Comprehensive Study Guide to the History and Development of the Periodic Table

Chapter Overview and Foundational Concepts

• Learning Objectives:

  • Understand the historical contributions of John Dalton and the development of atomic weights.
  • Learn about the initial classification of chemical elements into groups.
  • Comprehend how elements are arranged systematically within the periodic table.
  • Understand that the atomic number of an element depends directly on its atomic structure.
  • Utilize the periodic table to predict the internal structure of an atom.
  • Examine the specific physical and chemical properties of Group $1$ elements.

• Foundational Knowledge Review:

  • Atomic Structure: Atomic structure describes the internal configuration of an atom, including its subatomic particles and arrangement. Atoms bond together in multiple ways to form chemical compounds.
  • Element Definition: An element is a pure chemical substance consisting of atoms that all have the same number of protons.
  • Periodic Table: The periodic table is a tabular arrangement of chemical elements organized by their atomic structure, electron configurations, and recurring chemical properties.

John Dalton and Atomic Weights

• Historical Context of John Dalton:

  • John Dalton ($1766-1844$) was an English chemist who constructed the modern atomic theory and made the earliest attempts to arrange chemical elements in systematic order.
  • Dalton measured the masses of elements liberated when chemical compounds were decomposed.
  • During Dalton's era, the modern term 'mass' was not in use; scientists used the terms 'weight' and 'atomic weight' instead.

• Dalton's Benchmark System and Water Decomposition:

  • Dalton selected the weight of hydrogen as the standard baseline unit ($1$) to compare against the weights of all other chemical elements.
  • Upon decomposing the compound water into hydrogen and oxygen, Dalton observed that the weight of the recovered oxygen was $7$ times greater than the weight of the recovered hydrogen.
  • Dalton assumed that atoms always combine in a simple $1:1$ ratio (one atom of one element combining with exactly one atom of another element).
  • Based on this $1:1$ assumption, Dalton concluded that the atomic weight of hydrogen was $1$ and the atomic weight of oxygen was $7$.

• Errors and Revisions in Atomic Weight Calculations:

  • Dalton was not a highly accurate experimenter, leading to erroneous observations.
  • Subsequent measurements by other scientists demonstrated that the weight of oxygen produced when water splits is actually $8$ times greater than the weight of hydrogen.
  • Water is composed of two hydrogen atoms bonded to one oxygen atom ($H_2O$), yielding a $2:1$ atomic ratio rather than a $1:1$ ratio.
  • Correcting for this ratio established that the true atomic weight of oxygen is $16$

• Early Balance Scales and Measurement Procedures:

  • Quantitative weights were measured using mechanical balance scales.
  • Calibration required adjusting levelling screws to ensure the balance mechanism was perfectly horizontal.
  • A plumb bob hanging down adjacent to the central support was used to verify that the balance pointer aligned vertically downwards with the midpoint of the scale.
  • Operation involved placing chemical samples in one pan and adding reference weights to the opposing pan until the pointer returned precisely to the scale's central midpoint.

• Dalton's Chemical Symbols ($1805$):

  • Dalton was the first scientist to introduce visual symbols for chemical elements, representing elements as customized circles and combining these circles to illustrate compound molecules.
  • In $1805$, Dalton published a table of substances he believed to be elements along with their assigned atomic weights:
  • Hydrogen (circle with central dot): atomic weight $1$
  • Azote (circle with vertical bar; historical name for nitrogen): atomic weight $4$
  • Carbon (filled black circle): atomic weight $5.4$
  • Oxygen (open circle): atomic weight $7$
  • Phosphorus (circle with three radiating lines): atomic weight $9$
  • Sulphur (circle with internal cross): atomic weight $13$
  • Magnesia (circle with internal 'S' or vertical line): atomic weight $20$
  • Lime (circle with diagonal line): atomic weight $24$
  • Soda (circle with horizontal line): atomic weight $28$
  • Potash (circle with three vertical lines): atomic weight $42$
  • Strontian: atomic weight $46$
  • Barytes: atomic weight $68$
  • Iron (circle with 'I'): atomic weight $50$
  • Zinc (circle with 'Z'): atomic weight $56$
  • Copper (circle with 'C'): atomic weight $56$
  • Lead (circle with 'L'): atomic weight $90$
  • Silver (circle with 'S'): atomic weight $190$
  • Gold (circle with 'G'): atomic weight $190$
  • Platina (circle with 'P'; historical name for platinum): atomic weight $190$
  • Mercury (circle with internal dots): atomic weight $167$
  • Six of the entries in Dalton's $1805$ list were actually chemical compounds rather than elements: Water, Lime, Soda, Potash, Magnesia, and Barytes (along with Strontian).

