How to remember atomic masses?

 You need to remember about 1 to 30 atomic no. elements

The elements with odd atomic number have to be multiplied with two and add 1 to it.

The elements with even atomic number have to be multiplied by 2.

However there are some exceptions. These exception are Hydrogen, Beryllium, Nitrogen, Chlorine and Argon.

We know that hydrogen have no neutron so its atomic mass have to be 1.

Beryllium is having an even atomic number but follows the rule of odd atomic numbers.

Nitrogen it is having an odd atomic number but follows the role of even atomic number.

Chlorine follows the normal role of on atomic number but 0.5 is added to the answer.

Argon also follows the even atomic number rule but 4 is added to the answer.