the periodic table

• groups

  vertical columns in the periodic table
• periods

  horizontal rows
• metalloids

  Elements that have some properties of non metals or properties that are intermediate between those of a metal and non metal
• noble gas

  group 18
• the reactions of an elements are determined by the number of electrons in the outer shell of their atoms
• elements in the same group in the periodic table have the same number of electrons in their outer they have similar chemical properties
• lanthanoids are above actinoids
• elements in the same group will form compounds with the same formula
• periodicity

  used to describe the trends in properties that occur across a period and down a group in the periodic table
• the larger the atomic radius the larger the atom
• atomic radius

  half the intermolecular distance between two atoms of the same element which are covalently bonded
• nuclei are far more massive and dense than the electrons
• atomic radius increases down a group as the atoms have increasingly more electron shells
• the electrons in the inner electron shells shield the outer electrons from the full attractive force of the nucleus
• the number of inner shells that cause shielding increases down the group, and this mostly balances out the increase in nuclear charge
• atomic radius decreases across a period
• why does the atomic radius decrease across a period?

  nuclear charge increases from left to right across the period but there is no significant change in shielding because each atom has the same number of inner shells of electrons
• the amount of shielding depends on the amount of inner shells and nuclear charge
• la

  lanthanoids
• ac 


  actinoides
• effective nuclear charge

  number of protons - number of inner shell electrons
• ionic radius

  a measure of the size of an ion
• ionic radius increases down a group
• why does the ionic radius increase down a group

  as we go down a group the atoms have increasingly more electron shells
• the ionic radii of positive ions are smaller than their atomic radii
• the electrons are pulled in until the attraction from the nucleus balances with the repulsion between electrons- with less repulsion the electrons can be pulled in closer to the nucleus and the radius is smaller
• the ionic radii of negative ions are greater than their atomic radii
• halogen ions are larger than the atoms because it has more electrons for the same nuclear charge and greater repulsion between electrons
• variation in ionic radius across a period 


  for positive ions there is a decrease in ionic radius as the charge on the ion increases but for negative ions the size decreases as the charge decreases
• Mendeleev's periodic table arrangement

  grouped elements by their chemical properties. if an element did not fit the pattern he left a gap
• modern periodic table arrangement

  • Group (number of electrons in the valence energy level)
  • Period (the number of occupied energy levels)
  • By increasing atomic number
• periodicity

  The trend in chemical and physical properties that is

  repeated across a period.
• trend in atomic radius across period 3

  • As you move from left to right across the period the proton number (nuclear charge) increases
  • This means that the attraction between the valence electrons and the nucleus would increase
  • Therefore, the atomic radius decreases
• Atomic radii increase down a group as the number of occupied energy levels increases.
• the valence electrons do not experience the full

  nuclear charge due to electron shielding. (valence electrons are repelled by inner electrons)
• why is effective nuclear charge less than the nuclear charge


  the valence electrons do not experience the full

  nuclear charge due to electron shielding. (valence electrons are repelled by inner electrons)
• Ionic radius is measured as the half distance between a neighbouring anion and cation.
• How do the ionic radii of ions compare

  to their parent atoms for anions and

  for cations?

  • Positive ions are smaller than their parent atoms.
  • Negative ions are larger than their parent atoms.
• Why are Positive ions are smaller than their parent

  atoms.

  • Lost electrons
  • Effective nuclear charge increases
  • Greater attraction
  • Smaller radii
• The ionic radii decrease from Groups 1 to 13 for

  positive ions.