Question

What are isotones?
A. Atoms having same number of protons but different number of neutrons
B. Atoms having same number of nucleons but different number of electrons
C. Atoms having same A but different $A - Z$
D. Atoms having the same $A - Z$ but different Z.

Answer 1

An atom consists of electrons, neutrons and protons. Apart from them, they also consist of sub-atomic particles such as muon, up quark and many more. In some atoms, a similarity is observed between the numbers of electrons, protons and neutrons. One such similarity is isotones.

Complete answer:
Atoms are known as the fundamental unit of a matter. Most of the mass in an atom is found in the nucleus. Atoms are composed of a nucleus which is at the center of the atom. In the nucleus we have protons and neutrons with other subatomic particles.

The atomic number of an element is denoted by the symbol Z. The mass number of an element is denoted by the alphabet A. The atomic number is the number of electrons or the number of protons present in an atom while atomic mass number is the sum of masses of nucleons. Nucleons are protons and neutrons put together.
We have $A = Z + N$
Where N is the number of neutrons.
$\therefore N = A - Z$
Atoms having the same number of neutrons are called isotones.

Therefore, atoms having the same $A - Z$ values are called isotones. Thus, option D is the correct option.

Note: Atomic pairs can be classified in following types: Isobars, isotones, isomers.
Isobars are the pair of atoms that have the same A.Isotopes have the same $Z$ but different $N$. Isomers have the same $Z$ but in different excited states.Isodiaphers have the same $N-Z$ values.Isotones have the same $N$ but different $Z$.Examples of isotones are: $Boron - 12$ and $Carbon - 13$ as both of them have seven neutron each.