Chapter 2: Atomic Structure

Explain · page 28: their mass number may vary depending upon the number of neutrons present in the nucleus. Atoms of the same element having different number of neutrons in their nuclei are called isotopes. For example, element carbon has three isotopes as its atoms have six, seven and eight neutrons in their nuclei. These isotopes are represented as '<C, '2C, '2C. Similarly, hydrogen exists as three isotopes, Hydrogen, Deuterium and Tritium represented by }H, {H and ;H. Hydrogen (/H) is the only

Core idea Atoms of the same element can have different numbers of neutrons, so their total mass (mass number) changes. These atoms are called isotopes (آیزوٹوپ) and behave almost the same chemically because their electrons are unchanged.

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What you need to remember

• Isotopes are atoms of one element that differ only in the number of neutrons.
• Mass number = protons + neutrons; it tells you how heavy the atom is.
• Example for carbon:
• $^{12}\text{C}$ (6 protons + 6 neutrons)
• $^{13}\text{C}$ (6 protons + 7 neutrons)
• $^{14}\text{C}$ (6 protons + 8 neutrons)
• Example for hydrogen:
• $^{1}\text{H}$ (1 proton + 0 neutrons) – normal hydrogen
• $^{2}\text{H}$ (deuterium) – 1 proton + 1 neutron
• $^{3}\text{H}$ (tritium) – 1 proton + 2 neutrons
• $^{1}\text{H}$ has no neutron, the only atom that does.
• Chemical behaviour is governed by electrons, so all isotopes of an element react the same way, even though their masses differ.
• Mass relationships:
• $^{2}\text{H}$ ≈ 2 × $^{1}\text{H}$
• $^{3}\text{H}$ ≈ 3 × $^{1}\text{H}$

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Everyday Pakistan example

When you boil water for tea or cooking, the water is mostly $^{1}\text{H}_2\text{O}$, but it also contains a tiny fraction of heavy water $^{2}\text{H}_2\text{O}$ (deuterium). You don’t notice any difference in taste or boiling point because the chemical behaviour is the same; only the mass is slightly higher.

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Common mistake

Students often think that different isotopes will act differently in chemical reactions. Correction: The number of neutrons does not change the electrons, so the chemical properties stay the same; only physical properties like mass and density change.

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In short: Isotopes are atoms of the same element with different neutron counts, giving them different masses but almost identical chemical behaviour.