On Earth, there is no naturally occurring element with an atomic number of zero or closest to zero. The atomic number of an element corresponds to the number of protons in its nucleus, and protons are essential for defining an element.
However, it's worth mentioning that there is an element called neutronium, which hypothetically consists only of neutrons with no protons. Neutronium is not found naturally on Earth but is believed to exist in extreme conditions such as neutron stars. Neutron stars are incredibly dense celestial objects composed mostly of tightly packed neutrons.
In a neutron star, the pressure is so high that electrons combine with protons, resulting in a dense "soup" of neutrons. Although neutronium is not considered an element in the traditional sense, it is sometimes mentioned in the context of exotic matter or hypothetical scenarios.
In summary, while there is no naturally occurring element on Earth with an atomic number of zero or closest to zero, the hypothetical concept of neutronium is associated with extreme conditions such as those found in neutron stars.