Question

What element has a mass number of 138 and protons equal to 57?

Answer 1

Lanthanum  is your answer i believe.
~ thank you for letting me answer your question here on brainly! much love from england xx

Answer 2

Basic Information
Name: Lanthanum 
Symbol: La 
Atomic Number: 57 
Atomic Mass: 138.9055 amu 
Melting Point: 920.0 °C (1193.15 K, 1688.0 °F) 
Boiling Point: 3469.0 °C (3742.15 K, 6276.2 °F) 
Number of Protons/Electrons: 57 
Number of Neutrons: 82 
Classification: Rare Earth 
Crystal Structure: Hexagonal 
Density @ 293 K: 6.7 g/cm3 
Color: white