Total number of elements which do not form hydrides are Mo, Ca, Fe, Pd, Co, Ru, W, Cr

Answer

To determine the total number of elements from the given list that do not form hydrides, we will follow these steps:

### Step 1: Identify the Elements and Their Group Numbers
The elements provided are:
- Molybdenum (Mo)
- Calcium (Ca)
- Iron (Fe)
- Palladium (Pd)
- Cobalt (Co)
- Ruthenium (Ru)
- Tungsten (W)
- Chromium (Cr)

Now, let's assign the group numbers to each element:
- Molybdenum (Mo) - Group 6
- Calcium (Ca) - Group 2
- Iron (Fe) - Group 8
- Palladium (Pd) - Group 10
- Cobalt (Co) - Group 9
- Ruthenium (Ru) - Group 8
- Tungsten (W) - Group 6
- Chromium (Cr) - Group 6

### Step 2: Understand the Hydride Gap
The hydride gap refers to the observation that elements in groups 7, 8, and 9 typically do not form hydrides. Therefore, we need to identify which of the listed elements fall into these groups.

### Step 3: Identify Elements That Do Not Form Hydrides
From the group numbers:
- **Group 8**: Iron (Fe), Ruthenium (Ru)
- **Group 9**: Cobalt (Co)
- **Group 10**: Palladium (Pd)

These elements do not form hydrides.

### Step 4: Check Group 6 Elements
For Group 6, we have:
- Molybdenum (Mo)
- Tungsten (W)
- Chromium (Cr)

Among these, it is known that Chromium (Cr) can form hydrides, while Molybdenum (Mo) and Tungsten (W) do not.

### Step 5: Compile the List of Elements Not Forming Hydrides
From our analysis, the elements that do not form hydrides are:
1. Molybdenum (Mo)
2. Tungsten (W)
3. Iron (Fe)
4. Cobalt (Co)
5. Ruthenium (Ru)
6. Palladium (Pd)

### Conclusion
Thus, the total number of elements from the provided list that do not form hydrides is **6**.

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