Idea of atoms

• Atom


  Made up of three particles: protons, neutrons, electrons
• Proton


  • Relative mass 1, relative charge +1
• Neutron


  • Relative mass 1, relative charge 0
• Electron


  • Relative mass 1/1840, relative charge -1
• Atomic number


  Number of protons
• Atoms of the same element have the same number of protons
• Mass number


  Sum of protons and neutrons
• Atoms of the same element can have different numbers of neutrons. These are called isotopes
• Radioactive isotopes


  • Unstable, usually heavy nuclei like uranium and plutonium, spontaneously disintegrate and can emit alpha, beta, and gamma radiation
• Alpha (α) particles


  Positively charged helium nuclei, mass of four units, stopped by a piece of paper, strongly ionising
• Beta (β) particles


  Negatively charged electrons, negligible mass, stopped by 0.5 cm of aluminium
• Gamma (γ) radiation


  Very high energy electromagnetic radiation, > 2cm of lead required to stop it, weakly ionising
• Radioactive decay


  1. Alpha decay

  2. Beta decay

  3. Positron emission

  4. Electron capture

  5. Gamma emission
• Half-life


  The time taken for the radioactivity of a radioactive isotope to fall to half of its initial value, or the time taken for half the atoms in a radioactive isotope to decay
• The half-life of a radioisotope is characteristic of each radioisotope and unaffected by catalysts or changes in temperature
• Half-life calculations


  1. Finding the time taken for the radioactivity to fall to a certain fraction of its initial value

  2. Finding the mass of a radioactive isotope remaining after a certain length of time given the initial mass
• Penetrating power/danger of radioactive emissions


  Depends on whether the source is outside or inside the body
• Ionising radiation can damage the DNA of a cell, leading to changes in cell function, mutations, and cancer or cell death
• Beneficial uses of radioactivity

  • Treatment of cancer
  • Tracer
  • Calculating age of plant and animal remains
  • Estimating geological age of rocks
  • Production of electricity
  • Measuring the thickness of metal foil
• Electrons


  Involved in the changes that happen during chemical reactions
• Electrons within atoms


  • Occupy fixed energy levels/shells which are numbered 1, 2, 3 etc (principal quantum numbers, n)
  • The lower the value of n, the closer the shell is to the nucleus and the lower the energy level
• Atomic orbitals


  • Areas of space around the nucleus where there is a high probability of finding an electron of a given energy
  • Orbitals of the same type are grouped together in a subshell (s, p, d, f)
• Electron spin


  For two electrons to exist in the same orbital they must have opposite spins (reduces the effect of repulsion)
• s orbital


  • Spherical
  • Can hold up to two electrons
• p orbital


  • Dumb-bell shaped lobes at right angles (px, py, pz)
  • Can hold up to six electrons in total (two in each orbital)
• d orbital


  • Five different orbitals
  • Can hold up to 10 electrons in total (two in each orbital)
• f orbital


  • Seven different orbitals (do not need to recognise/draw)
  • Can hold up to 14 electrons in total (two in each orbital)
• Filling shells and orbitals

  1. Electrons fill orbitals from lower to higher energy

  2. A maximum of two electrons can occupy any orbital, each having an opposite spin

  3. If two or more orbitals of equal energy are available, electrons will occupy them singly at first with parallel spins before filling in pairs
• Electronic configuration or electronic structure


  The way in which the electrons are arranged in an atom
• Electronic structure


  • K atom, 19 electrons: 1s22s22p63s23p64s1
  • Cu atom, 29 electrons: 1s22s22p63s23p63d104s1
  • Cu2+ ion, 27 electrons: 1s22s22p63s23p63d9
• Ionisation


  The process of removing electrons from an atom
• Successive ionisation energies always increase
• Reasons for increasing ionisation energies

  • There is a greater 'effective' nuclear charge as the same number of protons are holding fewer and fewer electrons
  • As each electron is removed, each shell will be drawn slightly closer to the nucleus
  • As the distance of each electron from the nucleus decreases, the nuclear attraction increases
• The first ionisation energy of elements varies going across a period and down a group
• Factors affecting ionisation energy

  • The size of the positive nuclear charge
  • The distance of the outer electron from the nucleus
  • The shielding effect of electrons in fully occupied inner shells
• Light


  A form of electromagnetic radiation that travels as waves
• Frequency (f), wavelength (λ) and speed of light (c)

  Related by the equation: c = fλ
• Electron excitation and emission

  1. If atoms are given energy through heating or from an electrical field, electrons are pushed up from a lower energy level to a higher energy level (promoted)

  2. When the source of energy is removed, the electrons fall from their higher energy level (excited state) to a lower energy level and energy lost is released as a photon (quantum of energy)
• Frequency (f) and energy (E)

  Related by the equation: E = hf (h is a constant called Planck's constant (6.63 × 10–34 J s))
• Frequency is proportional to energy and wavelength is inversely proportional to energy