Is 1.234 is a not rational number?

First of all , do not use the phrase ' not rational' . The Word in the English language, and mathematics is 'IRRATIONAL'.

Net 1.234 is rational; it can be converted to a ratio(fraction).

For Irrational Numbers the decimals go to inifinity and there is no regular order in the decimal digits.

'pi' is probably the most well known irrational numbers at pi = 3.141592654...

The three terminl dots/stops indicate to mathemticians, that the number goes to infinity. Notice also, there is no regular order of digits in the decimal moiety.

Since the number given ends at '4' , then it does not go to infinity, hernce it is rational .

To convert to a ratio (fraction)

1.234 = 1 234/1000

Cancel down by '2'

1 117/500 (THis is the fraction in its lowest terms).