride
• magnesium oxide
• calcium fluoride

Question 11

Metallic bonding

Answer 11

• the electrostatic attraction between the positive metal ions and sea of delocalized electrons
• metal atoms are tightly packed together in lattice structures and the electrons in their outer shells are free to move throughout the structure (delocalised electrons)

Question 12

Covalent bonding

Answer 12

• the electrostatic attraction between the nuclei of 2 atoms and a shared pair of electrons

Question 13

Bonds and number of electrons shared

Answer 13

C-C 2
C=C 4
Triple C 6

Question 14

Expanding the octet rule (elements in period 3)

Answer 14

• when the central atom of a covalently bonded molecule can accommodate more than 8 electrons in its outer shell
• SO2, PCI5, SF6

Question 15

Electron deficient

Answer 15

• accommodating less than 8 electrons in the outer shell

Question 16

Dative covalent bonding

Answer 16

• Ammonia + hydrogen chloride gas = NH4+
• AI2CI6

Question 17

Sigma bonds

Answer 17

• formed by direct overlap of orbitals between bonding atoms
• electron density is symmetrical
• the pair of electrons are found between the nuclei of the 2 atoms
• the electrostatic attraction between the electrons and nuclei bonds the atoms to each other

Question 18

Pi bonds

Answer 18

• formed by the sideways overlap of adjacent p orbitals above and below the sigma bond
• maximises overlap of p orbitals

Question 19

Hybridisation

Answer 19

• mixing of atomic orbitals to form covalent bonds
• p atomic orbitals can also overlap end-on to form sigma bonds, but for them to do this they need to first become modified in order to gain s orbital character
• the orbitals are therefore slightly changed in shape to make one of the p orbital lobes bigger

1s + 3p = sp3 hybridised
1s + 2p= sp2 hybridised
1s + 1p orbital = sp hybridised

Question 20

Bond energy

Answer 20

• energy required to break 1 mole of a particular covalent bond in the gaseous state
• the larger the bond energy, the stronger the covalent bond is

Question 21

Bond length

Answer 21

• in internuclear distance of 2 covalently bonded atoms
• the greater the forces of attraction between electrons and nuclei, the more the atoms are pulled closer to each other
• this decreases bond length and increases strength of covalent bond
• triple bonds are the shortest and strongest covalent bonds due to the large electron density between the nuclei of the 2 atoms
• increases force of attraction between electrons and nucleus therefore bond length is shorter + covalent bond is stronger

Question 22

Long bond length + smallest bond energy =

Answer 22

most reactive as it takes the least amount of energy to break apart

Question 23

Order of repulsion

Answer 23

Lone pair-lone pair > lone pair-bond pair > bond pair-bond pair

Question 24

Examples (7)
BF3
CO2
CH4
NH3
H20
SF6
PF5

Answer 24

BF3- trigonal planar 120
CO2- linear 180
CH4- tetrahedral 109.5
NH3- pyramidal 107
H2O- non-linear 104.5
SF6- octahedral 90
PF5- trigonal bipyramidal 90 + 120

Question 25

Hydrogen bonding

Answer 25

• strongest form of intermolecular bonding
• type of pd-pd bonding
• for it to take place a species which has a O or N (very electronegative) atom with an available lone pair of electrons and a species with an -OH or -NH group is needed (hydrogen bond is 180)
• examples are ammonia and water

Question 26

Concept of hydrogen to explain properties of water (ice and water)

Answer 26

1) High melting and boiling point- caused by strong intermolecular forces of H bonding and molecules are tightly held together, therefore lot of energy required to break water molecules apart

2) High surface tension (ability of a liquid surface to resist any external forces)- water molecules at the surface of liquid are bonded to other water molecules through H bonds and these molecules pull downwards on the surface molecules causing them to become compressed and more tightly together at the surface

3) Ice has a lower density than liquid water- ice is closely packed together as it is a solid in a 3D H-bonded network in a rigid lattice and each oxygen id surrounded by H atoms. Relatively long bond lengths of H bonds and this way of packing means the water molecules are slightly further away in the liquid form

Question 27

Non-polar and polar

Answer 27

Non-polar= when 2 atoms in a covalent bond have the same electronegativity

Polar- when 2 atoms in a covalent bond have different electronegativities. Electron distribution is asymmetric and the greater the difference in electronegativity, the more polar the bond becomes

Question 28

Dipole moment

Answer 28

• a measure of how polar a bond is
• the direction of the dipole moment is shown by an arrow which points to the partially negative end of the dipole

Question 29

Some molecules have polar bonds but are overall not polar, because…

Answer 29

The polar bonds in the molecule are arranged in such a way that the individual dipole moments cancel each other out
- example in notes

Question 30

van der Waals forces and dipoles

Answer 30

• the intermolecular forces between molecular entities other than those due to bond formation
• weakest type of van der Waals forces and occur between all atoms and molecules, whether they are polar or non-polar.
• they are caused by temporary fluctuations in the electron cloud around atoms, which create instantaneous dipoles

Question 31

Intermolecular
Intramolecular

Answer 31

• forces between a molecule
• forces within a molecule

Question 32

Instantaneous dipole-induced dipole forces (id-id)

Answer 32

• exist between all molecules/atoms
• electron cloud in non-polar molecules are constantly moving and during this the electron charge cloud can be more on 1 side than the other and this causes a temporary dipole to arise
• this temporary dipole can induce a dipole on the neighbouring molecules and when this occurs the delta + end of the dipole in 1 molecule is attracted to the delta - end of a neighbouring molecule
• because the electron clouds are always moving the dipole is only temporary

Question 33

id-id forces increase with

Answer 33

• increasing number of electrons (and atomic number) in the molecule
• increasing the places where the molecules come close together

Question 34

Permanent dipole-permanent dipole forces (pd-pd)

Answer 34

• polar (different electronegativities) molecules have permanent dipoles
• the molecules will always have a - and + charged end
• the delta + end of the dipole in one molecule is attracted to the delta - end of the dipole in a neighbouring molecule
• for smaller molecules with the same number of electrons, pd-pd forces are stronger than id-id forces

Question 35

Order of forces

Answer 35

Hydrogen bonding > pd-pd > id-id

Question 36

Drawings of water and ammonia

Answer 36

in notes

Question 37

Ionic, covalent and metallic bonding are stronger than ?

Answer 37

intermolecular forces

Question 38

Be able to illustrate ionic, covalent and coordinate bonding

Answer 38

in notes