Overview
This lecture covers the history and structure of the periodic table, focusing on Dmitri Mendeleev’s contributions and the periodic relationships among elements.
Mendeleev and the Creation of the Periodic Table
- Dmitri Mendeleev, a Russian chemist, organized known elements by recurring properties and atomic weights.
- He noticed periodic (repeating) trends in element reactivity and inserted gaps for undiscovered elements.
- Mendeleev predicted the properties of missing elements with remarkable accuracy before their discovery.
- Several contemporaries studied element periodicity, but Mendeleev's insight into missing elements and periodic relationships set him apart.
Organization and Groups of the Periodic Table
- Elements are grouped in the table by similar properties that repeat (are periodic) across periods (rows).
- Alkali metals (far left) are soft, shiny, and highly reactive, forming +1 cations and reacting strongly with halogens.
- Alkaline earth metals (next group) are less reactive than alkali metals, forming +2 cations.
- Transition metals (center block) include common metals like iron and gold; they are good conductors and less reactive.
- Halogens (right side, next to noble gases) are highly reactive nonmetals, forming -1 anions.
- Noble gases (far right) are completely unreactive.
- Lanthanides and actinides (bottom rows) are rare, similar metals often separated from the main table for space.
- Elements in the "staircase" gap (between metals and halogens) can be metals, metalloids, gases, or nonmetals.
Evolution and Design of the Table
- Mendeleev’s periodic table was not the only design—others, like de Chancourtois’s cylinder, were proposed.
- The current table’s layout is for convenience but doesn’t fully represent periodic relationships (like adjacent atomic numbers).
- The structure could be improved, for example, by connecting elements cyclically.
Mendeleev’s Legacy and Impact
- Mendeleev’s table served as a powerful tool for predicting unknown elements and guiding future discoveries.
- The answer to the table’s periodicity lies in the arrangement of electrons, explained after Mendeleev’s time.
Key Terms & Definitions
- Periodic Table — a chart organizing elements by increasing atomic number and recurring chemical properties.
- Periodicity — the recurring trends or patterns in element properties across the table.
- Alkali Metals — elements in group 1, highly reactive, form +1 cations.
- Alkaline Earth Metals — group 2 elements, less reactive, form +2 cations.
- Transition Metals — central block elements, typical metallic properties.
- Halogens — group 17 elements, highly reactive nonmetals, form -1 anions.
- Noble Gases — group 18 elements, inert and unreactive.
- Lanthanides/Actinides — bottom-row rare metals, often shown separately for space.
- Ion — an atom or molecule with an electric charge due to loss or gain of electrons.
- Cation — positively charged ion.
- Anion — negatively charged ion.
Action Items / Next Steps
- Prepare for the next lesson on the role of electrons in explaining periodic trends.