1.6 IONISATION ENERGY OBJECTIVES: To define the term ‘ionisation energy’ To describe and explain the trends in ionisation energy across period 3 and down group 2 FIRST THOUGHTS… What do you think we might mean by ionisation energy? KEY WORDS: IONISATION ENERGY NUCLEAR CHARGE TREND

1.6 IONISATION ENERGY WHAT IS IONISATION ENERGY? “The energy required to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions” Energy required to remove electrons Measured in kJmol-1 Given the abbreviation IE Values tell us a lot about the electronic configuration of elements

1.6 IONISATION ENERGY SUCCESSIVE IONISATION ENERGIES We can measure the energies required to remove each electron in turn, from outer electrons to inner electrons Values increase with each successive ionisation event Why might this be? Successive Ionisation Energies of Na

1.6 IONISATION ENERGY SUCCESSIVE IONISATION ENERGIES To form a positive ion, need energy to overcome attraction from nucleus. As each electron is removed from an atom, the remaining ion becomes more positively charged. Removing the next electron away from an increasing positive charge is more difficult and the ionisation energy is even larger.

1.6 IONISATION ENERGY SUCCESSIVE IONISATION ENERGIES Na (g)  Na+ (g) + e- 1st IE = + 496 kJmol-1 Na+ (g)  Na2+ (g) + e- 2nd IE = + 4563 kJmol-1 Na2+ (g)  Na3+ (g) + e- 3rd IE = + 6913 kJmol-1 Notice how successive ionisation energies are written Start with the end product of previous ionisation event MUST include the gaseous state!

1.6 IONISATION ENERGY SUCCESSIVE IONISATION ENERGIES From this graph we can see evidence for electron shells Na looks to have 1 electron that’s easy to remove  furthest from nucleus 8 nearer to the nucleus  bit harder to remove 2 very close to the nucleus  nearest to +ve charge and hardest to remove

1.6 IONISATION ENERGY FACTORS AFFECTING IONISATION ENERGY As electrons are negatively charged and protons in the nucleus are positively charged, there will be an attraction between them. The greater the pull of the nucleus, the harder it will be to pull an electron away from an atom. Nuclear attraction of an electron depends on: Atomic radius Nuclear Charge Electron shielding or screening

1.6 IONISATION ENERGY ATOMIC RADIUS: Greater the atomic radius, the smaller the nuclear attraction experienced by the outer electrons NUCLEAR CHARGE: The greater the nuclear charge, the greater the attractive force on the outer electrons Lithium has a greater nuclear charge so you may expect it to have a higher ionisation energy. However it is affected to a greater extent by the atomic radius and the electron shielding Hydrogen Helium Lithium 519 kJ mol-1 1310 kJ mol-1 2370 kJ mol-1

1.6 IONISATION ENERGY ELECTRON SHIELDING Inner shells of electrons repel the outer-shell electrons Known as ‘Electron shielding’ or ‘screening’ More inner shells  larger the screening and smaller the nuclear attraction of outer electrons Hydrogen Helium Lithium 519 kJ mol-1 1310 kJ mol-1 2370 kJ mol-1

1.6 IONISATION ENERGY TRENDS ACROSS PERIODS Look at the graph 1) Describe the patterns you see 2) Can you suggest an explanation for them

1.6 IONISATION ENERGY TRENDS ACROSS PERIODS There is a ‘general increase’ across a period before the value drops dramatically for the start of another period. This is because nuclear charge is increasing He Ne Ar Kr Xe

1.6 IONISATION ENERGY EXPLANATION HYDROGEN EXPLANATION Despite having a nuclear charge of only 1+, Hydrogen has a relatively high 1st Ionisation Energy as its electron is closest to the nucleus and has no shielding. 1st IONISATION ENERGY / kJmol-1 1s ATOMIC NUMBER 1

1.6 IONISATION ENERGY EXPLANATION HELIUM EXPLANATION Helium has a much higher value because of the extra proton in the nucleus. The additional charge provides a stronger attraction for the electrons making them harder to remove. 1st IONISATION ENERGY / kJmol-1 1s ATOMIC NUMBER 2

1.6 IONISATION ENERGY EXPLANATION LITHIUM EXPLANATION There is a substantial drop in the value for Lithium. Despite the increased nuclear charge, there is electron shielding from the 1s orbital. The 2s electron is also further away from the nucleus. It is held less strongly and needs less energy for removal. 1st IONISATION ENERGY / kJmol-1 1s 1s 2s ATOMIC NUMBER 3

1.6 IONISATION ENERGY EXPLANATION BERYLLIUM EXPLANATION The value for Beryllium is higher than for Lithium due to the increased nuclear charge. There is no extra shielding. 1st IONISATION ENERGY / kJmol-1 1s 1s 2s 1s 2s ATOMIC NUMBER 4

1.6 IONISATION ENERGY EXPLANATION There is a DROP in the value for Boron. This is because the extra electron has gone into one of the 2p orbitals. The increased shielding makes the electron easier to remove It was evidence such as this that confirmed the existence of sub-shells. If there hadn’t been any sub-shell, the value would have been higher than that of Beryllium. 1s BORON 1st IONISATION ENERGY / kJmol-1 1s 1s 2s 1s 2s 2p 1s 2s ATOMIC NUMBER 5

