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💡 Same element, different weight
Hydrogen comes in three forms — protium, deuterium, tritium — all hydrogen, but each carries a different number of neutrons.
What are isotopes?
- Atoms of the same element — same atomic number (Z) .
- But different numbers of neutrons , so different mass numbers (A) .
- Same electrons → same chemical properties; different physical properties.
Examples: hydrogen — \({}^{1}_{1}\text{H}\), \({}^{2}_{1}\text{H}\), \({}^{3}_{1}\text{H}\); carbon — \({}^{12}_{6}\text{C}\), \({}^{13}_{6}\text{C}\), \({}^{14}_{6}\text{C}\). Each carbon isotope has 6 protons and 6 electrons but a different number of neutrons.
🔬 Ready to Go Beyond
Atoms are too tiny to weigh in grams, so scientists use a special unit — the unified atomic mass unit (u) . Earlier this was written as the atomic mass unit (amu).
🌎 Bridging Science and Society
- \({}^{235}_{92}\text{U}\) — fuel in nuclear reactors to generate electricity.
- \({}^{60}_{27}\text{Co}\) — used in radiation treatment for cancer.
- \({}^{131}_{53}\text{I}\) — used to treat goitre and thyroid cancer.
- \({}^{14}_{6}\text{C}\) — used in archaeology to date ancient fossils and artefacts.
NCERT Question 14 — Aman was discussing the structure
Aman was discussing the structure of atom with his classmates. During the discussion, he learnt that an element X has a mass number of 35 and contains 18 neutrons. Based on this information, answer the following questions: (i) How many electrons and protons does element X have? (ii) What is its atomic number? (iii) Identify the element X. (iv) Write its electronic configuration. (v) How many valence electrons does it have? (vi) What will be the mass number if two neutrons are added to its nucleus? (vii) What will be the relation of X with the new atom?
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Important Definitions
- Isotopes — atoms of the same element with the same atomic number but different mass numbers.
- Unified atomic mass unit (u) — the special unit used to measure the very small mass of atoms.
