Chemistry · Structure Of Atom · NEET
Look at the numbers in the symbol (superscript A on top = mass number, subscript Z below = atomic number = protons). ISOTOPES: same Z (same protons), different A. They are the SAME element. Example: hydrogen-1, hydrogen-2, hydrogen-3. ISOBARS: same A (same mass number), different Z. They are DIFFERENT elements. Example: 40-Ar (Z=18) and 40-Ca (Z=20). ISOTONES: same number of neutrons (neutrons = A minus Z), different Z. Example: 14-C (14-6=8 neutrons) and 16-O (16-8=8 neutrons). One quick line: isotopes = same protons, isobars = same mass, isotones = same neutrons.
Neutrons = mass number minus atomic number = A - Z. The top number is A, the bottom number is Z (protons). Do A - Z for each species. If two atoms give the SAME neutron value but have DIFFERENT Z, they are isotones. Example: for 13-C, Z=6, so neutrons = 13-6 = 7. For 14-N, Z=7, so neutrons = 14-7 = 7. Same neutrons (7), different protons, so they are isotones. NEET loves this small subtraction, so always compute A - Z first.
No. This is the most common mistake. Isotopes have the SAME atomic number (same protons, same element) but DIFFERENT mass numbers because they have different numbers of neutrons. Example: carbon-12 and carbon-14 both have 6 protons, but carbon-12 has 6 neutrons and carbon-14 has 8 neutrons. If two atoms had the same mass number but were different elements, those would be isobars, not isotopes.
No. Isobars are always DIFFERENT elements. 'Iso-bar' means same mass number A, not same element. Since they are different elements, they have different atomic numbers (different protons). Example: 40-Ca (20 protons) and 40-Ar (18 protons) are isobars because both have mass number 40, but they are two different elements. Only isotopes are the same element.
Match the last part of each word to what stays the same. isotoP -> same P (Protons). isobAr -> same A (mass number). isotoNe -> same N (Neutrons). So P for protons, A for mass, N for neutrons. Another way: isotopes are 'same top-left electron count' (same element), isobars share the 'bar' = the big top mass number, and isotones share the neutrons. Once you lock this, the numbers in the symbol tell you the answer in seconds.
Isotopes of an element show almost the SAME chemical properties, because chemical behaviour depends on the number of electrons (which equals protons in a neutral atom), and isotopes have the same protons and electrons. They differ mainly in physical properties that depend on mass (like density and rate of some reactions). Isobars and isotones are different elements, so they have different chemical properties.
Tritium, a radioactive isotope of hydrogen, emits which of the following particles?
The number of protons, neutrons and electrons in ¹⁷⁵₇₁Lu, respectively, are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Isotopes share the same number of protons (same atomic number Z). Isobars share the same mass number A. Isotones share the same number of neutrons. Each term keeps ONE thing constant.
Isotopes: ¹²C and ¹⁴C (both Z = 6). Isobars: ⁴⁰Ar (Z = 18) and ⁴⁰Ca (Z = 20), both A = 40. Isotones: ¹⁴C (8 neutrons) and ¹⁶O (8 neutrons), same neutrons, different Z.
A - Z gives the number of neutrons. Compute it for each atom. Same A - Z with different Z means isotones. It is a quick subtraction that NEET tests directly.
Only isotopes are the same element, because same protons (same Z) means same element. Isobars and isotones are always different elements.
Structure of Atom is a scoring chapter, and NTA repeatedly asks you to classify given atoms as isotopes, isobars or isotones. Getting the definitions and the A - Z step right earns easy, guaranteed marks.
