AS Level Chemistry - Unit 1 - Edexcel

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  • Created by: Fayadh56
  • Created on: 28-12-13 06:01

Ionisation Energy

First Ionisation Energy

  • The amount of energy that is required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 molde of gaeseous unipositive ions.

Second Ionisation Energy

  • The amount of energy that is required to remove 1 mole of electrons from 1 mole of gaseous unipositive ions to form 1 mole of gaseous dipositive ions.

Remark: The 2nd IE, and Succesive ones are more Endo. than the first.


1) Why is the 1st IE Endo? 

  • Energy is absorbed to overcome the nuclear attractions while removing 1 e- from the valency shell of a gaseous atoms.
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Ionisation Energy Questions

  • The Succesive IE are more Exothermic because more energy is required to overcome the extra nuclear attraction, whilst removing the 2nd/succesive e-s from the gaseous positive ions as its more tightly held by the nucleus.

The value of the 1st IE depends on the following factors:-

  • The Effective Nuclear Charge
  • The distance between the outer electron and the nucleus
  • The 'Shielding' produced by lower energy levels.

 1) Down the group: As we go down, Atomic Number Increases, No. of shells Increase, Shielding Increases, Nuclear Attraction Decreases, Atomic Size Increases. Less Energy is required to remove an electron from the valency shell. 1st IE decreases down a group.

2) Across a period: Atomic No Increases(by 1 unit), E-s are added to the same shell, shielding remains almost the same, Atomic Size Decreases, as the Nuclear Charge Increases, and Nuclear attraction gradually increases, therefore more energy is need to remove an e's, In conclusion 1st IE INCREASES. 

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Irregularities In IE (Groups 2-3, and 5-6)


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Atomic Radius

Atomic Radius: Half the distance b/w the nucleus of 2 identical atoms bonded together.


1) Across a Period, Atomic Radius Decreases, As electrons are being added to the same shell(shielding has no effect), Proton no Increases, Affective nuclear charge increases(more attraction). 

2) Down a group, Atomic Radius Increases, No of shells Increases, Shielding Increases, Forces of attraction between the nucleus, and valence e-s decreases, so affective nuclear attraction decreases.

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Trend in Chemical Properties of Elements

Chemical Properties of elements in the periodic table depends on their ability to gain or lose e-s

  • Metals have the ability to lose electrons, Non Metals have the ability to gain electrons

As we move across the period

  • Mettalic Character Decreases. (Ability to lose e-s decrease)
  • Non Metallic Character Increases. (Ability to gain e-s Increase)

As we move down a group

  • Metallic Character Increases (Ability to lose electrons Increase) 
  • Non Metallic Character Decreases (Ability to gain E-s Decreases)


  • Fr(Bottom Left) ---> Most Reactive Metal
  • F (Top Right)    ---> Most Reactive Non Metal

The Chemical Properties of an element, depend on its electron confgiuration.

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Electron Affinity

1st EA: It is the amount of energy released when 1 mole of electrons are added to 1 mole of gaseous atoms to form 1 mole of gaseous uninegative ions.

Why is the 1st IE Exothermic?

  • The elctron added to the gaseous atom experiences a net nuclear attraction. Due to this it is an exothermic process.

2nd EA: It is the amount of energy absorbed when 1 mole of e-s are added to 1 mole of uninegative ions to from 1 mole of gaseous dinegative ions.

Why is the 2nd and Succesive EA, always Endothermic?

  • More energy is spent to overcome the extra repulsions from the electron cloud of the -ve gaseous anion, and the added electron. So Energy is needed.


  • The Use of EA is almost only confined to the G6, and G7 elements. 
  • The sum of 1st EA + 2nd EA > 0 (+ve)
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Trends in Electron Affinity

In Genral, As we go down in a group (G5, 6, 7) 

  • 1st EA becomes less exothermic (less energy is given out when the -ve ion forms)
  • Flourine is an exception - Flourine is a small atom, you are putting the new electron in a small second shell, that is already crowded with electrons. There will be a significant amount of repulsion due to this. This repulsion lessens the attraction for the incoming electron and lessens the EA.

Trends in 1st EA 

  • Along a period, becomes more exothermic, Proton No Increases, Nuclear Charge Increases, Shieding Stays the Same, there is stronger attraction between the electron, and the protons.
  • Down a group, becomes less exothermic, Radius Increases, More Shielding, Less Nuclear attraction between the nucleus and added electrons, stronger repulsions. 
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On the sixth slide isn't it supposed to be why 1st EA is exothermic?

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