Study notes for Unit 1 Topic 2 - Ionisation Energy for Chemistry Edexcel A Level 2015. Focusing on ionisation energy, the various factors which affect it and any trends seen, as well as the proof for shell structure.
Ionisation:
● Ionisation is the removal of one or more electrons.
● First ionisation energy = the energy needed to remove 1 electron from each
atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.
● X (g) → X+(g) + e-
● Energy is put in to ionise an atom so it's an endothermic reaction.
● A high ionisation energy means there’s a strong attraction between the electron and
the nucleus, so more energy is needed to overcome the attraction and remove the
electron.
Factors affecting ionisation energy:
● Nuclear charge = the more protons in the nucleus, the more positively charged it is
and therefore the stronger the attraction for the electrons.
● Electron shell = an electron in a shell closer to the nucleus will be much more
strongly attracted to the nucleus than one in a shell further away.
● Shielding = if the number of electrons between the outer electrons and the nucleus
increases, the outer electrons feel less attraction towards the nuclear charge. There
is less of a pull towards the nucleus as the outer electrons are shielded.
First ionisation energy trend:
● As you go down a group, ionisation energies decrease because it gets easier to
remove outer electrons.
● Elements further down a group have extra electron shells - the atomic radius is larger
- outer electrons are further away from the nucleus - less attraction.
● Extra inner shells shield the outer electrons from the attraction of the nucleus.
● Whilst the positive charge of the nucleus is increasing (more protons) this effect is
overridden by the effect of the extra shells.
Second ionisation energy:
● Each time you remove an electron, there’s a successive ionisation energy. You can
remove all the electrons from an atom until only the nucleus is left.
● Second ionisation energy = the energy needed to remove 1 electron from each ion in
1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions.
● Second ionisation energy, X+ (g) → X2+(g) + e-
● nth ionisation energy, X(n-1)+ (g) → Xn+(g) + e-
Proof of shell structure:
● A graph of successful ionisation energies provide evidence for the shell structure of
atoms.
● Within each shell, successful ionisation energies increase, because electrons are
being removed from an increasingly positive ion (it’s increasingly positive because
there’s less repulsion between the remaining electrons so they’re held more tightly).
● The big jumps in ionisation energy happen when a new shell is broken into.
● Each shell closer to the nucleus requires more energy.
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