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

The number of elements in the first transition series that have an oxidation number equal to their group number is 5.

The first transition series elements and their relevant properties are:

1. Scandium (Sc): Group 3, maximum oxidation state +3. (+3 = Group 3)
2. Titanium (Ti): Group 4, maximum oxidation state +4. (+4 = Group 4)
3. Vanadium (V): Group 5, maximum oxidation state +5. (+5 = Group 5)
4. Chromium (Cr): Group 6, maximum oxidation state +6. (+6 = Group 6)
5. Manganese (Mn): Group 7, maximum oxidation state +7. (+7 = Group 7)
6. Iron (Fe): Group 8, common oxidation states +2, +3. Maximum observed is +6 (e.g., in ferrates), but not +8.
7. Cobalt (Co): Group 9, common oxidation states +2, +3. Maximum observed is +4, but not +9.
8. Nickel (Ni): Group 10, common oxidation states +2, +3. Maximum observed is +4, but not +10.
9. Copper (Cu): Group 11, common oxidation states +1, +2. Not +11.
10. Zinc (Zn): Group 12, common oxidation state +2. Not +12.

Thus, Scandium, Titanium, Vanadium, Chromium, and Manganese are the five elements in the first transition series that exhibit an oxidation number equal to their group number.