chem bonding

Question 1

what is electronegativity

Answer 1

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

Question 2

trend of electronegativity (most electronegative ions)

Answer 2

increase across period
decrease down a group

(fluorine, oxygen and nitrogen)

Question 3

why does electronegativity increases across period

Answer 3

• no. of protons increase, there is an increasing nuclear charge
• shielding effect stays approximately constant -> additional e- added to same valence shell provides negligible shielding effect
• effective nuclear charge increases
• valence e- exp stronger nuclear attraction
  -> ability of atom in a molecule to attract the shared pair of electrons increase

Question 4

why does electronegativity decrease down the group

Answer 4

• increase in number of protons -> increase in nuclear charge
• increasing shielding effect as there is an increase in the number of inner shell electrons
• effective nuclear charge remains approximately constant
• valence e- are further away from the nuclear as there is an increase in the no. of PQS
• valence e- experience weaker nuclear attraction
• ability of the atom in a molecule to attract the shared pair of electrons decreases

Question 5

what is an ionic bond

Answer 5

strong electrostatic attraction between two oppositely charged ions, formed by the transfer of e- between the original atoms

Question 6

what is a covalent bond

Answer 6

strong electrostatic attraction between the positive nuclei of the two atoms and the bonding electrons shared between them

Question 7

what are the factors of a dative bond

Answer 7

• must have at least one lone pair of electrons
• for the atom to be an acceptor, it has to have at least one empty orbital
• only atoms in period 3 or higher than that have expanded octet
• central atom from period 2 have more than 8 e-

Question 8

why can’t elements in period 1 and 2 expand their octet

Answer 8

because there is an absence of energetically accessible/low-lying d-orbitals in their valence shells

Question 9

what are the 3 postulates of VSEPR theory? (valence shell electron pair repulsion)

Answer 9

1. electron pairs are similar in pair and tend to repel each other
   -> e- pair in valence shell will experience the least repulsion when they are far apart from each other as possible
2. repulsion decreases in e- in the order of:
   lp-lp repulsion > lp-bp repulsion > bp-bp repulsion
   this is because lp is attracted by only one positive nucleus while bp is attracted to 2 nuclei
   -> lp exp greatest repulsion
3. repulsion between bp increase with increasing electronegativity of central atom and decrease with increasing electronegativity of terminal atoms

Question 10

what is a sigma bond?

Answer 10

formed by head-on overlap between valence atomic orbitals
(s+s, s+p, p+p)

• has symmetry with respect to rotation about the bond axis
• most of the e- density is found between the two nuclei

Question 11

what is a pi bond

Answer 11

• formed after the formation of sigma bonds
• formed by the sideways overlap of parallel p-orbitals
• has a nodal plane about the nuclear axis
• e- density is concentrated above and below the plane

Question 12

why is sigma bond stronger than pi bond

Answer 12

there is a greater extent of overlap between the atomic orbitals in the sigma bond than the pi bond

Question 13

covalent character in an ionic bond

Answer 13

• partial transfer of e- electrons
• ions exist but e- cloud of anion is distorted

Question 14

ionic character in covalent bond

Answer 14

• unequal sharing of e-
• e- cloud distorted towards the more electronegative atom

Question 15

why is there covalent character in the ionic bond?

Answer 15

• due to the polarisation of the anion by the highly charged cation

Question 16

what are the factors of polarisation of anion

Answer 16

1. polarising power of the cation
2. polarisability of the anion

Question 17

how does the polarising ability of the cation affect the extent of polarisation in an ionic bond?

Answer 17

with a higher charge density (high charge small mass) of cation -> cation can polarise the anion to a greater extent -> leading to greater polarising power -> thus, degree of polarising power increases

ans format:
being highly positively charged, ____ (cation) draws the electron cloud of the _____ (anion) anion to itself. The cation is said to be polarising the anion.
as a result, part of the electron cloud of ____ (anion) gets drawn into the region between the two ions, and gets shared by them.
the ionic bond thus acquires a certain degree of covalent character since the electron cloud is being shared between _____ and _____.

Question 18

how does the polarisablity of the anion affect the extent of polarisation in an ionic bond?

Answer 18

size of anion
- larger anions are more easily polarised by cations

Question 19

what determines the polarity of the covalent bond with ionic character?

Answer 19

the difference in electronegativity between the two atoms

Question 20

what is bond energy?

