[nagisa.xx]

Ok, I have spent ages looking this up to no avail. I have absolutely no idea what the answer could be and I am seriously confused :S

The first part is:
The mineral galena that was used to the crystal in the first radios is formed when the group 4 element lead combines with the group 6 element sulphur.

then the question is:
Name three other pairs of elements that you would choose to investigate as alternative combinations to lead and sulphur.

Could someone please explain this to me? because I don't really understand how you are meant to find alternatives if you haven't really gone into much depth about each element? Do you just take any random ones from the same groups?

also another question I am having trouble with is:
Name two other pairs of elements that you would choose to investigate as alternatives to LED semiconductors.

Like the first question, I don't really get what they mean by 'alternatives'?

If you have a vague idea please *delete me*