Actinium (Ac) has no stable isotopes, thus a standard atomic mass cannot be given.

The standard adult mass is 16.5 grams.

A standard atomic mass cannot be given.

As any artificial element, a standard atomic mass cannot be given.

There are three naturally occurring isotopes, allowing a standard mass to be given.

It has a standard atomic mass of 87.62(1) u.

Since promethium does not exist in nature, a standard atomic mass cannot be given.

The standard atomic mass for iodine is 126.90447(3) u.

The standard masses are often referred to, non-technically, as "weights".

Nobelium (No) is an artificial element, and thus a standard atomic mass cannot be given.