Early Classification of Elements: Johann Wolfgang Döbereiner

• Döbereiner's Triads ($1829$):

  • Johann Wolfgang Döbereiner ($1780-1849$) was a German chemist who analyzed Dalton's work.
  • By $1829$, more than $12$ new chemical elements had been discovered.
  • Döbereiner classified elements into groups of three based on similarities in their atomic weights, physical properties, and chemical behaviors.
  • These three-element groupings were named Döbereiner's Triads.

• Specific Triad Groupings:

  • Alkali Metal Triad: Lithium ($Li$), Sodium ($Na$), Potassium ($K$).
  • Halogen Triad: Chlorine ($Cl$), Bromine ($Br$), Iodine ($I$).
  • Alkaline Earth Metal Triad: Calcium ($Ca$), Strontium ($Sr$), Barium ($Ba$).

Periodic Patterns in Element Properties: John Newlands

• Law of Octaves and Periodic Recurrence:

  • John Newlands ($1837-1898$) was an English chemist who attempted to organize the elements after an additional $10$ elements were discovered beyond Döbereiner's work.
  • Newlands arranged all known elements in strict order of increasing atomic weight, starting from the lightest.
  • He observed that chemical and physical properties repeated periodically every eight elements.

• Order of Elements by Atomic Weight:

  • Newlands' ordered sequence: Lithium, Beryllium, Boron, Carbon, Nitrogen, Oxygen, Fluorine, Sodium, Magnesium, Aluminium, Silicon, Phosphorus, Sulfur, Chlorine, Potassium, Calcium.

• Periodic Table Rows 2, 3, and 4 Data:

  • Row 2: Lithium ($Li$, atomic weight $6.94$), Beryllium ($Be$, atomic weight $9.01$), Boron ($B$, atomic weight $10.8$), Carbon ($C$, atomic weight $12.0$), Nitrogen ($N$, atomic weight $14.0$), Oxygen ($O$, atomic weight $16.0$), Fluorine ($F$, atomic weight $19.0$), Neon ($Ne$, atomic weight $20.2$).
  • Row 3: Sodium ($Na$, atomic weight $23.0$), Magnesium ($Mg$, atomic weight $24.3$), Aluminium ($Al$, atomic weight $27.0$), Silicon ($Si$, atomic weight $28.1$), Phosphorus ($P$, atomic weight $30.1$), Sulfur ($S$, atomic weight $32.1$), Chlorine ($Cl$, atomic weight $35.5$), Argon ($Ar$, atomic weight $39.9$).
  • Row 4: Potassium ($K$, atomic weight $39.1$), Calcium ($Ca$, atomic weight $40.1$).

• Post-Newlands Discoveries: Insertion of Noble Gases:

  • Neon ($Ne$) and Argon ($Ar$) were discovered after Newlands completed his initial arrangements.
  • Neon ($Ne$) was inserted between Fluorine ($F$) and Sodium ($Na$).
  • Argon ($Ar$) was inserted between Chlorine ($Cl$) and Potassium ($K$).
  • The number of elements spanning between Lithium and Sodium, as well as between Sodium and Potassium, demonstrates repeating periodic intervals.

Dmitri Mendeleev and the Development of the Periodic Table

• Mendeleev's Periodic System ($1834-1907$):

  • Dmitri Mendeleev was a Russian scientist who synthesized and expanded upon the findings of John Dalton and John Newlands.
  • Mendeleev arranged elements in order of increasing atomic weight while simultaneously analyzing their combining capacities (valencies).

• Atomic Combining Ratios and Valency Patterns:

  • Mendeleev disproved Dalton's assumption of universal $1:1$ combining ratios, recognizing that atoms of an element could combine with $2$, $3$, $4$, or more atoms of another element.
  • Mendeleev tracked combining ratios across the element sequence:
  • Lithium ($Li$): combines with $1$ atom.
  • Beryllium ($Be$): combines with $2$ atoms.
  • Boron ($B$): combines with $3$ atoms.
  • Carbon ($C$): combines with $4$ atoms.
  • Subsequent elements: combining capacity falls systematically to $3$, then $2$, then $1$ before rising again.
  • This established a cyclic, periodic rise-and-fall pattern of atomic combining capacities ($1 \rightarrow 2 \rightarrow 3 \rightarrow 4 \rightarrow 3 \rightarrow 2 \rightarrow 1$).

• Handwritten Early Manuscripts:

  • Mendeleev's early handwritten draft titled "Essai d'une système des éléments d'après leurs poids atomiques et fonctions chimiques par M. D. Mendeleeff" illustrates his continuous revisions of atomic weights and element placements as accurate empirical data became available.

