Which lanthanide element has the lowest atomic mass?

• A. Ce (Cerium)
• B. La (Lanthanum)
• C. Nd (Neodymium)
• D. Sm (Samarium)

Answer: (B)

Verified step by step guidance

1. Understand that the lanthanide series consists of 15 elements with atomic numbers from 57 (Lanthanum) to 71 (Lutetium).
2. Recall that atomic mass generally increases as you move from left to right across the periodic table and as atomic number increases.
3. Identify the atomic numbers of the given elements: La (57), Ce (58), Nd (60), and Sm (62).
4. Since Lanthanum (La) has the lowest atomic number among the options, it will also have the lowest atomic mass.
5. Therefore, the lanthanide element with the lowest atomic mass among the choices is La (Lanthanum).