Atomic Structure
Ionisation Energy
OxfordAQA International AS and A-level Chemistry
First ionisation energy and its equation
First ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms, forming one mole of gaseous 1+ ions.
- The electron is written e−.
- The nth ionisation energy starts from the ion that has already lost n−1 electrons, and takes one more electron off it.
- Write the equation for a single atom or ion. Never write it doubled, as 2Mg+(g) → 2Mg2+(g) + 2e−.
Successive ionisation energies
The second ionisation energy is the energy needed to remove one electron from each ion in one mole of gaseous 1+ ions, forming one mole of gaseous 2+ ions, and every later one follows the same pattern.
Why the second is greater than the first
More energy is required to remove an electron from a positively charged ion than from an atom.
Never write the second electron feels different shielding from the first. The first two electrons leave the same shell as each other, so the shielding has not changed between them. What has changed is that the second electron is being pulled off an ion rather than an atom, and that there are now more protons than electrons.
The big jump
- Every successive ionisation energy is larger than the one before it.
- A large jump comes where the next electron has to be taken from a shell closer to the nucleus.
- The number of electrons removed before the jump is the number of electrons in the outer shell, and so the group. Count the values before the jump, then name the element or the group.
- Working the other way, to name the ionisation that produced a given ion, count how many electrons it has lost from the neutral atom's configuration: three gone is the third ionisation.
Which element has the highest second or third ionisation energy
The nth ionisation energy is highest for the element whose ion has just reached a noble gas arrangement after n−1 electrons have gone, because the next electron then has to come from the shell inside.
| Highest value | Element |
|---|---|
| Second ionisation energy in Period 3 | sodium |
| Third ionisation energy in Period 3 | magnesium |
| Third ionisation energy from lithium to nitrogen | beryllium |
Sketching a successive ionisation energy graph
Plot ionisation energy against the number of electrons removed. The sketch rises at every step, and it has a sharp increase wherever a shell empties. Sodium is 1s2 2s2 2p6 3s1, so its outer shell empties after one electron and its second shell empties after nine.
On a plot of second ionisation energy across Period 3, sodium sits far above every other element, and sulfur sits above phosphorus.
Comparing the same ionisation energy in two elements
A comparison names where the electron is removed from in each element, and why that makes it easier or harder to remove. Which reason to give depends on whether the two electrons come from the same shell or from different ones.
| Comparison | The two electrons come from | Where it is removed from | Why |
|---|---|---|---|
| Second ionisation energies of silicon and aluminium | Two sub-shells of the same shell | the electron is removed from a 3p orbital in Si and a 3s orbital in Al | the 3p orbital is higher in energy, so the electron is more easily lost |
| Second ionisation energies of sodium and magnesium | Two different shells | the electron is removed from the 2p sub-shell | the electron removed is less shielded than a 3s electron, or closer to the nucleus than a 3s electron |
Either name the sub-shell in each of the two elements, or end the reason with a comparison against the other. Where one element sits below the other, the electron that leaves it comes from an atom with more shells, so the electron removed is further from the nucleus and there is more shielding.
Every species named in a comparison like this is an atom or an ion. Never call one a molecule.
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