atomic structure

• Atomic theory proposes that everything is made up of tiny particles that cannot be broken down further and are separated by empty space
• Democritus from ancient Greece around 500 BC originally proposed the idea of atomic theory
• John Dalton described atoms as solid spheres and suggested that different types of spheres make up different elements
• J.J. Thompson introduced the plum pudding model, showing that atoms contain negatively charged particles known as electrons
• Ernest Rutherford and his students discovered the nucleus of the atom by firing positively charged alpha particles at a thin sheet of gold, leading to the nuclear model
• Rutherford's nuclear model proposed a compact nucleus containing all the positive charge of the atom, with electrons orbiting around it in shells
• Niels Bohr suggested that electrons orbit the nucleus in a similar way to planets orbiting the sun, preventing the collapse of the atom
• Experiments by Rutherford revealed that the nucleus contains small discrete particles known as protons
• James Chadwick provided evidence for neutral particles in the nucleus, which we now call neutrons
• Structure of an atom:

  • Nucleus contains protons (positively charged) and neutrons (neutral)
  • Electrons orbit around the nucleus in shells
  • Electrons are about 2000 times smaller than protons or neutrons and have a negative charge
• Information in a periodic table box:

  • Elemental symbol (e.g. Li for lithium)
  • Atomic number (number of protons)
  • Mass number (total number of protons and neutrons)
• Isotopes:

  • Atoms with the same number of protons but different numbers of neutrons
  • Example: Lithium-6 (3 neutrons) and Lithium-8 (5 neutrons) are isotopes of Lithium
• Radioactive decay:

  • Unstable isotopes decay into other elements by emitting radiation (alpha, beta, gamma, or neutrons)
• Electron energy levels:

  • Electrons are arranged in shells around the nucleus
  • Electrons can jump to higher energy levels if they gain enough energy (excited state)
  • Electrons emit energy as electromagnetic radiation when falling back to lower energy levels
• Ionization:

  • Outermost electron can absorb enough energy to leave the atom
  • Atom becomes a positive ion with more protons than electrons
  • Ionizing radiation can knock electrons off atoms
• Isotopes of an element have the same number of protons but a different number of neutrons
• Only one or two isotopes of an element are stable, while the rest are unstable and can undergo radioactive decay
• Radioactive materials consist of unstable isotopes that can decay
• There are four types of nuclear radiation: alpha particles, beta particles, gamma rays, and neutrons
• Alpha radiation:

  • Made up of two protons and two neutrons
  • Represented by helium's nuclear symbol He
  • Have an overall charge of 2+
  • Easily stopped by collisions with other molecules
  • Can only travel a few centimeters in the air and are absorbed by a single sheet of paper
  • Strongly ionizing, can easily knock electrons off atoms
• Beta radiation:

  • Consists of electrons
  • Have a charge of -1
  • Moderately ionizing and penetrate moderately far into materials
  • Require several meters of air or about five millimeters of aluminum to stop
• Gamma radiation:

  • Waves of electromagnetic radiation
  • Weakly ionizing
  • Can penetrate far into materials before being stopped
  • Require thick sheets of lead or multiple meters of concrete to stop
• Neutron emission:

  • Nucleus can emit a neutron to increase stability if it contains too many neutrons
• Alpha particles consist of two protons and two neutrons, like the nucleus of a helium atom
• Representation of alpha particles: helium 4 2 or the Greek letter alpha
• During alpha decay, an unstable nucleus like uranium-238 emits an alpha particle, losing two protons and two neutrons
• To determine the decay product after alpha decay, subtract 4 from the mass number and 2 from the atomic number of the original nucleus
• Example of alpha decay: Radium-226 (atomic number 88) decays to form an element with a mass number of 222 and an atomic number of 86, which is radon
• Beta decay involves a neutron turning into a proton and emitting a fast-moving electron (beta particle)
• During beta decay, the atomic number of the nucleus increases by 1, while the mass number remains the same as a neutron is converted into a proton
• Example of beta decay: Carbon-14 decays to form nitrogen with an atomic number of 7
• Gamma radiation is pure energy with no mass or charge, causing no change in the nucleus during decay
• Example of gamma decay: Thorium-234 undergoing gamma decay remains as thorium-234
• Neutron emission involves the emission of a neutron from the nucleus, resulting in a decrease of one in the mass number
• Example of neutron emission: Beryllium-9 decays into beryllium-8 plus a neutron
• Radioactive materials contain unstable isotopes that decay by emitting radiation like alpha particles, beta particles, or gamma rays
• Activity of a radioactive sample is the overall rate of decay of all the isotopes in the sample, measured in becquerels where one becquerel represents one decay per second
• Half-life can be defined as the time taken for the number of radioactive nuclei in a sample to halve or the time taken for the activity to halve
• Decay process of radioactive isotopes is random, but with a large enough sample, we can determine the activity and half-life
• Half-life is correlated with the number of radioactive nuclei remaining and the activity of the sample