Laboratory 05 Periodic Trends Densities in the Chromium

Laboratory 05 Periodic Trends : Densities in the Chromium Family of Transition Metals 1

Objectives • Measure the bulk densities and calculate atomic densities of the three transition elements in the same raw of the periodic table. • Relate measured density to atomic size, a periodic trend. 2

Periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number. Horizontal rows – periods Vertical columns – groups

Periodic table blocks The s-block comprises the first two groups (alkali metals and alkaline earth metals) as well as hydrogen and helium. The p-block comprises the last six groups which are groups 13 through 18 in IUPAC (contains all of the semimetals). The d-block comprises groups 3 through 12 in IUPAC (contains all of the transition metals. ) The f-block, usually offset below the rest of the periodic table, comprises the lanthanides and actinides. 4

Atomic number (z) Each chemical element has a unique atomic number Z = The number of protons in the nucleus = number of electrons in the atom Atomic mass = Z + the number of neutron in the nucleus 5

Groups A group or family is a vertical column in the periodic table. Elements in the same group show patterns in atomic radius, ionization energy, and electronegativity. From top to bottom in a group, the atomic radii of the elements increase. Since there are more filled energy levels, valence electrons are found farther from the nucleus. From the top, each successive element has a lower ionization energy because it is easier to remove an electron since the atoms are less tightly bound. 6

Periods A period is a horizontal row in the periodic table. Elements in the same period show trends in atomic radius, ionization energy, electron affinity, and electronegativity. Left to right across a period, atomic radius usually decreases. This occurs because each successive element has an added proton and electron which causes the electron to be drawn closer to the nucleus. This decrease in atomic radius also causes the ionization energy to increase when moving from left to right across a period. The more tightly bound an element is, the more energy is required to remove an electron. Electronegativity increases in the same manner as ionization energy because of the pull exerted on the electrons by the nucleus 7

Bulk density is a property of powders, granules and solids. It is defined as the mass of many particles of the material divided by the total volume they occupy. The total volume includes particle volume, inter-particle void volume and internal pore volume. 8

Atom Density The atom density is the number of atoms of a given type per unit volume of the material. N = n = ρ NA V M n= The number of atoms M = Atomic weight NA= Avogadro number = 6. 022× 1023 atoms per gram-mole) ρ = Density V = Volume 9

% Volume filled = Bulk density Atom density 68 % of volume filled by atoms 74 % of volume filled by atoms 10