Answer 20

the energy absorbed when one mole of a given covalent bond in a gaseous compound is broken into gaseous atoms or molecules

Question 21

what factors affect covalent bond strength?

Answer 21

• bond order
  triple bond > double bond > single bond

-> no. of bp of e decreases
-> this results in weaker electrostatic attraction of the ____ nucleus for the bonding e-
-> bond length increases when bond strength decreases

• size of bonding atom
  smaller the atomic size, larger extent of overlapping between atomic orbitals, covalent bond strength increases

-> when atomic size increases, orbitals become more diffused
-> extent of effective orbital overlap decreases
-> bond energies decreases, bond strength decreases

• electronegativity difference
  greater electronegativity diff, greater bond polarity hence greater bond strength

-> polar bond is strengthened by some ionic character, greater energy to break bond hence greater bond strength

Question 22

what does polarity of simple molecules depend on

Answer 22

• bond polarity
• shape of molecule

Question 23

what are the characteristics of simple molecular structure?

Answer 23

• molecules held together by weak intermolecular forces of attraction
• within the molecule there are strong covalent bonds between atoms

Question 24

What types of intermolecular forces are there?

Answer 24

1. instantaneous dipole-induced dipole (id-id)
   -> non-polar
2. permanent dipole-permanent dipole (pd-pd)
   -> polar
3. hydrogen bonding
   -> polar

Question 25

how does id-id arise?

Answer 25

- due to instantaneous dipoles for **non-polar** molecules - e- are mobile -> displaced towards one end of the molecule at any instant - that end becomes partially -tive - other end temporarily short of e- and becomes partially +tive - sets up an induced dipole in another molecule **weak attraction present between all particles**

Question 26

what factors affect strength of id-id interactions?

Answer 26

**size of e- cloud/no. of e-** -> larger e- cloud more polarisable (easily distorted) than a smaller e- cloud -> id-id more easily formed -> strength of id-id increases with size of e- cloud/no. of e- (usually affects boiling point) **surface area of contact between molecules** -> molecules with comparable no. of e- but different spatial arrangement of atoms hv diff id-id strengths -> strength of id-id will increase with greater surface area of contact -> straight chain/linear molecules have greater surface area of contact than branched molecules (w same no. of e-) -> stronger id-id (usually compare organic chem)

Question 27

how does pd-pd arise?

Answer 27

- occur between polar molecules -> attraction between permanent dipoles in the structure - electrostatic attraction between the partially -tive end of a molecule and partially -tive end of the other molecule causes pd-pd interactions - increases with increasing **polarity of molecule** (bc of diff in electro-tivity)

Question 28

how do hydrogen bonds arise?

Answer 28

- H atom must be bonding directly to a very **small, highly electronegative atom** with lone electron pairs (usually F,O,N) - highly electronegative atom causes the electron density to be so greatly withdrawn (e- all go to more electronegative side) that the H atom is left w a significant amt of partial +tive charge criteria: - hydrogen atom bonded directly to F, O or N - lp of e- in a neighbouring molecule can attract the partial positive charge on H atom

Question 29

what does the strength of hydrogen bond depend on?

Answer 29

**extensiveness of H bond** -> no. of H bond formed per molecule -> depends on no. of lp of e- available on electronegative atom & no. of H atom available **polarity of bond** -> dependent on difference in electronegativity between H and electronegative atom

Question 30

structure of ice?

Answer 30

- one water molecule forms 4 hydrogen bonds to 4 other water molecules - rigid but open structure -> a lot of empty space between molecules -> ice less dense than water - when ice melts, some of the H bonds are overcome and the water molecules move into the empty spaces that were in ice struture - water molecules are closer together in liquid water than in ice

Question 31

what is the electrical conductivity in simple molecules?

Answer 31

no free mobile charge carriers (no delocalised electrons & free mobile ions) in all states -> cannot conduct electricity

Question 32

how is the mp and bp of simple molecules?

Answer 32

- weaker intm forces of attraction -> broken when molecule is melted or boiled ≠ **covalent bonds broken** strength of itm forces hydrogen bonds > pd-pd > id-id stronger itm forces -> higher bp/mp -> more energy required to overcome interaction and change state

Question 33

how is the solubility of simple compounds?

Answer 33

non-polar molecules are more soluble in non-polar solvents polar molecules are more soluble in polar solvents substances that form H bond w water molecules are more soluble in water energy given out through dissolving -> sufficient to compensate?/insufficient to compensate for the greater amt of energy to overcome H bonds -> sol./insol. in water