Ionisation Energy
First Ionisation Energy Trends
The enthalpy change when one electron is removed from each atom in a mole of gaseous atoms, producing one mole of gaseous ions.
Br(g) → Br+(g) + e–
The nuclear charge increases but as both Li and Be’s highest energy electrons are in the 2s orbital, they experience the same amount of shielding.
Although the nuclear charge increases from Be to B, their highest energy electrons are in different shells. Boron’s highest energy electrons are in the 2p subshell which is higher energy than the Beryllium’s highest energy electrons, which are in the 2s subshell.
Although nuclear charge increases going from nitrogen to oxygen, the highest energy electron in oxygen is in an orbital that has two electrons in it. These electrons experience repulsion, which reduces their ionisation energy.
The amount of shielding stays the same due to the atoms all having the same number of shells. The nuclear charge increases as you go across the period, resulting in a stronger electrostatic force of attraction between the nucleus and the outer shell electrons. This causes the radius to decrease.
Nitrogen has the highest first ionisation energy. It has more protons than Lithium but doesn’t experience repulsion like oxygen.
Helium. It has only one shell whereas Boron and Beryllium have two.
Argon. It has more protons than magnesium and phosphorous.
Magnesium. Calcium has more shells than Mg and Al, so Calcium has the lowest. Al has one extra electron, which is found in the 2p subshell, which is higher energy than the 2s electrons found in Mg.
Second Ionisation Energy Trends
- Na+: 1s22s2 2p6 (Same as Ne)
- Mg+: 1s22s22p63s1 (Same as Na)
- Cl2+: 1s22s22p63s23p3 (Same as P)
- O2+: 1s22s22p2 (Same as C)
Na+(g) → Na2+(g) + e–
You are removing an electron from a positively charged ion rather than an atom. As the attraction between the outer shell electron and the nucleus is strong in an ion, the second ionisation energy is more endothermic.
Another way of viewing this is that the other 3s electron partially shields the electron when it is an atom. Once that electron has been removed, the amount of shielding is decreased.
Li+ has an electron configuration of 1s2 whereas Be+ has an electron configuration of 1s22s1. There is an extra shell of electrons in Be+ so the distance between the nucleus and the outer shell is much greater and there is more shielding. This means that lithium’s 2nd ionisation energy is higher than Beryllium.
Oxygen has a higher 2nd ionisation energy because it has a greater nuclear charge than nitrogen. Additionally, neither the O– ion nor the N– ion experience any repulsion due to paired electrons unlike the oxygen atoms, so the only factor is the nuclear charge.
Neon has a higher second ionisation energy than fluorine. This is because both of their highest energy electrons are 2p electrons that experience repulsion. The only factor is the nuclear charge, which is higher in the case of Ne.
Lithium. It has the smallest number of shells.
Lithium. It has the smallest number of shells.
Lithium. It has fewer shells than sodium but the same number as helium. It has more protons than helium.
Mix and Match Ionisation Energy Trends
Cl2+(g) → Cl3+(g) + e–
Second ionisation energy of magnesium. Both have the same electron configuration so the only factor is nuclear charge. Mg has more protons, so 2nd ionisation energy of Mg is higher than 1st ionisation energy of Na.
Second ionisation energy of sodium. Both have the same electron configuration so the only factor is nuclear charge. Na has more protons, so 2nd ionisation energy of Na is higher than 1st ionisation energy of Ne.
Third ionisation energy of oxygen. Both have the same electron configuration so the only factor is nuclear charge. O has more protons, so 3rd ionisation energy of O is higher than 1st ionisation energy of C.