In nuclear reaction equations, the atomic number and atomic mass are written reversed compared to their usual arrangement in the periodic table. This reversal is done for practical reasons and conventions in representing nuclear reactions.
In nuclear reactions, it is common to represent the nuclides involved using a shorthand notation called nuclear symbols. These symbols consist of the element's chemical symbol, its atomic number (Z), and its mass number (A). The mass number represents the total number of protons and neutrons in the nucleus, while the atomic number represents the number of protons specifically.
To write nuclear reactions conveniently, the convention is to write the mass number as a superscript to the left of the chemical symbol and the atomic number as a subscript to the left of the chemical symbol. For example, the nuclear symbol for helium-4 is ^4_2He, where the mass number 4 is the superscript and the atomic number 2 is the subscript.
This convention helps to distinguish the nuclides involved in a nuclear reaction and provide a concise representation of the reaction. It is different from the arrangement in the periodic table, where the atomic number is usually written above the chemical symbol and the atomic mass below it.