Which out of the following belong to 3d-series?

Options

A. Copper
B. Cobalt
C. Gold
D. Silver

Answer

To determine which elements belong to the 3d series, we need to identify the elements from the provided options and check their positions in the periodic table. The 3d series consists of the transition metals that fill the 3d subshell. The elements in the 3d series are:

1. Scandium (Sc)
2. Titanium (Ti)
3. Vanadium (V)
4. Chromium (Cr)
5. Manganese (Mn)
6. Iron (Fe)
7. Cobalt (Co)
8. Nickel (Ni)
9. Copper (Cu)
10. Zinc (Zn)

Now, let's analyze the options provided:

1. **Copper (Cu)**: Copper is in the 3d series. Its electronic configuration is [Ar] 4s¹ 3d¹⁰.

2. **Cobalt (Co)**: Cobalt is also in the 3d series. Its electronic configuration is [Ar] 4s² 3d⁷.

3. **Gold (Au)**: Gold is not in the 3d series; it belongs to the 5d series. Its electronic configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s¹.

4. **Silver (Ag)**: Silver is not in the 3d series; it belongs to the 4d series. Its electronic configuration is [Kr] 4d¹⁰ 5s¹.

From this analysis, we can conclude that the elements that belong to the 3d series from the given options are **Copper (Cu)** and **Cobalt (Co)**.

### Final Answer:
Copper (Cu) and Cobalt (Co) belong to the 3d series.