The lowest possible value for an element's atomic number is 1, which corresponds to the element hydrogen.
The atomic number represents the number of protons present in the nucleus of an atom. Since protons are fundamental particles and are required for an atom to be electrically neutral, it is not possible for an element to have an atomic number of zero.
If an atom had zero protons, it would not have any positive charges to balance the negative charges of the electrons, resulting in an unbalanced, positively charged particle. Such a particle would not exhibit the properties of an element as we understand them. Therefore, the concept of an element with an atomic number of zero is not valid within the framework of our current understanding of atomic structure.