Detailed Structure of Atoms and Nuclei

Structure of Atoms and Nuclei

1. Introduction

The atom is the fundamental unit of matter, consisting of a dense central nucleus surrounded by electrons in specific regions called orbitals. Understanding atomic and nuclear structure helps explain chemical properties, radioactivity, and nuclear reactions.

2. Atomic Models Over Time

  • Dalton’s Model: Atom is a solid indivisible sphere.
  • Thomson’s Plum Pudding Model: Atom is a positive sphere with embedded electrons.
  • Rutherford’s Model (1911): Most mass and positive charge concentrated in the nucleus; electrons orbit the nucleus.
  • Bohr’s Model (1913): Electrons orbit nucleus in fixed energy levels or shells.
  • Quantum Mechanical Model: Electron positions described by probability clouds called orbitals.
Nucleus

3. Structure of the Atom

The atom consists of:

  • Nucleus: Contains protons and neutrons.
  • Electrons: Negatively charged particles orbiting the nucleus in shells.

Atomic Number (\( Z \)) is the number of protons in the nucleus.
Mass Number (\( A \)) is the total number of protons and neutrons.

4. Nuclear Forces and Stability

Inside the nucleus, protons repel each other due to positive charges, but they remain bound together by the strong nuclear force, which acts only over very short distances and is much stronger than the electrostatic repulsion.

Neutrons help to stabilize the nucleus by increasing the strong force without adding repulsive charge.

5. Nuclear Notation and Equations

The general notation for an atom is:

\( \displaystyle {}^{A}_{Z}X \)

Where:
- \( X \) = chemical symbol
- \( A = Z + N \) (mass number)
- \( Z \) = atomic number (number of protons)
- \( N \) = number of neutrons

6. Radioactive Decay Equations

Many nuclei are unstable and decay by emitting particles or radiation. Common decay types include alpha (\( \alpha \)) and beta (\( \beta \)) decay.

\(\displaystyle {}^{A}_{Z}X \rightarrow {}^{A-4}_{Z-2}Y + {}^{4}_{2}\alpha \) (Alpha decay)
\(\displaystyle {}^{A}_{Z}X \rightarrow {}^{A}_{Z+1}Y + \beta^{-} + \overline{\nu}_e \) (Beta minus decay)

7. Example 1: Nuclear Notation

Write the nuclear notation for an atom with 15 protons and 16 neutrons.

Solution:
Atomic number \( Z = 15 \) (Phosphorus, P)
Number of neutrons \( N = 16 \)
Mass number \( A = Z + N = 15 + 16 = 31 \)

Nuclear notation:
\( \displaystyle {}^{31}_{15}P \)

8. Example 2: Alpha Decay

Uranium-238 undergoes alpha decay. Write the decay equation.

Solution:
\( \displaystyle {}^{238}_{92}U \rightarrow {}^{234}_{90}Th + {}^{4}_{2}\alpha \)

9. Sizes and Scales

  • Atom size is approximately \(10^{-10}\) meters (0.1 nm).
  • Nucleus size is about \(10^{-15}\) meters (1 fm), which is ~100,000 times smaller than the atom.
Atom (~10⁻¹⁰ m) Nucleus (~10⁻¹⁵ m)

10. Summary

  • Atoms have a small, dense nucleus made of protons and neutrons.
  • Electrons orbit the nucleus in defined energy levels or orbitals.
  • Nuclear forces keep the nucleus stable despite proton repulsion.
  • Radioactive decay changes the nucleus by emitting particles.
  • Nuclear notation represents an atom’s composition succinctly.

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structure of atoms and nuclei