The reason Argon is the last element in its period is that it has the highest atomic number in its period, and that's the way all periods are arranged.
the element of argon:Atomic Number: 18Atomic Weight: 39.948Melting Point: 83.80 K (-189.35°C or -308.83°F)Boiling Point: 87.30 K (-185.85°C or -302.53°F)Density: 0.0017837 grams per cubic...