Is ammonium ion a bronsted base

Unlikely. It's unlikely for ammonium ion to accept a proton and act as a Bronsted-Lowry Acid.

What's the definition of Bronsted-Lowry acids and bases?

Ammonium ions are positive. Protons are also positive.

Positive charges repel each other, which means that it will be difficult for to accept any additional protons. As a result, it's unlikely that will accept any proton and act like a Bronsted-Lowry Base.

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.

pie chart because it is in percents ( % )