Composition of Nucleus — Revision Notes

⚡ 30-Second Revision

Nucleus: — Dense, positively charged core of an atom.
Protons ($p$): — Positive charge ($+e$), mass $approx 1.672 \times 10^{-27},\text{kg}$. Number of protons = Atomic Number ($Z$).
Neutrons ($n$): — No charge, mass $approx 1.674 \times 10^{-27},\text{kg}$. Number of neutrons = $N$.
Nucleons: — Collective term for protons and neutrons.
Mass Number ($A$): — Total nucleons ($A = Z + N$).
Nuclide Notation: — $^A_Z X$.
Isotopes: — Same $Z$, different $N$ (different $A$).
Isobars: — Same $A$, different $Z$ (different $N$).
Isotones: — Same $N$, different $Z$ (different $A$).
Strong Nuclear Force: — Short-range ($< 10^{-15},\text{m}$), attractive, charge-independent, strongest fundamental force.
Nuclear Radius: — $R = R_0 A^{1/3}$, where $R_0 approx 1.2 \times 10^{-15},\text{m}$.
Nuclear Density: — Approximately constant for all nuclei ($approx 2.3 \times 10^{17},\text{kg/m}^3$).

2-Minute Revision

The atomic nucleus is composed of protons (positive charge, $Z$) and neutrons (no charge, $N$), collectively called nucleons. The atomic number ($Z$) defines the element, while the mass number ($A = Z+N$) represents the total nucleons.

Nuclides are denoted as $^A_Z X$. Key classifications include isotopes (same $Z$, different $N$), isobars (same $A$, different $Z$), and isotones (same $N$, different $Z$ and $A$). The strong nuclear force is paramount for nuclear stability; it's a short-range, attractive, charge-independent force, far stronger than electromagnetic repulsion.

Nuclear radius follows $R = R_0 A^{1/3}$, leading to a remarkably constant and extremely high nuclear density across all nuclei. Remember Chadwick's discovery of the neutron resolved inconsistencies in earlier nuclear models.

Focus on these definitions and properties for quick recall in NEET.

5-Minute Revision

Revisiting the composition of the nucleus is crucial for NEET. At the heart of every atom lies the nucleus, a compact structure made of protons and neutrons, collectively known as nucleons. Protons carry a positive elementary charge ($+e$) and their count defines the atomic number ($Z$), which uniquely identifies an element and dictates its chemical properties.

Neutrons are electrically neutral but contribute significantly to the nuclear mass and, more importantly, to its stability. The total number of protons and neutrons gives the mass number ($A$). A specific nucleus is represented as $^A_Z X$.

Understanding the relationships between nuclides is vital: Isotopes have the same $Z$ but different $N$ (e.g., $^1_1 \text{H}, ^2_1 \text{H}$). Isobars have the same $A$ but different $Z$ (e.g., $^{40}_{18} \text{Ar}, ^{40}_{19} \text{K}$). Isotones have the same $N$ but different $Z$ and $A$ (e.g., $^{39}_{19} \text{K}$ and $^{40}_{20} \text{Ca}$ both have 20 neutrons).

The stability of the nucleus against the electrostatic repulsion between protons is maintained by the strong nuclear force. This force is the strongest of the four fundamental forces, but it is extremely short-range (acting only over femtometers), attractive, and remarkably charge-independent (acting equally between p-p, n-n, and p-n pairs). Its saturating nature means each nucleon interacts only with its immediate neighbors.

Nuclear size is characterized by the radius formula $R = R_0 A^{1/3}$, where $R_0 approx 1.2 \times 10^{-15},\text{m}$. This relationship implies that the nuclear density is approximately constant for all nuclei, an astonishingly high value of about $2.3 \times 10^{17},\text{kg/m}^3$. This constant density is a key concept. Remember that the discovery of the neutron by Chadwick in 1932 was pivotal in establishing the current proton-neutron model of the nucleus, resolving earlier inconsistencies.

Prelims Revision Notes

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Nucleus Composition: — Protons ($p^+$) and Neutrons ($n^0$). Collectively called Nucleons.
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Protons: — Charge $= +e$, Mass $approx 1.00727,\text{u}$. Determines element identity.
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Neutrons: — Charge $= 0$, Mass $approx 1.00866,\text{u}$. Stabilizes nucleus.
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Atomic Number ($Z$): — Number of protons. Defines the element. (e.g., for Carbon, $Z=6$).
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Neutron Number ($N$): — Number of neutrons.
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Mass Number ($A$): — Total number of nucleons ($A = Z + N$). Represents approximate atomic mass.
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Nuclide Notation: — $^A_Z X$, where $X$ is the chemical symbol.

* Example: $^{12}_6 \text{C}$ has 6 protons, 6 neutrons, 12 nucleons.

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Isotopes: — Same $Z$, different $N$ (and thus different $A$). Same chemical properties, different physical properties. (e.g., $^1_1 \text{H}, ^2_1 \text{H}, ^3_1 \text{H}$).
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Isobars: — Same $A$, different $Z$ (and thus different $N$). Different elements, different properties. (e.g., $^{40}_{18} \text{Ar}, ^{40}_{19} \text{K}, ^{40}_{20} \text{Ca}$).
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Isotones: — Same $N$, different $Z$ and $A$. Different elements, different properties. (e.g., $^{39}_{19} \text{K}$ ($N=20$) and $^{40}_{20} \text{Ca}$ ($N=20$)).
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Strong Nuclear Force (Strong Interaction):

* Nature: Extremely strong attractive force. * Range: Very short-range ($< 10^{-15},\text{m}$). Rapidly falls to zero beyond this. * Charge Dependence: Charge-independent (acts equally between p-p, n-n, p-n). * Saturation: Each nucleon interacts only with immediate neighbors. * Spin Dependence: Depends on the relative orientation of nucleon spins.

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Nuclear Size: — Nuclear radius $R = R_0 A^{1/3}$, where $R_0 approx 1.2 \times 10^{-15},\text{m}$ (Fermi).
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Nuclear Density: — Approximately constant for all nuclei, $ho approx 2.3 \times 10^{17},\text{kg/m}^3$. This is extremely high.
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Nuclear Stability: — For light nuclei, $N approx Z$. For heavy nuclei, $N > Z$ for stability (more neutrons needed to overcome proton repulsion). Plot of $N$ vs $Z$ forms a 'belt of stability'.
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Discovery of Neutron: — James Chadwick (1932) via $alpha$-particle bombardment of Beryllium ($^9_4 \text{Be} + ^4_2 \text{He} \to ^{12}_6 \text{C} + ^1_0 n$).

Vyyuha Quick Recall

Protons are Positive, Neutrons are Neutral. Atomic number is Z (like 'zee' for protons). All Nucleons are A (Mass Number). Isotopes are Identical elements (same Z), Inconsistent neutrons. Strong force is Short-range, Strong, and Sticky (attractive) for Spinning nucleons.