Structure, Bonding and Equilibria

Question 1

Electronegativity

Answer 1

The ability of an atom to attract an electron to itself.

Question 2

Electronegative elements

Answer 2

F
O
Cl

Top right of periodic table

Question 3

Electropositive elements

Answer 3

Cs
Ba
Rb

Bottom left of periodic table

Question 4

Ionic bonding when…

Answer 4

Difference in electronegativity >2

Question 5

Covalent bonding when …

Answer 5

0.5< difference in electronegtivity <2

Question 6

Metallic bonding when…

Answer 6

Difference in electrnegativity <0.5

Question 7

ΔH atomisation

Answer 7

X(standard) -> X(g)

always endothermic

Question 8

1st ionisation enthalpy

Answer 8

X(g) -> X⁺(g) + e⁻

always endothermic

Question 9

Electron gain enthalpy

Answer 9

X(g) + e⁻ -> X⁻(g)

usually exothermic (or close to it)

Question 10

Lattice enthalpy

Answer 10

X⁺(g) + Y⁻(g) -> XY(s)

always exothermic (when defined in this way)

Question 11

General trend in I.E. across a period.
Why?

Answer 11

Overall, ionisation energy increases across a period.
Increasing nuclear charge and reduced electron shielding ability increases effective nuclear charge.
Electrons experience greater attraction to nucleus, so more energy input required to remove them.

Question 12

Zeff

Answer 12

Effective nuclear charge - the actual amount of positive charge experienced by an electron.
Zeff = Z (atomic number) - S (shielding constant)

Question 13

Electron shielding

Answer 13

Partial neutralisaiton of nuclear charge by core electrons + electron-electron repulsion reduces Zeff felt by outer shell electrons.

Question 14

Electron penetration

Answer 14

An electron’s relative electron density near the nucleus - i.e. proximity to which an electron can approach the nucleus.
Penetrating power depends on shell and subshell
1s>2s>2p>3s>3p>4s>3d etc

Question 15

In the second period, why is there a dip in 1st I.E at B and O?

Answer 15

Boron - the outer 2p electron does not penetrate the 2s electrons effectively. Therefore easier to remove.

Nitrogen - repulsion between paired 2p electrons makes them easier to remove.

Question 16

General trend in I.E down a group.
Why?

Answer 16

Overall, I.E decreases down the group.
Principle quantum number (number of shells) increases. Valence electrons spend more time futher from nucleus - attraction to nucleus is lower - electrons are easier to remove.

Question 17

In group 1, why is there an uneven trend in I.E between K and Rb?

Answer 17

Between K and Rb the 3d orbitals are filled.
D orbital electrons do not shield the nuclear charge as effectively. I.E for Rb is higher than would be expected as Zeff for valance electrons is higher.

note - I.E for Rb is still lower than for K. However, the difference is smaller than would be expected.

Question 18

General trend in electron gain enthalpy across a period.
Why?

Answer 18

Overall, electron gain enthalpy becomes more negative across the period.
Zeff increases across the period as nuclear charge increases and electron shielding is less effective.
Makes it easier for atom to accept electron as incoming electron experiences greater attraction.

Question 19

In period 2, why is there an increase in electron gain enthalpy at N?

Answer 19

The incoming electron has to pair up in a 2p orbital. Must overcome repulsion with paired electron.

Question 20

Lattice enthalpy is proportional to…

Answer 20

∣z⁺∣∣z⁻∣/(r⁺+r⁻)

absolute product of charge on the anion and cation / sum of radius of anion and cation

Question 21

Lattice enthalpy increases when…

Answer 21

…ion size decreases
… charge on either ion increases

Question 22

Bond energy is…

Answer 22

…the energy required to break a specific bond via homolytic fission
units: kjmol-1
always positive

Question 23

Bond dissociation energies

Answer 23

Standard enthalpy change for the reaction in which the bond in question is broken.
*Average * values

Question 24

Bonds to larger atoms tend to be…

Answer 24

…weaker and longer.

Question 25

Bonds to more electronegative atoms tend to be…

Answer 25

…shorter

Question 26

Equilibrium bond length

Answer 26

Internuclear distance that minimises repulsion between nuclei, but maximises attraction between nuclei and electrons.

Question 27

Hypervalent species

Answer 27

Species with >8 valence electrons.
Found further down the periodic table.

Question 28

To which elements is the octet rule not applicable?

Answer 28

Transition metals.

Question 29

Formal charge

Answer 29

= (e⁻ in valence shell) - (no. of bonds to atoms) - (no. of unshared e⁻)