Topic 3: Periodicity

• Every chemical reaction can be explained in terms of atoms
• Alpha particles were used in the development of the nuclear model of the atom that was first proposed by Rutherford
• The subatomic particle theory of matter represents a paradigm shift in science that occurred in the late 1800s
• Atoms


  Contain a positively charged dense nucleus composed of protons and neutrons (nucleons)
• Electrons


  Occupy the space outside the nucleus
• Mass spectrometer


  Used to determine the relative atomic mass of an element from its isotopic composition
• Leucippus and Democritus stated around 440 BC that matter was composed of indivisible particles termed atomos
• John Dalton developed an atomic model of matter that was supported by experimental data in 1808
• Dalton's atomic theory


  • Postulate 1: All matter (materials) consists of very small particles called atoms
  • Postulate 2: An element consists of atoms of one type only
  • Postulate 3: Compounds consist of atoms of more than one element and are formed by combining atoms in whole-number ratios
  • Postulate 4: In a chemical reaction atoms are not created or destroyed
• Law of definite proportions


  A compound always has the same proportion of elements by mass
• Law of conservation of matter


  Matter cannot be created or destroyed. The total mass of matter following a chemical reaction is equal to the total mass of matter before the start of the reaction
• Law of multiple proportions


  If two elements X and Y combine in different ways to form more than one compound, the masses of X that combine with a fixed mass of Y can be expressed as a ratio of small whole numbers
• J.J. Thomson proposed the "plum-pudding" model of the atom in 1906
• "Plum-pudding" model of the atom

  The atom was similar to a plum pudding, with negatively charged particles (like raisins) embedded in a positive region (the "pudding") of the atom
• Ernest Rutherford conducted the gold foil experiment in 1909 to test Thomson's "plum-pudding" model
• Rutherford's gold foil experiment


  A thin gold metal foil was placed in an evacuated chamber and bombarded with alpha particles
• Most alpha particles went through the gold foil and some were deflected slightly


  Some particles were deflected by very large angles and some even bounced straight back towards the source
• Rutherford's explanation


  The gold foil consists of thousands of gold atoms. When the beam of positively charged alpha particles bombarded the foil, the majority of the particles passed through undeflected, since the atom consists mainly of empty space. However, at the core of the atom lies a dense region of positive charge called the nucleus. When an alpha particle came close to the nucleus of a gold atom it deflected through a large angle, and when it hit the nucleus it reflected back along its initial path.
• The nucleus occupies a tiny volume of the atom and the diameter of an atom is approximately 100000 times the diameter of the nucleus
• Picometre (pm)


  Unit used to describe the dimensions of atoms, 1 pm = 10^-12 m
• The relative volume of open space in the atom is vast and our simple representation of Rutherford's atomic model is obviously unrealistic
• Atom


  Extremely small, diameter in the range 1 × 10^-10 to 5 × 10^-10 m
• Angstrom (Å)


  Unit used in X-ray crystallography for atomic dimensions, 1 Å = 10^-10 m
• The atomic radius of the fluorine atom is 60 pm (6.0 × 10^-1 Å)
• The periodic table is split into periods, with the period number equal to the principal quantum number, n, of the highest occupied energy level
• Metals, non-metals, and metalloids

  • Metals
  • Non-metals
  • Metalloids
• Metals


  • Good conductors of heat and electricity
  • Malleable
  • Ductile
  • Have lustre
• Mercury, Hg, is a liquid and can dissolve many other metals to form amalgams
• Non-metals


  • Poor conductors of heat and electricity
  • Typically gain electrons in chemical reactions
• Metalloids


  • Boron
  • Silicon
  • Germanium
  • Arsenic
  • Antimony
  • Tellurium
  • Astatine
• Some metalloids like silicon and germanium are semiconductors
• Main-group elements


  • Group 1 (excluding H)
  • Group 2
  • Groups 13-18
• Transition elements


  • Groups 3-11
• Blocks of the periodic table

  • s-block
  • p-block
  • d-block
  • f-block
• The number of valence electrons for s- and p-block elements can be found from the group number
• Deducing electron configuration from position in periodic table

  1. State number of protons and electrons

  2. State group

  3. State number of valence electrons

  4. State number of protons and electrons in anion

  5. Deduce full electron configuration

  6. Deduce condensed electron configuration

  7. Draw orbital diagram
• Position within a defined orbit

  75
• The Bohr model of the atom is highly simplistic and electrons are in fact located in atomic orbitals, which are regions of space where there is a high probability of finding an electron
• The position of the electron is not fixed, so we cannot measure the radius of the atom in the same way as we measure the radius of a circle
• Atoms cannot be represented as spheres with fixed boundaries