Periodic Classification Of Elements

To determine which element is found in the free state in nature, we must consider the reactivity and occurrence of each given element:

• Zinc (Zn): Zinc is quite reactive and is typically found in nature as compounds, not in its elemental form. It usually exists as sulfides or carbonates.

• Gold (Au): Gold is a noble metal, known for its low reactivity, which allows it to be found in nature in its elemental or free state. It is typically found as nuggets or dust in alluvial deposits.

• Iron (Fe): Iron is prone to oxidation and is almost always found in ores like hematite or magnetite, not in its free state.

• Sodium (Na): Sodium is highly reactive, especially with water, and is never found in its elemental form in nature. It is primarily found as compounds, such as halite (sodium chloride).

Conclusion: Among the given options, Gold is the element that naturally occurs in the free state.

Therefore, the correct option is: Gold.

To determine the total number of rare earth metals, we need to understand their classification in the periodic table.

1. Definition of Rare Earth Metals:
Rare earth metals are a set of 17 chemically similar metallic elements, consisting of:
- The 15 lanthanides (atomic numbers 57-71)
- Scandium (Sc, atomic number 21)
- Yttrium (Y, atomic number 39)

2. Breakdown of the Elements:
a) Lanthanides (15 elements):
- La (57), Ce (58), Pr (59), Nd (60), Pm (61), Sm (62), Eu (63)
- Gd (64), Tb (65), Dy (66), Ho (67), Er (68), Tm (69), Yb (70), Lu (71)
b) Other rare earths (2 elements):
- Scandium (Sc, 21)
- Yttrium (Y, 39)

3. Why These Elements Are Grouped Together:
- Similar chemical properties
- Often occur together in mineral deposits
- Comparable ionic radii
- Similar extraction and separation challenges

4. Common Misconceptions:
- Despite the name "rare", most are relatively abundant in Earth's crust (except Pm)
- They're not all equally rare - cerium is more abundant than lead
- Technically not all are "earth" metals in the traditional sense

Final Answer:
The total number of rare earth metals is 17 (15 lanthanides + scandium + yttrium).