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Exploring Density and Atomic Size Trends in Transition Metals

This experiment investigates the relationship between bulk densities and atomic densities of selected transition metals—Chromium (Cr), Molybdenum (Mo), and Tungsten (W). Students will measure bulk densities using a graduated cylinder and relate these measurements to atomic size and periodic trends. Safety precautions, including wearing gloves and goggles, are emphasized throughout the experiment. Understanding how atomic mass and atomic structure influence the physical properties of elements will enrich knowledge about the periodic table and the work of chemists like Mendeleev and Moseley.

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Exploring Density and Atomic Size Trends in Transition Metals

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Presentation Transcript

  1. Lab 4 Periodic Trends

  2. Goals of the Experiment • Measure bulk densities & calculate atomic densities of some transition metals • Relate density to atomic size (a periodic trend)

  3. 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

  4. Safety • Gloves • Safety goggles/glasses

  5. Background • Dmitri Mendeleev arranged the Periodic Table in groups and in order of increasing atomic mass .

  6. Background • Henry Moseley rearranged the Periodic Table in order of increasing atomic number (Z).

  7. Background • Atomic number (Z) • Number of electrons and protons • Ex: Cr 24 • Atomic mass (M) • Z + number of neutrons • Ex: MCr = 52 amu

  8. 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)

  9. Background (Periodic table) • Property of an element depends on the location (family = vertical column; period = horizontal row).

  10. 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

  11. 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

  12. 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

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