M3 Ionisation Energy Q&A
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State and explain the trend in first ionisation energies down a group.
• First ionisation energy decreases • Outer electron is in different shell further from nucleus so atomic radius increases • Outer electron experiences more shielding • (Increased shielding outweighs increased nuclear charge) • Outer electron is less strongly attracted to the nucleus so less energy is needed to remove the electron
State and explain the trend in first ionisation energies across a period.
• First ionisation energy increases • Number of protons increases, so nuclear charge increases • Outer electrons in the same shell so experience the same shielding • Atomic radius decreases • Outer electron is more strongly attracted to nucleus so more energy is needed to remove the electron
Explain the decrease in first ionisation energy between group 2 and 3 (e.g. Be –>B, Mg –> Al)
• Electron in (e.g.) Boron is removed from (2)p sub-shell and electron in (e.g.) Beryllium is removed from (2)s sub-shell • (2)p sub-shell is higher in energy than the (2)s sub-shell • Less energy is needed to remove the (2)p electron
Explain the decrease in first ionisation energy between group 5 and 6 (e.g. N –> O, P —> S)
• In (e.g.) Nitrogen, each p orbital contains 1 electron; in (e.g.) Oxygen, one p orbital contains two paired electrons • Paired electrons repel each other, so less energy is needed to remove the electron from (e.g.) Oxygen
Explain the increase in successive ionisation energies, for example, shown by the element D below:
• Once an electron has been removed, there are the same number of protons but fewer electrons • Proton : electron ratio increases • Remaining electrons are being removed from a positive ion • Remaining electrons are more strongly attracted to the nucleus • More energy is needed to remove each electron from the positive ion
Use the ionisation energies of element D below, to deduce which group it
• Largest increase is between 4th and 5th ionisation energies • 5th electron is in new shell closer to the nucleus, experiencing less shielding and more attraction to the nucleus • 4 electrons in the outer shell – element is in group 4