ICSE Class 10 Answered

Fluorine has highest electronegativity as it has the largest nuclear charge and the least atomic shell, so it has the largest nuclear pull force.

Chlorine has highest electron affinity - Fluorine is a small atom with a small amount of space available in its 2p orbital. Therefore, any new electron trying to attach to fluorine experiences lower electron affinity from the electrons already living in the element's 2p orbital. Since chlorine's outermost orbital is  3p orbital, so there is more space. The electrons in this orbital are inclined to share this space with an extra electron. So, chlorine has a higher electron affinity than fluorine, and this orbital structure causes it to have the highest electron affinity of all of the elements.

Francium has lowest electronegativity and lowest electron affinity.

Helium has highest ionisation potential - 

Helium has 2 electrons in the 1s orbital. These electrons are as close to the nucleus as possible. The nucleus has a positive charge, pulling these negative electrons very tightly. It takes a lot of energy to break this attraction. Having 2 electrons in the 1s orbital also means that it is very stable. Other elements with full outer shells or with electrons in other orbitals having a smaller first ionisation energy than Helium. This is because the electrons in previous orbitals shield the valence (outer) electrons, so the charge of the nucleus isn't felt as much. Hence, it is easier to break the attraction.

Caesium has lowest ionisation potential.