Two atoms have the same mass number but different atomic numbers. They are:

1. Isotopes share atomic number, differ in mass number. 2. Isobars share mass number, differ in atomic number. 3. Allotropes are different physical forms of the same element. 4. So same A and different Z is the definition of isobars. _Source: Selina Concise Chemistry ICSE Class 9, Ch 4 'Atomic Structure'_