When a proton is lost by an atom, the atomic number decreases by 1.
The atomic number (Z) represents the number of protons in the nucleus of an atom. Each element has a unique atomic number, which determines its identity. By losing a proton, the atom no longer has the same number of protons as before, resulting in a decrease in the atomic number.
For example, if an atom with an atomic number of 7 (nitrogen) loses a proton, its atomic number becomes 6 (carbon). This change in the number of protons affects the element's properties and its position in the periodic table.