How many of the following are lanthanides?
Uranium, praseodymium, erbium, gadolinium, cerium, hafnium, osmium, iridium

Answer

To determine how many of the given elements are lanthanides, we need to identify each element and check if it belongs to the lanthanide series.

1. **List of Elements**:
- Uranium (U)
- Praseodymium (Pr)
- Erbium (Er)
- Gadolinium (Gd)
- Cerium (Ce)
- Hafnium (Hf)
- Osmium (Os)
- Iridium (Ir)

2. **Identify Lanthanides**:
- The lanthanides are a series of 15 elements from Lanthanum (La, atomic number 57) to Lutetium (Lu, atomic number 71). The lanthanides include:
- Lanthanum (La)
- Cerium (Ce)
- Praseodymium (Pr)
- Neodymium (Nd)
- Promethium (Pm)
- Samarium (Sm)
- Europium (Eu)
- Gadolinium (Gd)
- Terbium (Tb)
- Dysprosium (Dy)
- Holmium (Ho)
- Erbium (Er)
- Thulium (Tm)
- Ytterbium (Yb)
- Lutetium (Lu)

3. **Check Each Element**:
- **Uranium (U)**: Not a lanthanide (it is an actinide).
- **Praseodymium (Pr)**: Is a lanthanide.
- **Erbium (Er)**: Is a lanthanide.
- **Gadolinium (Gd)**: Is a lanthanide.
- **Cerium (Ce)**: Is a lanthanide.
- **Hafnium (Hf)**: Not a lanthanide (it is a d-block element).
- **Osmium (Os)**: Not a lanthanide (it is a d-block element).
- **Iridium (Ir)**: Not a lanthanide (it is a d-block element).

4. **Count the Lanthanides**:
- The lanthanides from the list are Praseodymium (Pr), Erbium (Er), Gadolinium (Gd), and Cerium (Ce).
- Total count of lanthanides = 4.

**Final Answer**: There are **4 lanthanides** in the given list.

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