Question

when atoms of different elements combine what do they form

Answer 1

When atoms of different elements combine together in a fixed proposition then they form compound. example Hydrogen and oxygen combine to form water.

Answer 2

Compounds are formed when atoms of different elements combine in simple whole number ratio.
Example:
Carbon Monoxide (CO), Carbon dioxide (CO2) hence carbon and oxygen Combine in the ratio 1: 1 and  1: 2 ratios respectively to give two different compounds.