Which of the following element(s) was/were not known when Mendeleev gave his classification?

Options

A. Ge
B. Ga
C. Sc
D. All of these

Explanation

To solve the question "Which of the following element(s) was/were not known when Mendeleev gave his classification?", we can follow these steps:

### Step 1: Understand Mendeleev's Periodic Table
Mendeleev created his periodic table in 1869. At that time, not all elements that we know today had been discovered.

### Step 2: Identify the Elements in Question
The question asks us to identify which elements were unknown during Mendeleev's time. The elements mentioned in the video transcript are:
- Germanium (Ge)
- Gallium (Ga)
- Scandium (Sc)

### Step 3: Research the Discovery Dates of the Elements
- **Germanium (Ge)**: Discovered in 1886 by Clemens Winkler.
- **Gallium (Ga)**: Discovered in 1875 by Paul Emile Lecoq de Boisbaudran.
- **Scandium (Sc)**: Discovered in 1879 by Lars Fredrik Nilson.

### Step 4: Determine Which Elements Were Unknown
From the discovery dates:
- Germanium was discovered after Mendeleev's classification, so it was unknown to him.
- Gallium was discovered shortly after Mendeleev's classification, but it was known to him as he predicted its existence.
- Scandium was also discovered after Mendeleev's classification, so it was unknown to him.

### Step 5: Conclusion
The elements that were not known to Mendeleev when he created his periodic table are Germanium and Scandium. Gallium was known to him, as he predicted its existence.

### Final Answer
The elements that were not known when Mendeleev gave his classification are:
- Germanium
- Scandium