Lanthanum is element number 57, with the symbol La. Its name comes from the Greek for “to lie hidden.” It is the first of the lanthanoids.
Atomic number 57138.9
LaLanthanumランタン (rantan)
Symbol
La
Atomic weight
138.9
Place in the table
Period 6 · Lanthanoids
Atomic weight from the Chemical Society of Japan, “Table of Standard Atomic Weights with Four Significant Figures (2026)”.
The “hidden” element
Lanthanum was discovered in 1839. It was named because something that had been hidden was “finally found.”
Where it sits on the table
The fifteen elements from lanthanum to lutetium are called the lanthanoids. On the Room of the Elements table they are in the upper of the two rows set apart at the bottom.
The lanthanoids take their name from this first element, lanthanum.
Think about it
Q1What would it take to find a “hidden” element?
Q2Count the fifteen lanthanoids on the Room of the Elements table.
Q3Krypton (“hidden”) and lanthanum (“lying hidden”) have similar meanings. What do you think of that?
There is no single right answer. Talking it over with someone at home is a good way to learn.
Did you know? — Two element names are about hiding: lanthanum and krypton. Both were hard to find.