I guess they mean atomic number, number of protons in nucleus.
Elements are arranged in terms of increasing atomic number in the periodic table. The atomic number of an element represents the number of protons in the nucleus of an atom of that element. The periodic table is organized in such a way that elements with similar properties are placed in the same column, called a group, and elements with different properties are placed in different groups. Moving from left to right across a period (horizontal row), the atomic number increases, indicating the addition of more protons and electrons. Therefore, the periodic table's arrangement is based on the increasing atomic number.
