Question

1-what is the 1 Atom unit ?

Answer 1

One atomic unit is equal to the mass of a nucleon ( proton or neutron )
It is numerically equal to
1g /mol
symbol 'u' is used to represent it

Answer 2

An atomic mass unit (symbolized AMU  is defined as precisely 1/12 the mass of an atom of carbon-12. The carbon-12 (C-12) atom has six protons and six neutrons in its nucleus. In imprecise terms, one AMU is the average of the proton rest mass and the neutron rest mass.