Which of these substances is a compound 1.carbon

2. chlorine

3. uranium

4. ammonia

The answer to that would be magnesium

The answer is  d.  hydrogen 

So the main goal for the elements is stability which is a complete/full valence shell and typically the magic number for a stable element is 8 valence electron but not always. so elements to the left of the periodic table tend to be way less stable then those on the right, so the elements on the left are much more reactive.