how do you say 0.00000005 in word form?

Answer 1

not five tenths
not five hundredths
not 5 thousandths
not 5 tenthousandths
not 5 hundredthousandths
not 5 millionths
not 5 tenmillionths
BUT
Five hundredmillionths

Answer 2

.....or maybe just say five times ten to the minus eight :)