The question asks which element has the maximum first ionisation potential. Ionisation potential is the energy required to remove an electron from a gaseous atom or ion. It is a measure of the attraction between the electron and the nucleus.
Several factors affect the ionisation potential of an element:
- Nuclear charge: Higher nuclear charge increases the ionisation potential as it strengthens the attraction between the positively charged nucleus and the negatively charged electron.
- Atomic size: Larger atomic size decreases the ionisation potential because the outer electrons are further away from the nucleus and are less strongly attracted.
- Electron shielding or screening: Inner shell electrons can shield outer electrons from the full effect of the nucleus, lowering the ionisation potential.
Now, let's consider the given options:
- Cesium (Cs): Being an alkali metal with a large atomic size and a relatively low effective nuclear charge, it has a low ionisation potential.
- Fluorine (F): It is highly electronegative and has a high ionisation potential, but it is not the highest.
- Sodium (Na): This is another alkali metal like Cs, and thus it has a low ionisation potential.
- Helium (He): As a noble gas with a complete outer shell, it has a very high nuclear charge-to-size ratio. This makes the ionisation energy significantly higher.
Among these elements, helium has the highest first ionisation potential due to its small size and the absence of electron shielding, making it difficult to remove an electron from its stable electron configuration.
Therefore, the correct answer is:
He