Atoms with Similar Properties—Classification Schemes

  1. Introduction
  2. Objectives
  3. Materials
  4. Vocabulary
  5. Presentation

    Table 2 represents the (slightly enhanced) facts about the chemical elements as known to Mandeleev in 1868. The instructor must provide each student (or group of students) with a pile of index cards. Each card should contain the data from one of the cells, possibly formatted as in Figure 1. The card pile should have 67 different cards representing every element known to Mandeleev. After a little introduction (triplets and octets section below), ask the students to arrange the cards in some order that will enhance our understanding about the elements. Since the students probably know about the periodic table already, they should be able to arrange the cards quickly. Then ask them to devise some interpretations and uses for the table as it existed in Mendeleev's era.

    Triplets and Octets

    In 1829, Johann Wolfgang Döbereiner observed that many of the elements could be grouped into triads (groups of three) based on their chemical properties. Lithium, sodium, and potassium, for example, were grouped together as being soft, reactive metals. Döbereiner also observed that, when arranged by atomic weight, the second member of each triad was roughly the average of the first and the third. This became known as the Law of Triads.

    English chemist John Newlands produced a series of papers in 1864 and 1865 that described his own classification of the elements: he noted that when listed in order of increasing atomic weight, similar physical and chemical properties recurred at intervals of eight, which he likened to the octaves of music This Law of Octaves, however, was ridiculed by his contemporaries, and the Chemical Society refused to publish his work. Nonetheless, Newlands was able to draft an atomic table and use it to predict the existence of missing elements, such as germanium. The Chemical Society only acknowledged the significance of his discoveries some five years after they credited Mendeleev.

    Symbol: F
    Name: Florine
    Atomic Weight: 19
    Properties: Halogen

    Figure 1. Card format for each card in the deck.

    The properties are indicated by a single word and the instructor should provide the definition of each of the words as listed below:

