Physics · nuclei · NEET
| Quantity that stays same | Isotopes: Z (protons) | Isobars: A / Isotones: N |
| Element identity | Isotopes: same element | Isobars & Isotones: different elements |
| Example | Isotopes: 1-1-H, 2-1-H, 3-1-H | Isobars: 40-18-Ar, 40-20-Ca ; Isotones: 3-1-H, 4-2-He |
Use the nuclide symbol A-Z-X where Z is protons (atomic number), A is mass number (protons + neutrons), and N = A - Z is neutrons. Isotopes: same Z, different N (so different A). Isobars: same A, different Z. Isotones: same N, different Z (so different A). Only one quantity is fixed in each case; the others change.
Match the capital letter in the name to the quantity that stays the same. IsotoPe -> same P (protons, so same Z). IsobAr -> same A (mass number). IsotoNe -> same N (neutrons). This one trick covers all three, so you never have to guess in the exam.
Isobars are always DIFFERENT elements. They have the same mass number A but different atomic number Z, so they have different numbers of protons. Example: argon-40 (Z=18) and calcium-40 (Z=20) are isobars. Because Z is different, the chemical identity is different.
Yes. Isotopes have the same number of protons and therefore the same number of electrons and the same electron arrangement, so their chemical properties are almost identical. They differ only in the number of neutrons, which changes the mass, not the chemistry. Example: carbon-12 and carbon-14 both behave chemically as carbon.
N is the neutron number. From the symbol A-Z-X, subtract the atomic number from the mass number: N = A - Z. For example, for carbon-14 written 14-6-C, N = 14 - 6 = 8 neutrons. You need N to identify isotones, since isotones are nuclei with the same N.
Isotopes: 1-1-H, 2-1-H, 3-1-H (all hydrogen, Z=1, N = 0,1,2). Isobars: 40-18-Ar and 40-20-Ca (same A=40, different Z). Isotones: 3-1-H and 4-2-He (both have N=2). Memorising one clean example of each makes the definitions stick.
Isotopes have the same atomic number Z, isobars have the same mass number A, and isotones have the same neutron number N.
No. Deuterium 2-1-H has N = 1 and helium-3 3-2-He has N = 1, so actually they ARE isotones (both N=1). A safer stated pair is 3-1-H and 4-2-He, both with N=2.
Isobars and isotones are always different elements because their atomic number Z is different. Isotopes are always the same element because Z is the same.
Because they have the same protons (same Z) but a different number of neutrons. Since mass number A = Z + N, changing N changes A.
Yes. Definitions of isotopes, isobars and isotones are basic NCERT points in the Nuclei chapter and are often tested as direct memory or example-matching questions.
