Carbon-12 is the most common isotope of carbon and has a mass number of 12. however, the average atomic mass of carbon found on a periodic table is slightly more than 12 daltons. why?

Carbon has 3 isotopes in nature: carbon-12, carbon-13 and carbon-14.

The average atomic mass of carbon, which is the one found in the periodic table, can be calculated as follows:
(average mass of 12C x its percentage of abundance) + 
(average mass of 13C x its percentage of abundance) + 
(average mass of 14C x its percentage of abundance) 

Since carbon-12 is the most abundant one, therefore, the average mass is closest to 12, however, other isotopes (carbons having more than 12 neutrons)  are also taken into consideration.

Acovalent bond is formed between the two hydrogen bonds