    1. Hydrogenic—Hydrogen sometimes behaves like an alkali metal, sometimes like a halogen and sometimes like a transition metal. Because of its schizophrenic behavior, we assign it a class of its own.
    2. Alkali metal—The alkali metals are all highly reactive and are never found in elemental forms in nature. Because of this, they are usually stored in mineral oil or kerosene (paraffin oil). They also tarnish easily and have low melting points and densities. Physically, the alkali metals are mostly silver-colored, except for metallic cesium, which can have a golden tint. These elements are all soft metals of low density. Chemically, all of the alkali metals react aggressively with the halogens to form ionic salts. They all react with water to form strongly alkaline hydroxides. The vigor of reaction increases down the group.
    3. Alkaline earth metal—The alkaline earth metals are silver colored, soft metals, which react readily with halogens to form ionic salts, and with water, though not as rapidly as the alkali metals, to form strong alkaline (basic) hydroxides. Beryllium is an exception: It does not react with water or steam. The alkaline earth metals are named after their oxides, the alkaline earths, whose old-fashioned names were beryllia, magnesia, lime, strontia, and baryta. "Earth" is an old term applied by early chemists to nonmetallic substances that are insoluble in water and resistant to heating—properties shared by these oxides.
    4. Transition metal—The transition metals generally form compounds in many oxidation states. They from paramagnetic compounds. They often act as catalysts for other chemical reactions. They are conductors of electricity and generally posses a high density, high melting point and high boiling point. Mercury is a notable exception.
    5. Post-transition metal—The post-transition metals have melting and boiling points generally lower than that of the transition metals. Their electronegativity is higher and they are softer than transition metals.
    6. Metaloid—The metaloids are solid with a metallic luster but brittle. The have densities lower than post-transition metals but higher than non-metals. They are intermediate in their electrical and thermal conductivity.
    7. Non-metal—The non-metals are poor conductors of heat and electricity. they form acidic oxides. In solid form they are dull and brittle. They usually have lower densities than metals. They have significantly lower melting and boiling points then metals (with the exception of Carbon).
    8. Halogen—Halogens are highly reactive, and as such can be harmful or lethal to biological organisms in sufficient quantities. Fluorine is one of the most reactive elements in existence, attacking otherwise inert materials such as glass, and forming compounds with the heavier noble gases. It is a corrosive and highly toxic gas. The halogens all form binary compounds with hydrogen known as the hydrogen halides, a series of particularly strong acids.
    H
    Hydrogen
    1
    Hydrogenic
    Li
    Lithium
    7
    Alkali metal
    Be
    Beryllium
    9.4
    Alkaline earth metal
    B
    Boron
    11
    Metaloid
    C
    Carbon
    12
    Non-metal
    N
    Nitrogen
    14
    Non-metal
    O
    Oxygen
    16
    Non-metal
    F
    Fluorine
    19
    Halogen
    Na
    Sodium
    23
    Alkali metal
    Mg
    Magnesium
    24
    Alkaline earth metal
    Al
    Aluminum
    27.3
    Post-transition metal
    Si
    Silicon
    28
    Metaloid
    P
    Phosphorus
    31
    Non-metal
    S
    Sulfur
    32
    Non-metal
    Cl
    Chlorine
    35.
    Halogen
    Fe
    Iron
    56,
    Transition Metal
    K
    Potassium
    39
    Alkali metal
    Ca
    Calcium
    40
    Alkaline earth metal
    ?
    ?
    44
    Transition Metal
    Ti
    Titanium
    48
    Transition Metal
    V
    Vanadium
    51
    Transition Metal
    Cr
    Chromium
    52
    Transition Metal
    Mn
    Manganese
    55
    Transition Metal
    Co
    Cobalt
    59
    Transition Metal
    Cu
    Copper
    63
    Transition metal
    Zn
    Zinc
    65
    Transition Metal
    ?
    ?
    68
    Post-transition metal
    ?
    ?
    72
    Metaloid
    As
    Arsenic
    75
    Metaloid
    Se
    Selenium
    78
    Non-metal
    Br
    Bromine
    80
    Halogen
    Ni
    Nickel
    59
    Transition Metal
    Rb
    Rubidium
    85
    Alkali metal
    Sr
    Strontium
    87
    Alkaline earth metal
    ?Yt
    Yttrium
    88
    Transition metal
    Zr
    Zirconium
    90
    Transition metal
    Nb
    Niobium
    94
    Transition metal
    Mo
    Molybdenum
    96
    Transition metal
    ?
    ?
    100
    Transition Metal
    Ru
    Ruthenium,
    104
    Transition metal
    Ag
    Silver
    108
    Transition metal
    Cd
    Cadmium
    112
    Transition metal
    In
    Indium
    113
    Post-transition metal
    Sn
    Tin
    118
    Post-transition metal
    Sb
    Antimony
    122
    Metaloid
    Te
    Tellurium
    125
    Metaloid
    J
    Iodine
    127
    Halogen
    Rh
    Rhodium
    104
    Transition metal
    Cs
    Cesium
    133
    Alkali metal
    Ba
    Barium
    137
    Alkaline earth metal
    ?Di
    ?
    138
    Lanthanide
    ?Ce
    Cerium
    140
    Lanthanide
    Pd
    Palladium
    106
    Transition metal
    ?Er
    Erbium
    178
    Lanthanide
    ?La
    Lanthanum
    180
    Lanthanide
    Ta
    Tantalum
    182
    Transition metal
    W
    Tungsten
    184
    Transition metal
    Os
    Osmium, 
    195
    Transition metals
    Au
    Gold
    199
    Transition metal
    Hg
    Mercury
    200
    Transition metal
    Tl
    Thallium
    204
    Post-transition metal
    Pb
    Lead
    207
    Post-transition metal
    Bi
    Bismuth
    208
    Post-transition metal
    Ir
    Iridium, 
    197
    Transition metals
    Pt
    Platinum
    198
    Transition metals
    Th
    Thorium
    231
    Actinide
    U
    Uranium
    240
    Actinide

    Table 2. This table represents the knowledge of the elements as known to Mandeleev in 1861.

    Just for fun, here is Mandeleev's original table.

    Mandeleev's original periodic table
  6. Evaluation