5.4 The Structure of the Atom

Atoms are the building blocks of everything around us. They're made up of tiny particles - protons, neutrons, and electrons - arranged in specific ways. Understanding how atoms work is key to grasping the nature of matter and energy in the universe.

Electrons in atoms can jump between energy levels, absorbing or releasing light in the process. This behavior explains why different elements have unique colors and spectra. It's also crucial for understanding how stars shine and how we can study distant objects in space.

Atomic Structure

Structure of atoms

• Atoms fundamental building blocks of matter composed of subatomic particles (protons, neutrons, electrons)
• Nucleus located at the center of the atom contains protons with positive charge and neutrons with no charge
  • Nucleus contains vast majority of atom's mass (99.9%)
• Electrons negatively charged particles orbit nucleus in shells or energy levels
  • Electron configuration in outermost shell determines atom's chemical properties and behavior (reactivity, bonding)
  • The number of electrons in the outermost shell, known as valence electrons, is particularly important for chemical reactions

Electron energy transitions

• Electrons occupy discrete energy levels or shells around nucleus labeled with integers (n = 1, 2, 3)
  • Lower energy levels closer to nucleus (n = 1) have lower potential energy
  • Each energy level consists of one or more orbitals, which are regions where electrons are likely to be found
• Electrons transition between energy levels by absorbing or emitting energy
  • Absorption: electron moves from lower to higher energy level requires input of energy equal to difference between levels
  • Emission: electron moves from higher to lower energy level releases energy as photon with specific wavelength
• Electron transitions result in formation of atomic spectra
  • Emission spectra: unique patterns of light emitted by element (hydrogen spectrum)
  • Absorption spectra: dark lines in continuous spectrum caused by electrons absorbing specific wavelengths (Fraunhofer lines in solar spectrum)

Isotopes and atomic properties

• Isotopes are atoms of same element with different numbers of neutrons but same number of protons and electrons
  • Isotopes denoted by mass number (A) = number of protons + neutrons (Carbon-12 has 6 protons + 6 neutrons, Carbon-14 has 6 protons + 8 neutrons)
• Isotopes have similar chemical properties but may differ in physical properties
  • Some isotopes unstable and undergo radioactive decay (carbon-14, uranium-235)
    • Alpha decay: emission of alpha particle (helium-4 nucleus)
    • Beta decay: emission of beta particle (electron) and antineutrino
    • Gamma decay: emission of high-energy photons
• Isotopic abundances used to determine average atomic mass of element
  • Calculated by weighted average of masses of element's isotopes (chlorine-35 at 75.8%, chlorine-37 at 24.2%)

Atomic models and quantum mechanics

• The Rutherford model proposed a nuclear atom with electrons orbiting a small, dense nucleus
• The Bohr model introduced the concept of discrete electron energy levels but was later superseded
• Quantum mechanics provides a more accurate description of atomic behavior, including the probabilistic nature of electron positions and the concept of wave-particle duality
• The atomic number, which is the number of protons in an atom's nucleus, uniquely identifies each element