1.6 IONISATION ENERGY EXPLANATION CARBON EXPLANATION The value increases again for Carbon due to the increased nuclear charge. 1st IONISATION ENERGY / kJmol-1 1s 1s 2s 2p 1s 2s 1s 2s 2p 1s 2s ATOMIC NUMBER 6

1.6 IONISATION ENERGY EXPLANATION NITROGEN EXPLANATION The value increases again for Nitrogen due to the increased nuclear charge. As before, the extra electron goes into the vacant 2p orbital. There are now three unpaired electrons. 1s 2s 2p 1st IONISATION ENERGY / kJmol-1 1s 1s 2s 2p 1s 2s 1s 2s 2p 1s 2s ATOMIC NUMBER 7

1.6 IONISATION ENERGY EXPLANATION There is a DROP in the value for Oxygen. The extra electron has paired up with one of the electrons already in one of the 2p orbitals. The repulsive force between the two paired-up electrons means that less energy is required to remove one of them. 1s OXYGEN 1s 2s 2p 1st IONISATION ENERGY / kJmol-1 1s 1s 2s 2p 1s 2s 2p 1s 2s 1s 2s 2p 1s 2s ATOMIC NUMBER 8

1.6 IONISATION ENERGY EXPLANATION FLUORINE EXPLANATION The value increases again for Fluorine due to the increased nuclear charge. The 2p orbitals are almost full. 1s 2s 2p 1s 2s 2p 1st IONISATION ENERGY / kJmol-1 1s 1s 2s 2p 1s 2s 2p 1s 2s 1s 2s 2p 1s 2s ATOMIC NUMBER 9

1.6 IONISATION ENERGY EXPLANATION NEON EXPLANATION The value increases again for Neon due to the increased nuclear charge. The 2p orbitals are now full so the next electron in will have to go into the higher energy 3s orbital. 1s 2s 2p 1s 2s 2p 1s 2s 2p 1st IONISATION ENERGY / kJmol-1 1s 1s 2s 2p 1s 2s 2p 1s 2s 1s 2s 2p 1s 2s ATOMIC NUMBER 10

1.6 IONISATION ENERGY EXPLANATION There is a substantial drop in the value for Sodium. This is because the extra electron has gone into an orbital in the next energy level. This means there is an extra shielding effect of filled inner 1s, 2s and 2p energy levels. 1s SODIUM 1s 2s 2p 1s 2s 2p 1s 2s 2p 1st IONISATION ENERGY / kJmol-1 1s 1s 2s 2p 1s 2s 2p 1s 2s 1s 2s 2p 1s 2s 1s 2s 2p 3s ATOMIC NUMBER 11

1.6 IONISATION ENERGY EXPLANATION The value for Magnesium is higher than for Sodium due to the increased nuclear charge. There is no extra shielding. The trend is similar to that at the start of the 2nd period. 1s MAGNESIUM 1s 2s 2p 1s 2s 2p 1s 2s 2p 1st IONISATION ENERGY / kJmol-1 1s 1s 2s 2p 1s 2s 2p 1s 2s 1s 2s 2p 3s 1s 2s 2p 1s 2s 1s 2s 2p 3s ATOMIC NUMBER 12

1.6 IONISATION ENERGY TREND IN PERIOD 3 Draw a graph for the first ionisation energy of elements in period 3 For each point draw the box and arrow electron configuration For each point explain why the first ionisation energy either increases or decreases Na Mg Al Si P S Cl Ar 496 738 578 789 1012 1000 1251 1521

1.6 IONISATION ENERGY TREND IN PERIOD 3 Drop from Mg  Al: Mg: 1s2 2s2 2p6 3s2 Al: 1s2 2s2 2p6 3s2 3p1 Drop from P  S: P: 1s2 2s2 2p6 3s2 3p3 S: 1s2 2s2 2p6 3s2 3p4 The outer electron in Al has moved into the 3p orbital. It takes less energy to remove an electron from here than the 3s In P each of the 3p orbitals has 1 electron. In S one must have 2. The repulsion between these 2 paired electrons makes it easier to remove one

1.6 IONISATION ENERGY TREND DOWN GROUPS General trend is a decrease as we go down a group The outer electron is in a level that gets further from the nucleus Nuclear charge increases however this is more than cancelled out by the electron shielding from lower energy shells

1.6 IONISATION ENERGY EXTENSION QUES: Which has the higher value, the 1st I.E. of sodium or the 2nd I.E. of magnesium? A: The 2nd I.E. of magnesium The 1st I.E. of sodium involves the following change Na(g) Na+(g) 1s2 2s2 2p6 3s1 1s2 2s2 2p6 The 2nd I.E. of magnesium involves the same change in electron configuration… Mg+(g) Mg2+(g) 1s2 2s2 2p6 3s1 1s2 2s2 2p6 However, magnesium has 12 protons in its nucleus, whereas sodium only has 11. The greater nuclear charge means that the electron being removed is held more strongly and more energy must be put in to remove it.

1.6 IONISATION ENERGY HOMEWORK: Revise for an end of chapter test next lesson There are revision materials on the VLE  Exam questions and mark scheme

1.6 IONISATION ENERGY I CAN… I AM… State what is meant by ionisation energy C Describe the trend across period 3 and down group 2 B Explain why this trend occurs A Suggest how these trends provide evidence for the existence of electron levels and sub-levels A* How low can you go?? Write what you can do and what grade this is  show some proof you can do this!