Mendeleev and the Periodic Table

Primary Lesson Objective: To explain and evaluate the principles used by Dmitri Mendeleev to arrange the periodic table of elements.
Key Vocabulary and Concepts: * Atomic Mass: The basis upon which Mendeleev originally ordered the elements. * Periodic Table: The tabular display of the chemical elements. * Periodicity: The quality of occurring at regular intervals; in this context, the repeating nature of chemical and physical properties. * Elements: Pure substances consisting of only one type of atom. * Groups and Periods: The vertical columns and horizontal rows used to categorize elements. * Gaps / Predictions: The intentional spaces left for undiscovered elements and the subsequent estimation of their traits.
Cross-Curricular Links (CCL): The study of the periodic table integrates Knowledge from Mathematics, History, and English.
Tiered Success Criteria: * Description (Basic): Most students should be able to describe how Mendeleev arranged elements by increasing atomic mass and identified repeating patterns. * Explanation (Intermediate): Some students should be able to explain why Mendeleev left gaps for undiscovered elements and how he predicted their specific properties. * Evaluation (Advanced): Few students will be able to evaluate why Mendeleev’s periodic table was revolutionary compared to earlier classification systems used by scientists.

Comparative Element Table: * Lithium: Atomic Mass = $7$ ; Key Property: Soft, reacts with water. * Sodium: Atomic Mass = $23$ ; Key Property: Soft, reacts with water. * Missing Element: Unknown; Key Property: Unknown. * Potassium: Atomic Mass = $39$ ; Key Property: Soft, reacts with water.
Analysis Questions: 1. Identify Patterns: Note the pattern in atomic mass (increasing from $7$ to $23$ to $39$ ) and the consistency in key properties (all are soft and reactive with water). 2. Prediction of the Missing Element: * Atomic Mass: Based on the arithmetic mean or the progression between $23$ and $39$ , students should estimate a value. * Chemical Properties: Given the group characteristics, the missing element would likely be soft and react with water. 3. Reasoning: Logical deduction based on the observed periodicity of the established elements.

The 1869 Organization: Dmitri Mendeleev organized the $63$ known elements in $1869$ .
Criteria for Arrangement: The primary organizational logic was based on increasing atomic mass and the similarity of chemical properties.
Observations on Periodicity: Mendeleev recognized periodic patterns, which are chemical properties that repeat at regular intervals throughout the list of elements.
The Use of Gaps: Mendeleev famously left gaps in his table for elements that were not yet discovered. He used the properties of surrounding elements to predict the characteristics of these missing substances.
Validation of the System: The subsequent discovery of elements like Gallium and Germanium proved the accuracy of Mendeleev's system, as their actual properties closely matched his predictions.
Modern Adaptation: In contemporary science, periodic tables are arranged by atomic number rather than atomic mass. However, the modern table still strictly follows Mendeleev’s original logic of periodicity.
Supplemental Resource: A visual explanation is available via the YouTube link: https://www.youtube.com/watch?v=fPnwBITSmgU (Mendeleev’s Periodic Table Explained).

Quick Check Questions: 1. Who developed the first periodic table of elements? (Answer: Dmitri Mendeleev). 2. On what basis did Mendeleev arrange the elements? (Answer: Atomic mass and chemical properties). 3. What was unique about Mendeleev's periodic table? (Answer: It was predictive and allowed for missing information through gaps). 4. Why were gaps left in his table? (Answer: To preserve the alignment of elements with similar properties when no known element fit the next mass increment). 5. Which property replaced atomic mass in the modern periodic table? (Answer: Atomic number).

Task 1: Fill-in-the-Blanks Summary: Dmitri Mendeleev was the first scientist to create a table of elements based on their chemical and physical characteristics. He organized or arranged the elements in order of increasing atomic mass. Mendeleev noticed that certain properties of elements repeated at regular intervals. This repeating pattern is known as periodic behavior. When an element did not fit the pattern, Mendeleev left gaps in his table. He predicted the properties of elements that had not yet been discovered.
Task 2: Identification of Gaps: Students must identify specific locations where Mendeleev left gaps and suggest theoretical properties for those missing elements.
Task 3: Comparative Analysis: Compare Mendeleev’s original periodic table with the modern version, explaining the transition from atomic mass to atomic number and the reasons for this change.
Extension Research: Investigate elements such as Gallium or Germanium. Students should describe the specific details of Mendeleev's predictions (e.g., "eka-aluminium" or "eka-silicon") and how they matched the empirical data upon discovery.

Element Prediction 1: Hint: I am a shiny metal that melts in your hand. Mendeleev predicted me as eka-aluminium, and I sit below aluminum in the periodic table. (Identity: Gallium).
Element Prediction 2: Hint: I was predicted as eka-silicon. I am shiny like a metal but brittle like a non-metal, and I form compounds similar to silicon dioxide ( $SiO_2$ ). (Identity: Germanium).
Element Prediction 3: Hint: I came right after sodium. My atomic mass is approximately $24$ , and I react slowly with acids to form a white oxide. (Identity: Magnesium).
Interactive Quiz: Additional assessment is available at https://wayground.com/admin/quiz/6971ab6bcd3618ae5ec847.

Critical Questions: A specific question the student still has regarding the lesson.
Prior Knowledge: Two facts or concepts the student already knew before the lesson started.
Future Predictions: Three things the student guesses will be covered in the next lesson.