Lab 4

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) ZCr = 24

Mo (35 to 40 g) ZMo = 42

W (55 to 60 g) ZW = 74 Forceps or tongs Water 10 ml graduated cylinder

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