Chapt 3 Atomic structure

Question 1

Define First ionisation energy

Answer 1

First ionisation energy (of an element) is:

• The amount of energy needed to remove 1 mole of electrons
• from 1 mole of gaseous atoms of the element
• to form 1 mole of singly-charged gaseous cations

Question 2

Define second ionisation energy

Answer 2

• The amount of energy needed to remove 1 mole of electrons
• from 1 mole of singly-charged gaseous cations of the element
• to form 1 mole of doubly-charged gaseous cations

Question 3

Factors influencing IE and How they Affect IE
(NUCLEAR CHARGE)

Answer 3

1. Nuclear charge (proton number)

• Positively charge nucleus exerts an attractive force on the negatively charged electrons
• As no. of protons rises, NC increases.
• so ENC increases
• There is stronger electrostatic attraction between more positively charged nucleus and neg charged electrons
• So energy needed to remove e-
• IE HIGHER
  _________________________________

Question 4

IE formula

Answer 4

ENC (Effective nuclear charge / IE)
= Nuclear charge — shielding effect

ENC is a measure of net attraction experienced by outermost electrons

Question 5

IE trends across a period

Write ENC

Answer 5

• Across a period, IE generally increases

EXP:
- As proton number increases, nuclear charge increases

• As successive electrons are added to the same electron shell, shielding effect stays roughly the same.
• Hence, effective nuclear charge increases
  SO
• Net attraction between nucleus and outermost electrons increases
• more energy needed to remove outermost electron
• HENCE 1st ionisation energy generally rises across the period

Question 6

IE trend across period EXCEPTIONS: Be and B

Answer 6

Be: 1s^2 2s^2
B: 1s^2 2s^2 2p^1

Actual 1st IE
IE of B is lower then Be

Excepted:
IE of B should be higher than Be becuz B has higher nuclear charge.

Reason IE of B is lower than Be

• Less energy is needed to remove the electron in the 2p orbital
• becuz it is in a higher energy 2p subshell

Also
- 2s electrons are closer to the nucleus than 2p
- 2s provide shielding to the 2p

• So less energy needed to remove 2p electron in B
• becuz it has increased shielding from the fully filled 2s subshell

Question 7

IE trend across period EXCEPTIONS: N and O

Answer 7

Expectation
IE of O should be higher than N

Reality
IE of O is lower than N

O: 1s^2 2s^2 2p^4
N: 1s^2 2s^2 2p^3

• _ Less energy_ needed to remove the paired electron in the 2p_x orbital of Oxygen
• becuz it experiences electron repulsion

Question 8

IE trends down the group

DON’t write ENC for down the group bc NC & SE both rise

Answer 8

• IE generally decreases down the group

____________
- As number of protons increases , nuclear charge
increases

• Number of electron shells increases,
• shielding effect increases significantly.
  ——
• Outermost electron is further away from the nucleus
  ——
• Net attraction between nucleus & outermost electron decreases
  —-
• Less energy needed to remove valence electron
• HENCE first ionisation energy generally decreases down the group
  By

Question 9

Electronic configurations of Cr
Proton number =24

Answer 9

4s² Half filled
1s^2 2s^2 2p^6 3s^2 3p^6
(6 electrons left)
_ 3d⁵ 4s¹_

Reason:

3d^5 4s^1 configuration is more stable than 3d^4 4s^2

Question 10

Electronic configuration of Cu
Proton number 29

Answer 10

4s² always Half filled

[Ar]
(11 electrons left)
**_3d^{10 41_**}

Reason :
3d^10 4s^1 configuration is more stable than 3d^9 4s^2

Question 11

IE trend between periods
Don’t write ENC bc NC & SE both rise

Answer 11

IE between Group 18 of a period and the Group 1 of the next period decreases significantly
__
Eg
- Na has 1 more proton so Na nuclear charge is higher than Ne
- Na has 1 more electron shell than Ne, so Na shielding effect is significantly higher
__
- Na Outermost electron further away from nucleus
__
- Net attraction between outermost electron & nucleus of Na is lower
- Hence, IE decreases significantly down the graph

Question 12

Factors influencing IE and How they Affect IE
(SHIELDING EFFECT)

Answer 12

2. Shielding effect (number of inner electron shells & number of electrons)

• Electrons on inner electron shell cancels out some of attraction forces that the nucleus exerts on the outermost electrons
• More e shells leads to greater shielding effect experienced by outermost electrons

Question 13

Successive IE trend

Answer 13

• Generally, The more electrons removed, the higher the IE

REASON 1
- As more electrons are removed from the outershell, there are less electrons than protons
- shielding effect decreases
__
- Since nuclear charge is constant,
- there is stronger electrostatic attraction between the remaining electrons and the nucleus.
- More energy is needed to overcome the stronger forces of attraction.
_____________________
REASON 2
- as More electrons are removed
- the ion also becomes increasingly positively charged

• More energy is needed to remove a negatively charged electrons from the more positively charged ion