chemical bonding(1)

Question 1

define metallic bonding and its properties

Answer 1

it is the electrostatic forces of attraction between a lattice of cations and sea of delocalised electrons in giant metallic substances.

metallic bonds are strong and non-directional(extend in all direction)

strength of metallic bond is proportional to
(no. of valence electrons available for delocaisation per atom)/(size of metal cation).

Question 2

define giant metallic lattice structures

Answer 2

they consist of a lattice of metal cations in a** sea of delocalised electrons** held together by strong electrostatic forces of attraction.

Question 3

why do giant metallic lattice structures have high melting & boiling point?

Answer 3

a large amt of energy is needed to overcome the strong metallic bonds(electrostatic forces of attraction between cations and sea of delocalised electrons) during melting or boiling

Question 4

why are giant metalllic lattice structures good electrical conductors(even in solid statr)

Answer 4

when a potential difference is applied actross the metal, the **delocalised electrons which are free and mobile can function as charge carriers and move in the direction of the positive potential.**

Question 5

why are GMLS malleable(easily pressed into shape) and ductile(easily strentched)

Answer 5

1. As a force is applied to a metal, the metal ions are pushed and they rearrange in a different order to give the desired shape **w/o breaking the metallic bonds. **
2. similarly, as the metal is stretched, the metal ions are pulled and they **rearrange based on the direction of the applied force w/o breaking the metallic bonds. **

Question 6

why are GMLS good thermal conductors

Answer 6

1. when heated at one end, the electrons and the metallic cations take in thermal energy.
2. the electrons move faster and more randomly, colliding with other electrons and passing the energy to them.
3. At the same time, the metallic cations also vibrate faster, passing the energy to the** adjacent cations** when colliding with them.

Question 7

define ionic bonds

Answer 7

they are electrostatic forces of attraction between oppositely-charged ions in giant ionic substances.

formation of ionic compounds involves electron transfer between two species.

strength of ionic bonds dependent on lattice energy(larger LE means larger IB)

they are non-directional

Question 8

lattice energy

Answer 8

the energy released when one mole of the solid ionic compound is formed from its constituent gaseous ions.

magnitude of LE is proportional to (Q1xQ2)/(R1+R2)

Question 9

what are giant ionic lattice structures

Answer 9

they consist of a lattice of oppositely-charged ions held tgt by strong electrostatic forces of attraction.

Question 10

why do GILS have high melting point/boiling point

Answer 10

A large amt of energy is needed to overcome the strong ionic bonds during melting/boiling

Question 11

why are GILS non-electrical conductors in solid state, but good conductors in molten/aqueous state

Answer 11

1. in solid state, the ions can only vibrate about fixed positions and are not mobile to conduct electricity
2. in molten or aqueous state, the ions are free and mobile to function as charge carriers when a** potential difference is applied. **

Question 12

why are GILS hard and brittle

Answer 12

1. the regular arrangement of ions in the lattice resists any applied force as ions of the same charge** becomes closer and repels one another**.
2. if the applied force is sufficiently strong to slide one layer of ions over another, the resultant repulsive forces will **shatter the lattice. **

Question 13

why are GILS soluble in water and polar solvents

Answer 13

the formation of ion-dipole interactions between the ions and water molecules/polar solvent molecules, releases sufficient energy to overcome the strong ionic bonds and allow detachment of the ions from the ionic lattice for hydration/solvation.

** need to know how to draw a diagram for ion-dipoleinteraction)

except:
ionic compounds with very exothermic lattice energies are not soluble in water as the energy released from ion-dipole interactions is insufficient to overcome the strong ionic bonds to allow detachment of ions from the ionic lattice.

Question 14

why are GILS are not soluble in non-polar solvents.

Answer 14

weak interactions formed between the ions and the non-polar solvent molecules release insufficient energy to overcome the strong ionic bonds between the ions.

Question 15

define covalent bonds

Answer 15

they are electrostatic forces of attraction between the shared pair of electrons and positively charged nuclei of 2 atoms in simple and giant molecular substances.

a covalent bond is typically formed between 2 non-metallic atoms when their valence orbitals overlap. the shared pair of electrons is said to be localised between the 2 nuclei.

Question 16

define sigma bonds

Answer 16

a head-on overlap of orbitals with one overlapping region

stronger than a Pi bond.

can be S-S orbital overlap, S-P orbital overlap or P-P orbital overlap.

Question 17

define Pi bond and state its conditions

Answer 17

A side-on overlap or orbitals with 2 overlapping regions

a Pi bond is formed only if a sigma bond already exists.

only P orbitals involved.

Question 18

what constitutes a double bond and triple bond

Answer 18

double bond: 1 Sigma and 1 Pi bond
Triple bond: 1 sigma and 2 Pi bond

Question 19

define coordinate(dative) bond

Answer 19

the electrostatic forces of attraction that exist between the** shared pair of electrons** and the nuclei of the donor and acceptor atoms.

