1. Why Do We Need a New Mass Unit? 🤔
Atoms are tiny; a single carbon-12 atom weighs only \(1.992647 \times 10^{-26}\,\text{kg}\). Instead of kilograms, scientists choose the atomic mass unit (u): \[ 1\,\text{u}\;=\;\frac{1}{12}\,m_{^{12}\mathrm C}\;=\;1.660539 \times 10^{-27}\,\text{kg} \tag{13.1} \]:contentReference[oaicite:0]{index=0} This handy unit keeps atomic masses in easy-to-grasp numbers (usually close to whole numbers).
2. Isotopes 🧬
- Isotopes = atoms of the same element with identical chemical behavior but different masses.
- Chlorine offers a classic example:
- \(^{35}\mathrm{Cl}\): \(34.98\,\text{u}\) (75.4 % abundance)
- \(^{37}\mathrm{Cl}\): \(36.98\,\text{u}\) (24.6 % abundance)
- Hydrogen boasts three isotopes:
- Protium (\(^{1}\mathrm H\)): \(1.0078\,\text{u}\)
- Deuterium (\(^{2}\mathrm H\)): \(2.0141\,\text{u}\)
- Tritium (\(^{3}\mathrm H\)): \(3.0160\,\text{u}\) (radioactive)
3. Meet the Nucleons 🔍
| Nucleon | Symbol | Key facts |
|---|---|---|
| Proton | p | Charge \(+e\); mass \(m_p = 1.67262 \times 10^{-27}\,\text{kg} = 1.00727\,\text{u}\). The proton is the nucleus of \(^{1}\mathrm H\). :contentReference[oaicite:2]{index=2} |
| Neutron | n | Charge 0; mass \(m_n = 1.6749 \times 10^{-27}\,\text{kg} = 1.00866\,\text{u}\). James Chadwick discovered it in 1932. Free neutrons live ≈1000 s before turning into a proton, electron, and antineutrino. :contentReference[oaicite:3]{index=3} |
4. Counting Things Inside the Nucleus 📊
- \(Z\) – atomic number = number of protons.
- \(N\) – neutron number = number of neutrons.
- \(A\) – mass number = \(Z + N\) (total nucleons).
- Nucleus shorthand: \({}^{A}_{Z}\!X\). Example: \({}^{197}_{79}\mathrm{Au}\) contains 79 protons and 118 neutrons. :contentReference[oaicite:4]{index=4}
5. Family Terms You’ll Love 👪
- Isotopes: same \(Z\), different \(N\). Example: \(^{2}\mathrm H,\;^{3}\mathrm H\).
- Isobars: same \(A\), different \(Z\). Example: \(^{3}\mathrm H\) & \(^{3}\mathrm{He}\).
- Isotones: same \(N\), different \(Z\). Example: \(^{198}\mathrm{Hg}\) & \(^{197}\mathrm{Au}\).
6. How Do We Measure Atomic Masses? ⚖️
Scientists fire atoms through a mass spectrometer. The path bends in a magnetic field; heavier isotopes curve differently, so researchers pin down each mass precisely. :contentReference[oaicite:5]{index=5}
🔑 High-Yield Ideas for NEET 🚀
- Definition of the atomic mass unit (u) and its exact value.
- Isotope concept plus average atomic mass calculations (e.g., chlorine).
- Properties and masses of protons and neutrons (key constants you can quote).
- Quick recall of \(Z\), \(N\), and \(A\) — and the notation \({}^{A}_{Z}\!X\).
- Difference between isotopes, isobars, and isotones with classic examples.
🎉 You’re all set — dive in and conquer those NEET questions with confidence! 🎉
