Let's look at the Periodic Table of the Elements again. Now let's look at some terms and then we will look at more general properties. You should know the terminology above.
Size goes up with atomic number for any individual group.
Ionization energy goes down with atomic number for any individual group. Electrons are held in shells . The first shell holds only 2 electrons in what is called the 1s orbital . Thus helium has its first shell filled. There is no more room for electrons, so it can't react by picking up another electron.