Answer:
period 6
Explanation:
If the lanthanides were inserted into the periodic table, they would go into periodic 6.
Their atomic number is between 57 - 71 from element lanthanum to lutetium.
The elements that makes up the series are:
Lanthanum
Cerium
Praseodymium
Neodymium
Promethium
Samarium
Europium
Gadolinium
Terbium
Dysprosium
Holmium
Erbium
Thulium
Ytterbium
Lutetium