What percentage of the earth's atmosphere is hydrogen?
A fraction of a percent.
The top 3 gases of the dry atmosphere are Nitrogen, Oxygen and Argon. Together they make up 99.96% of the atmosphere. All the remaining gases make up the remaining 0.04% of the atmosphere and Hydrogen is not even in the top 10. According to Wikipedia hydrogen makes up 0.000055% of the atmosphere but I wouldn't assume that it is all that accurate as it is such a small amount.