To determine which statements about transition metals are true, let's analyze each statement one by one.
### Step 1: Analyze Statement 1
**Statement:** Transition metals form alloys.
**Analysis:**
Transition metals have similar atomic sizes, which allows them to replace each other in a lattice structure. This similarity in size facilitates the formation of solid solutions, or alloys, when different transition metals are mixed. Therefore, this statement is **true**.
### Step 2: Analyze Statement 2
**Statement:** Transition metals form complexes.
**Analysis:**
Transition metals are known for their ability to form complexes due to their small size, high nuclear charge, and the presence of vacant d-orbitals that can accommodate lone pairs of electrons from ligands. This characteristic is a defining feature of transition metals. Thus, this statement is also **true**.
### Step 3: Analyze Statement 3
**Statement:** Zn, Cd, and Hg are transition metals.
**Analysis:**
Zinc (Zn), Cadmium (Cd), and Mercury (Hg) have completely filled d-orbitals in their ground state and oxidation states. Since transition metals are defined as those that have partially filled d-orbitals, these elements do not qualify as transition metals. Therefore, this statement is **false**.
### Step 4: Analyze Statement 4
**Statement:** K₂[PtCl₆] is a well-known compound, but the corresponding nickel compound is not known.
**Analysis:**
Potassium hexachloroplatinate (K₂[PtCl₆]) is a stable compound because platinum can accommodate a coordination number of 6 due to its larger size and vacant orbitals. In contrast, nickel cannot form a similar compound (K₂[NiCl₆]) due to its smaller size, which limits its ability to coordinate with six ligands effectively. Therefore, this statement is **true**.
### Conclusion
Based on the analysis:
- Statement 1: True
- Statement 2: True
- Statement 3: False
- Statement 4: True
The true statements are 1, 2, and 4. Therefore, the correct answer is option 3 (1, 2, and 4).
### Summary of True Statements:
- **True Statements:** 1, 2, and 4
- **False Statement:** 3