Classification Activity: Physical and Chemical Properties of Nine Elements

• Properties of Test Elements (Table 7.1):

  • Element A: Soft, silvery solid; fizzes vigorously when added to water.
  • Element B: Green gas; does not conduct electricity.
  • Element C: Forms chemical compounds that burn with a dark red (crimson) flame in a Bunsen burner.
  • Element D: Red liquid; does not conduct electricity.
  • Element E: Soft, silvery solid; sets on fire upon contact with water and may explode.
  • Element F: Forms chemical compounds that burn with a green/yellow flame in a Bunsen burner.
  • Element G: Forms chemical compounds that burn with a bright red flame in a Bunsen burner.
  • Element H: Soft, silvery solid; fizzes when added to water and produces an orange flame.
  • Element I: Black solid; does not conduct electricity.

• Classification Groupings derived from Table 7.1:

  • Group 1 (Reactive Soft Metals / Alkali Metals): Elements A, E, and H.
  • Group 2 (Non-conducting Elements / Halogens): Elements B, D, and I.
  • Group 3 (Flame-Test Color Compounds / Alkaline Earth Metals): Elements C, F, and G.

Questions & Discussion

• Review Question: What is an atom?

  • Response: An atom is the basic unit of a chemical element, consisting of a nucleus surrounded by electrons.

• Review Question: Describe the structure of the atom.

  • Response: An atom consists of a central, dense nucleus containing protons and neutrons, orbited by electrons in defined energy shells.

• Review Question: Are all atoms the same? Explain your answer.

  • Response: No, atoms differ in their numbers of protons, neutrons, and electrons, which defines their distinct chemical elements and isotopes.

• Review Question: What is an element?

  • Response: An element is a pure substance made up of only one type of atom that cannot be broken down into simpler substances by chemical means.

• Review Question: What is the periodic table?

  • Response: The periodic table is a tabular arrangement of chemical elements ordered by atomic number, electron configuration, and recurring chemical properties.

• Question 1: What was the inaccurate observation that Dalton made?

  • Response: Dalton inaccurately observed that water is composed of one atom of hydrogen and one atom of oxygen in a $1:1$ ratio, measuring the mass ratio of oxygen to hydrogen as $7:1$ rather than $8:1$

• Question 2: What inaccurate conclusion did Dalton make from his studies?

  • Response: Dalton concluded that the atomic weight of oxygen was $7$ relative to hydrogen's atomic weight of $1$

• Question 3: How easy would it be to make errors using the balance scales shown in Figure 7.2? Explain your answer.

  • Response: It would be very easy to make errors because the apparatus required multiple manual adjustments. Operators had to precisely adjust levelling screws and visually align a hanging plumb bob with the central support to confirm that the pointer was pointed directly vertical (down) on the scale's exact midpoint before taking measurements.

• Challenge Yourself Exercise: Balance Scale Design:

  • Task: Construct a physical balance scale model inspired by early balance scales. Utilize household or lab items, produce a fully labelled diagram, and apply principles of moments ($\text{Torque} = \text{Force} \times \text{Distance}$) to ensure equal arm balancing and accurate weight measurement.

• Question 4: 'Azote' is another name for nitrogen and 'platina' is another name for platinum, but there are six names in Dalton's list that are actually compounds and not elements. Which are they?

  • Response: The six compounds in Dalton's $1805$ list are Water, Lime, Soda, Potash, Magnesia, and Barytes (along with Strontian).

• Question 5: Use secondary sources to find the symbols that we use today for the elements listed in Figure 7.3.

  • Response: Today's modern IUPAC chemical symbols for the elements listed are:
  • Hydrogen: $H$
  • Azote (Nitrogen): $N$
  • Carbon: $C$
  • Oxygen: $O$
  • Phosphorus: $P$
  • Sulphur: $S$
  • Iron: $Fe$
  • Zinc: $Zn$
  • Copper: $Cu$
  • Lead: $Pb$
  • Silver: $Ag$
  • Gold: $Au$
  • Platina (Platinum): $Pt$
  • Mercury: $Hg$

• Question 6: In what way was Dalton's work useful as evidence to Döbereiner and Newlands?

  • Response: Dalton's work established the concept of atomic weights and provided an initial quantitative baseline, giving Döbereiner and Newlands a numerical foundation to group elements and identify periodic patterns.

• Question 7: Could Döbereiner and Newlands attempt a more detailed sorting out of the elements than Dalton because they had more data? Explain your answer.

  • Response: Yes, because by the time of Döbereiner and Newlands, many more elements had been discovered (over $12$ additional elements for Döbereiner, and $10$ more for Newlands) and more accurate atomic masses had been experimentally determined.