What differentiates galaxy groups from clusters?

Clusters are larger than groups, although there is no sharp dividing line between the two. When observed visually, clusters appear to be collections of galaxies held together by mutual gravitational attraction.

Explanation:

HOPE THIS HELPED!...~DASH

As atomic number increases atomic radii also increase down group 1. ionisation energy down group 1 will also decrease because as atomic radii gets bigger there is less electrostatic force between nuclei and electrons so less energy needed to remove valence electron.