The number of elements in the first transition series that have oxidation number equals to its group number is ...... A. 4 B. 5 C. 7 D. 6 solve this

The first transition series (3d series) includes elements from Scandium (Sc) to Zinc (Zn). Their group numbers are:

• Scandium (Sc): Group 3
• Titanium (Ti): Group 4
• Vanadium (V): Group 5
• Chromium (Cr): Group 6
• Manganese (Mn): Group 7
• Iron (Fe): Group 8
• Cobalt (Co): Group 9
• Nickel (Ni): Group 10
• Copper (Cu): Group 11
• Zinc (Zn): Group 12

The maximum oxidation state for transition metals generally corresponds to the sum of their 4s and 3d electrons, up to Group 7.

• Scandium (Sc): Group 3. Electronic configuration: $[\text{Ar}] 3d^1 4s^2$. Maximum oxidation state: +3. (Matches group number)
• Titanium (Ti): Group 4. Electronic configuration: $[\text{Ar}] 3d^2 4s^2$. Maximum oxidation state: +4. (Matches group number)
• Vanadium (V): Group 5. Electronic configuration: $[\text{Ar}] 3d^3 4s^2$. Maximum oxidation state: +5. (Matches group number)
• Chromium (Cr): Group 6. Electronic configuration: $[\text{Ar}] 3d^5 4s^1$. Maximum oxidation state: +6. (Matches group number)
• Manganese (Mn): Group 7. Electronic configuration: $[\text{Ar}] 3d^5 4s^2$. Maximum oxidation state: +7. (Matches group number)
• Iron (Fe): Group 8. Electronic configuration: $[\text{Ar}] 3d^6 4s^2$. Maximum common oxidation state: +2, +3. Highest observed: +6 (e.g., in ferrates, $\ce{FeO4^2-}$). Does not reach +8. (Does not match group number)
• Cobalt (Co): Group 9. Electronic configuration: $[\text{Ar}] 3d^7 4s^2$. Maximum common oxidation state: +2, +3. Highest observed: +4. Does not reach +9. (Does not match group number)
• Nickel (Ni): Group 10. Electronic configuration: $[\text{Ar}] 3d^8 4s^2$. Maximum common oxidation state: +2. Highest observed: +4. Does not reach +10. (Does not match group number)
• Copper (Cu): Group 11. Electronic configuration: $[\text{Ar}] 3d^{10} 4s^1$. Maximum common oxidation state: +1, +2. Highest observed: +3. Does not reach +11. (Does not match group number)
• Zinc (Zn): Group 12. Electronic configuration: $[\text{Ar}] 3d^{10} 4s^2$. Maximum oxidation state: +2. Does not reach +12. (Does not match group number)

The elements that have an oxidation number equal to their group number are Scandium, Titanium, Vanadium, Chromium, and Manganese. There are 5 such elements.

The final answer is $\boxed{\text{5}}$.