Transcript Periodic Trends

 The chemical and physical properties of elements
recur in a systematic fashion because they are arranged
in increasing atomic number
 The size of atom is measured by examining the
distance between two nuclei of the same atom and
dividing by two… that is because the “edge” of an
electron cloud is difficult to determine.
 Going across a period the atomic radius decreases. In a
period each atom has the same number of shells, but
an increasing number of protons. The greater nuclear
charge holds the electrons closer to the nucleus.
 As you go down a group the number of shells increases
and therefore the electrons are further away from the
nucleus.
 Each period is larger than the period of atoms before
it, so atomic radii increases as period number
increases.
 Which atom is larger?
 Al or Cl?
 Mg or Ba?
 Al, B, or C?
 Compare an ion’s size to the size of its parent atom.
 Cations are smaller than the parent atom. Cations have
lost electrons and therefore dropped an outer shell.
Thus their size is smaller.
 Anions are larger than the parent atom. They gain
electrons and so the nuclear charge is spread out over
more electrons and each electron is held with less
force, therefore the ion expands a little.
 For instance,
 Calcium makes a 2+ charge. Calcium ion is smaller than
calcium atom
 Phosphorus makes a 3- charge. Phosphorus ion is larger
than phosphorus atom.
 Which is smaller?
 Potassium or its ion?
 Oxygen or its ion?
 Na, Na+, or Li?
 Energy needed to remove the outer most electron from
an atom
 Across a period: IE increases
 When atoms are smaller their electrons are closer to the
nucleus. The closer they are to the nucleus the more
difficult it is to remove them. Therefore, IE increases as
atomic size decreases
Down a Group
IE decreases down a group because the atomic size is
increasing. Electrons are further away from nucleus and
easier to remove.
 Examples:
 Which atom has the lowest ionization energy?
 Rb, As, Kr?
 How much an atom “wants” an electron. Atoms need a
full outer shell. EN is a measure of how much it wants
an additional electron to complete its outer shell.
 Across a period: EN increases until the Noble Gases.
Group 18. Group 18 elements have zero EN.
 Down a group: EN decreases
 Examples:
 Which atom is the lowest EN?
 Al, P, or Ar?
 Across a period: Metallic Properties decrease
 Down a Group Metallic Properties increase