Problem 138 - Chemistry A Molecular Approach

Question

Discuss these questions with the group and record your consensus answer. The table shown here includes data similar to those used by Mendeleev when he created the periodic table. On a small card, write the symbol, atomic mass, and a stable compound formed by each element. Without consulting a periodic table, arrange the cards so that atomic mass increases from left to right and elements with similar properties are above and below each other. Copy the periodic table you have invented onto a piece of paper. There is one element missing. Predict its mass and a stable compound it might form. $$ \begin{array}{ccc} \text { Element } & \text { Atomic Mass } & \text { Stable Compound } \\ \hline \text { Be } & 9 & \text { BeCI }_{2} \\ \hline \text { S } & 32 & \mathrm{H}_{2} \mathrm{~S} \\ \hline \mathrm{F} & 19 & \mathrm{~F}_{2} \\ \hline \mathrm{Ca} & 40 & \mathrm{CaCl}_{2} \\ \hline \mathrm{Li} & 7 & \mathrm{LiCl} \\ \hline \mathrm{Si} & 28 & \mathrm{SiH}_{4} \\ \hline \mathrm{Cl} & 35.4 & \mathrm{Cl}_{2} \\ \hline \mathrm{B} & 10.8 & \mathrm{BH}_{3} \\ \hline \mathrm{Ge} & 72.6 & \mathrm{GeH}_{4} \\ \hline \mathrm{N} & 14 & \mathrm{NF}_{3} \\ \hline \mathrm{O} & 16 & \mathrm{H}_{2} \mathrm{O} \\ \hline \mathrm{Ga} & 69.7 & \mathrm{GaH}_{3} \\ \hline \hline \text { As } & 75 & \text { AsF }_{3} \\ \hline \text { C } & 12 & \text { CH }_{4} \\ \hline \text { K } & 39 & \text { KCl } \\ \hline \text { Mg } & 24.3 & \text { MgCl }_{2} \\ \hline \text { Se } & 79 & \text { H }_{2} \text { Se } \\ \hline \text { Al } & 27 & \text { AlH }_{3} \\ \hline \text { Br } & 80 & \text { Br }_{2} \\ \hline \text { Na } & 23 & \text { NaCl } \\ \hline \end{array} $$

Step-by-Step Solution

Answer

Arrange the elements in order of increasing atomic mass, group by similarity in compounds, find the gap in the table, predict the missing element's mass and a compound it might form, and then draw the periodic table with your arrangement.

Step 1: Create element cards

Write the symbol, atomic mass, and a stable compound for each element on a small card. This will make it easier to arrange and rearrange the elements physically on a surface.

Step 2: Arrange by atomic mass

Starting with the element with the lowest atomic mass (Li), arrange the cards in a row with increasing atomic mass from left to right.

Step 3: Group by properties

Look for patterns among the stable compounds and atomic masses to group elements with similar properties above and below each other, forming columns.

Step 4: Identify the missing element

Identify a pattern or gap in the arrangement where a card seems to be missing. This will likely fall in a column of elements with similar properties and between elements with incrementally increasing atomic masses.

Step 5: Predict the missing element's properties

Predict the missing element's atomic mass based on the trend seen in the column and a stable compound it might form based on the stable compounds of the neighboring elements.

Step 6: Draw the invented periodic table

Copy the arrangement onto a piece of paper to form your own version of the periodic table, including the predicted missing element.

Key Concepts

Atomic Mass

The atomic mass, also known as atomic weight, is a fundamental property of elements that played a pivotal role in the creation of Mendeleev's periodic table. It represents the average mass of atoms of an element, typically expressed in atomic mass units (amu). In Mendeleev’s time, these values were determined by measuring the masses of elements that compose stable compounds.

When creating his periodic table, Mendeleev arranged the elements in order of increasing atomic mass. This early method of organization helped reveal the periodic nature of the elements, as it showed that after certain intervals, elements with similar properties would reappear. However, it's essential to note that modern tables are arranged by increasing atomic number, due to the discovery that atomic number unequivocally defines the identity of an element, rather than its atomic mass.

Stable Compounds

A stable compound is a chemical compound that is stable under specified conditions, meaning it won't change or decompose easily. These compounds are characterized by their distinct and predictable properties, which are consistent due to the specific configuration of their constituent atoms.

During Mendeleev’s era, analyzing how elements combine to form stable compounds was crucial for understanding the nature of the elements themselves. Compounds such as BeCl₂ and AlH₃ showcase the combining capacity, or valency, of the elements. While arranging elements in his periodic table, Mendeleev looked for patterns among the compounds to group elements with similar valency and reactivity, revealing a part of the periodic law that elements with similar properties reoccur periodically.

Element Properties

Element properties represent various characteristics that can be measured or observed, such as boiling and melting points, reactivity, and electron affinity. Mendeleev's genius was his ability to recognize patterns in these properties, even when the underlying reasons for these patterns were not understood at the time.

Identifying trends in thermodynamic properties like melting and boiling points, or chemical properties such as reactivity with oxygen, allowed Mendeleev to predict the characteristics of unknown elements. This aspect of his table demonstrated the power of periodic organization and was put to the test with the discovery of elements like gallium and germanium, which fit perfectly into the gaps left in Mendeleev’s table, confirming his predictions.

Periodic Trends

Periodic trends are the patterns observed within the periodic table that show a regular recurrence of certain elemental properties. This concept was central to Mendeleev's arrangement of the periodic table and is used today to predict the properties of unknown elements.

One trend is the change in atomic and ionic size across periods and down groups. Electronegativity, ionization energy, and electron affinity also vary in a predictable manner when moving across a period or down a group in the periodic table. By observing these trends, Mendeleev grouped elements with similar properties into columns known as groups or families. Today, these trends help us predict not only the behavior of an element but also how it will interact with others to form compounds.