Question

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

Answer 1

Hint: Atoms having the same number of neutrons are called isotones. The mass number (A) is the sum of the number of protons (Z), and the number of neutrons (N). The number of nucleons is the sum of neutrons and the number of electrons or protons. The number of nucleons defines the mass number of an atom.

Complete step by step solution:
Atoms are the fundamental unit of matter. Most of the mass in an atom is in the nucleus. Atom is composed of a nucleus in which electrons are bounded. Nucleus has protons and neutrons.
The atomic number denoted by Z is the number of protons in the nucleus. The mass number denoted by A is the sum of the number of protons and the number of neutrons, i.e. the number of nucleons.
${\text{A}} = {\text{Z}} + {\text{N}}$
Thus the number of neutrons ${\text{N}} = {\text{A}} - {\text{Z}}$
Alternatives of a chemical element can be of different types- isotopes, isobars, isotones, isomers. Isotopes are the atoms having the same number of protons and a different number of neutrons. Isotones are precisely vice versa. They are atoms having the same number of neutrons but a different number of protons.
A. Isotopes are the atoms having the same proton number and a different number of neutrons.
E.g. Protium, Deuterium, Tritium are the isotopes of hydrogen.
B. Isobars are the atoms having the same nucleon number and a varied number of electrons.
D. Atoms having the same A-Z means it has the same number of neutrons from the above equation and different Z means it has a different number of protons.

Thus the option is D.

Additional information: There are different examples of isotones. Boron-12 and Carbon-13 are one among them. Both of them have seven neutrons.

Note:
- Atomic species are classified as isotopes, isobars, isotones, isomers.
- Isotopes- Same Z and different N
- Isobars- Same A
- Isotones- Same N and different Z
- Isomers- Same Z, N but in different excited states
- Isodiaphers- Same N-Z