What does it mean if atoms have the same atomic number but different atomic mass?

Hint: We know that the term ‘iso’ means the same. We should know the terms involved in the word to answer the question. The molecular weight or atomic weight of an element is called mass number. Mass number is going to be obtained by doing the sum of the number of protons and neutrons present in the nucleus of the atom.

Complete answer: Also we know that two elements are said to be isobars of one another if they have different sets of electrons and the same number of nucleons. The term nucleon is a collective term used for the combination of neutron and proton altogether inside the nucleus of an atom. Due to some reason some atoms of the same atomic number have more neutrons than others and due to it their atomic mass becomes different then its species. Although it has same atomic number but different mass number. So this is called Isotopes. Example: carbon has two isotopes one has atomic mass \[12\] and other has atomic mass \[14\].
If the atoms have the same atomic number they are called isotopes. - If the atoms have the same number of neutrons then they are called isotones. - If the atoms have the same molecular formula then they are called isomers. - If the atoms have the same mass number then they are called isobars. - ‘Iso’ means the same and bar means mass number.

Note:
Remember that all these elements Sulphur, calcium, potassium, Argon, Chlorine have the same mass number then they are called isobars. Mass number of the elements is going to be written in the position of superscript to the element and the atomic number should be written in the subscript to the element.