The first ionisation energy  from top to bottom in group 1 of the periodic table.

Understanding Ionization Energy

1. Definition: Ionization energy is the amount of energy required to remove the most loosely held electron from an atom in its gaseous state.

2. Trends in Group 1: As we move down Group 1 (alkali metals) of the periodic table (from lithium to cesium), the first ionization energy generally decreases.

Factors Affecting Ionization Energy

3. Atomic Size: The atomic radius increases down the group because additional electron shells are added. This means that the outermost electron is further away from the nucleus, making it less tightly bound.

4. Shielding Effect: With the addition of more inner electron shells, the shielding effect increases. Core electrons repel outer electrons, which decreases the effective nuclear charge felt by the outermost electron.

Summary of Trends

5. Conclusion: Thus, the increased atomic radius and greater shielding effect down the group leads to lower first ionization energies for alkali metals, making it easier to remove the outermost electron as we move from lithium (higher ionization energy) to cesium (lower ionization energy).

In summary, the trend of ionization energy in Group 1 elements decreases from lithium to cesium due to increasing atomic size and shielding effect.