Lab 4
PERIODIC TRENDS
Goals of the Experiment
 Measure bulk densities & calculate atomic
densities of some transition metals
 Relate density to atomic size (a periodic
trend)
Materials
 Cr (25 to 30 g)
 Mo (35 to 40 g)
 W (55 to 60 g)
 Forceps or tongs
 Water
 10 ml graduated cylinder
ZCr = 24
ZMo = 42
ZW = 74
Safety
 Gloves
 Safety goggles/glasses
Background
 Dmitri Mendeleev
arranged the Periodic
Table in groups and in
order of increasing
atomic mass .
Background
 Henry Moseley
rearranged the Periodic
Table in order of
increasing atomic
number (Z).
Background
 Atomic number (Z)
 Number of electrons and protons
 Ex: Cr 24
 Atomic mass (M)
 Z + number of neutrons
 Ex: MCr = 52 amu
Background
 Atomic Mass (M)
 Some of the mass of an element is converted into
energy (nuclear binding ), E = mc2.
 Ex: Tungsten (Z = 74; MW = 184 amu).
Actual mass (isotope) = 183.95093129 amu.
1 amu = 1.66 x 10-27 kg.
(Show calculation)
Background
(Periodic table)
 Property of an element depends on the
location (family = vertical column; period =
horizontal row).
Background
(Periodic Trends)
 Atomic Radius
 Increases from R to L; Increases from Top to Bottom
(Show schematic view)
 Ionization Energy (IE) – Emin required to remove
1 e- from an atom/ion in its ground state and it
correlates to reactivity of metals (exceptions).
 Increases from L to R; Decreases as you go down a
family
 Smaller IE = more reactive the metal
Background
(Periodic Trends)
 Electron affinity (EA) - ∆E associated with the
addition of an electron to an atom/ion & it
correlates to the reactivity of nonmetals
(exceptions).
 Increases from L to R; Decreases as you go down a
family.
 Larger EA = more reactive nonmetal
Background
(Periodic Trends)
 Density also displays a periodic trend –
atomic density increases from top to bottom
but varies less significantly as one moves
from left to right across a period.
 Bulk density depends on 3 properties:
 Mass of the atoms
 Packing arrangement (crystal structure – body
centered, face centered, or simple cubic).
 Size of each atom