What is the next number in this sequence?

Can you determine the next number in the below sequence?

$1.0 , 4.0 , 6.9 , 9.0 , 10.8 , 12.0 , 14.0 , 16.0 , ?$

Bonus: Determine the next number that comes after '?'

Note: I'll post some hints if nobody gets the answer.

Hint #1: You won't get the relationship between the numbers by calculating.

Hint #2: These numbers have some relationship with science.

• Too broad; too many answers. Voted to close. Commented Jan 3, 2015 at 14:27
• The problem with this is that it's just a sequence. We could throw guesses at the wall all day and by the end be no closer to a solution. This type of puzzle is generally disliked on puzzling.SE, and moreover disliked as a whole. Commented Jan 3, 2015 at 14:48
• From seeing the answer: This is a puzzle where you either recognize the pattern or you don't. You can't get anywhere by thinking by about it.
– xnor
Commented Jan 3, 2015 at 16:32
• @xnor And it won't even be on OEIS (I think) because it isn't an integer sequence... Commented Jan 3, 2015 at 17:26
• @RobWatts Never mind. oeis.org/… but you'd have to round Commented Jan 3, 2015 at 21:49

The numbers represent

Atomic weights

The next 5 numbers are

19.0, 20.2, 23.0, 24.3, and 27.0