Laboratory 05
Periodic Trends : Densities in the
Chromium Family of Transition Metals
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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
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.
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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
Atomic mass = Z + the number of neutron in the nucleus
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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.
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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
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Bulk density
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.
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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
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% Volume filled = Bulk density
Atom density
68 % of volume
filled by atoms
74 % of volume
filled by atoms
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