1.2 Basic ideas about atoms

• Alpha particles are a helium nucleus (2 neutrons and 2 protons), they are stopped by a sheet of paper.
• Gamma rays are high energy photons that penetrate most materials except lead or concrete.
• Beta particles are high energy electrons, they can be stopped by 5mm of aluminium foil.
• When an element emits an α particle it’s mass number decreases by 4 and it’s atomic number decreases by 2.
• When an element emits a β particle its mass number stays the same but its atomic number increases by one.
• Electron capture is when an electron is captured by a proton, to form a neutron, the atomic number is decreased by 1, but the mass number is unchanged.
• Positron emission is when a proton is converted into a neutron while releasing a positron. The atomic number will decrease by 1.
• Half-life is the time taken for half the atoms in a radioisotope to decay.
• Ionisation radiation can cause mutations in DNA and can lead to cancer.
• Radiation can be used for

  • Radiotherapy (Cobalt-60) the high energy γ radiation is used to kill cancer cells.
  • Carbon dating of organisms (Carbon-14)
  • Product control in industry
• An atomic orbital is a region in an atom that can hold up to 2 electrons with opposing spins.
• • an s sub-shell can hold 2 electrons
  • a p sub-shell can hold 6 electrons
  • a d sub-shell can hold 10 electrons
• • Electrons fill orbitals in order of increasing energy (Aufbau principle)
  • A maximum of 2 electrons can occupy an orbital each with opposing spins (Pauli exclusion principle)
  • The orbitals will first fill with one electron each with parallel spins, before a second electron is added (Hund‘s rule)
• A 4s orbital will fill before a 3d orbital due to increasingly complex influences of nuclear attractions and electron repulsion’s upon individual electron.
• The first ionisation energy of an element is the energy required to remove one electron from each atom in one mole of its gaseous atoms.
• First ionisation energy: $X(g) =$$X^+$$(g) +$$e^-$
• The factors that affect ionisation energy are:

  • The size of nuclear charge: the greater the charge, the greater the attractive force on the electron to the nucleus
  • The distance of the outer electron from the nucleus: the force of attraction decreases as the distance increases.
  • The shielding effect by electrons filled in inner shells: the more filled inner or sub shells the smaller the attractive force due to increased repulsion.
• He>H as it has a greater nuclear charge in the same subshell so little extra shielding.
• He>Li since lithium‘s outer electron is in a new shell which has increased shielding and is further from the nucleus.
• Be>B since boron’s outer electron is in a new subshell of slightly higher energy level and is partly shielded by the 2s electrons.
• N>O since the electron-electron repulsion between the 2 paired electrons in one p orbital in oxygen makes one of the electrons easier to remove. Nitrogen does not contain paired electrons in its p orbital.
• He>Ne since neon‘s outer electron has increased shielding from inner electrons and is further from the nucleus.
• Succesive ionisation energies are a measure of the energy needed to remove each electron in turn until all the electrons are removed from an atom.
• The third ionisation energy is

  $X^2(g) =$$X^3(g) +$$e^-$
• Successive ionisation energies increase because:

  • There is a greater effective nuclear charge as the same number of protons are holding fewer and fewer electrons
  • As each electron is drawn there is less and less electron-electron repulsion and each shell is drawn in slightly closer to the nucleus
  • As the distance from the nucleus decreases, the nuclear attraction increases.
• Large jumps indicate a new shell being reached.

  
• Absorption spectra indicates the wavelengths absorbed that correspond to the energy taken in by the atoms to promote electrons from higher to lower energy levels.
• Emission spectra is where atoms are de-excited leading to an emission of a photon, at a specific frequency.
• The hydrogen spectrum is a series of lines that are produced when hydrogen atoms are excited to higher energy levels, and then emit the energy in the form of light.