Ionisation Energy & Electronic Configuration

First Ionisation Energy ($IE_1$) is defined as the energy required to remove one mole of electrons from one mole of gaseous atoms to provide one mole of gaseous $1+$ ions. This process is represented by the general equation: $X(g) \rightarrow X^+(g) + e^-$.

The state symbol (g) is critical because ionisation energy must be measured when atoms are in the gaseous state to ensure that no energy is used to overcome intermolecular forces, focusing solely on the attraction between the nucleus and the electron.

Successive Ionisation Energies refer to the energy required to remove the second, third, and subsequent electrons from the same atom (e.g., $X^+(g) \rightarrow X^{2+}(g) + e^-$). These values always increase because each subsequent electron is being removed from an increasingly positive ion, resulting in greater electrostatic attraction.

Across a Period

• General Trend: Ionisation energy increases across a period because the nuclear charge increases while the shielding remains relatively constant, leading to a stronger pull on the outer electrons.
• Subshell Anomaly (Group 2 to 13): A slight drop occurs because the electron in Group 13 is removed from a $p$-subshell, which is higher in energy and slightly shielded by the $s$-subshell electrons, making it easier to remove.
• Pairing Anomaly (Group 15 to 16): A drop occurs because Group 16 elements have a paired electron in a $p$-orbital. The mutual repulsion between these two electrons in the same orbital makes the first electron easier to remove than the unpaired electrons in Group 15.

Down a Group

• General Trend: Ionisation energy decreases down a group. Although nuclear charge increases, the effect is outweighed by the increase in atomic radius and the addition of new inner shielding shells, which significantly weaken the attraction to the valence electrons.

• First vs. Second IE: The second ionisation energy is always higher than the first because the electron is being removed from a positive ion, which holds its remaining electrons more tightly due to reduced electron-electron repulsion.

• Precision in Definitions: Always include 'one mole', 'gaseous atoms/ions', and 'to form one mole' in your definitions. Omitting the state symbols or the molar quantity often results in lost marks.

• Explaining Anomalies: When asked about the drop between Group 2 and 13, mention the specific subshells (e.g., $3s$ vs $3p$). For the drop between Group 15 and 16, explicitly mention 'electron pair repulsion' in the $p$-orbital.

• Successive IE Data: When identifying an element from a list of successive energies, look for the largest ratio between two consecutive values (e.g., $IE_4 / IE_3$). The number of electrons before the jump equals the group number.

• Comparison Questions: When comparing two elements, always structure your answer around three pillars: Nuclear Charge, Distance (Radius), and Shielding.