A dative bond is a covalent bond in nature.

Question 20

define adduct

Answer 20

a product of a direct addition of 2 different molecules which contains all constituent atoms of the 2 molecules

Question 21

define electronegativity and state its trends.

Answer 21

it is the ability of an atom to attract the shared pair of electrons towards itself in a covalent bond.

it increases across a period and decreases down a group

Question 22

state the order of elements following the trend of decreasing electronegativity

Answer 22

F,O,CL,N,Br,I,S,C,H=P,B,Si,Al<Be,Mg,Ca,Li,Na

Question 23

the concept of electronegativity applies

state the condition for polar and non-polar molecules to be formed

Answer 23

polar molecules:
atoms of different electronegativity
the more electronegative atom pulls the shared electron pair towards itself, resulting in a permanent dipole.

Non-polar molecules:
atoms of similar electronegativity.

Question 24

define bond dissociation energy and state what it affects

Answer 24

the energy required to break one mole of a partiular covalent bond in a specific molecule in the gaseous state to form gaseous atoms.

affects the strength of a covalent bond, where the higher the BDE, the stronger the covalent bond is.

Question 26

describe how bond energy affects thermal stability

Answer 26

thermal stability changes in the same direction as bond energy

Question 27

state the **3 exceptions** for the octet rule

Answer 27

1. electron deficient central atom: for polyatomic molecules, the central atom may have** less than eight valence electrons(BeCl2, AlCl3)**. 2. ability to expand octet structure: central atoms that are from **period 3 and above** may have more than eight valence electrons, due to **availability of vacant low-lying d orbitals to accommodate electrons for bonding.** However, **period 2 element only have a maximum of 8 electrons **in their valence shell, hence they cannot expand their octet structure since there are **no low-lying vacant d orbitals** in its valence shell. 3. unpaired electrons it is possible for some molecules to have an atom with **unpaired electrons**(1,3,5) like NO.

Question 28

define what the Valence Shell Electron Pair Repulsion(VSEPR) Theory and its principles

Answer 28

it is used to predict the molecular geometry of a molecule or molecular ion. ## Footnote must know how to draw dot-and cross diagram and lewis structure, as well as memorised fully all the different shapes and corresponding bond angles, quantities of bond pairs and loan pairs.

Question 29

state the 2 key principles of the VSEPR Theory:

Answer 29

1. electron pairs are arranged around the central atom in a molecule as far apart as possible so as to minimise repulsion between them. 2. the degree of repulsion differs btw the 2 types of electron pairs: Lp-Lp > Lp-Bp > Bp-Bp ## Footnote bond pairs refer to the electron pairs shared between 2 bonded atoms lond pairs refer to valence electron pairs of the central atom not involved in bonding each multiple bond considered as effectively one bond pair.

Question 30

state 2 ways in which the bond angle is influenced by the electronegativity of the atoms in the molecule

Answer 30

Electronegativity of central atom/surrounding atoms - when central atom is **more electronegative**, it **draws** the bond pairs of electrons **closer** to it, **increasing Bp-Bp repulsion** and hence **INCREASING bond angle**. - when **surrounding atoms are more electronegative**, they will **pull the bond pairs away** from the central atom, **reducing Bp-Bp repulsion** and hence **DECREASING** bond angle.

Question 31

state the way in which the bond angle is affected by the size of surrounding atoms bonded to central atom.

Answer 31

for **side atoms larger **than the central atom, the **mutual repulsion of their large electron clouds** **DECREASE** the bond angle.

Question 32

define the characteristics of ionic character in covalent bonds(polar bonds)

Answer 32

polar bonds are covalent bonds with ionic character. the shared electron pair is not equally shared between 2 atoms, giving rise to partial charges and hence a dipole moment. polar bonds are **stronger **than non-polar bonds with a **comparable bond length**, due to the **additional electrostatic attraction between the oppositely-charged dipoles. **

Question 33

state the factor affecting ioinic character

Answer 33

electronegativity: a **larger difference in electronegativity** will lead to a **greater extent of ionic character**, leading to a **larger dipole moment**.

Question 34

define covalent character in ionic bonds

Answer 34

it arises due to the **polarisation of the anioin electron cloud by neighbouring cations** in an ionic compound. the** electron density between oppositely-charged ions increases**, indicating some degree of covalent character in the ioinic bond. ## Footnote **IMPORTANT: it always results in disceprancies between theoretical and experiemntal lattice energies.**

Question 35

state the 2 factors affecting extent of covalent character.

Answer 35

1. polarising power of cation - ability of cation to distory the electron cloud of anion. - proportionalt to charge density (charge)/density. 2. Polarisability of the anion - refers to **how easily the electron cloud can be distorted** -** greater the size of the anion, the greater the polarisability. **