Back to: Inorganic Chemistry 100 Level
Welcome to class!
It’s wonderful to see you today. Let me ask: have you ever watched a secondary school inter-house sports competition? At first, it may seem like students are standing randomly on the field. But once they line up according to houses, colours, and events, everything begins to make sense. Chemistry went through a similar journey—scientists struggled for years to organise the known elements until they finally developed the periodic table we use today. Let’s take that journey together.
Development Of The Periodic Table
Early Attempts
In the early 1800s, only a few elements were known, and scientists wanted to classify them. They noticed that some elements shared similar properties. For example, lithium, sodium, and potassium all reacted violently with water. This suggested that elements could be grouped.
Dobereiner’s Triads (1829)
Johann Dobereiner grouped elements into sets of three called triads, based on their similar chemical properties. The atomic mass of the middle element was roughly the average of the other two. For example:
Lithium (Li), Sodium (Na), Potassium (K).
This was a brilliant start, but it couldn’t include all known elements.
Newlands’ Law of Octaves (1864)
John Newlands arranged elements in order of increasing atomic mass. He noticed that every eighth element had similar properties, just like musical notes repeat every eighth note in an octave. For example, sodium resembled lithium and potassium. However, his idea broke down when heavier elements were considered, and scientists mocked his “musical” approach.
Mendeleev’s Periodic Table (1869)
The real breakthrough came from Dmitri Mendeleev. He arranged elements in order of increasing atomic mass but also placed them in columns (groups) based on similar properties. What made Mendeleev a genius was that he left gaps for undiscovered elements, predicting their properties accurately. For example, he predicted the existence of germanium, which was later discovered with the exact properties he described.
Modern Periodic Table
Later, Henry Moseley (1913) improved Mendeleev’s table by arranging elements in order of atomic number (protons), not atomic mass. This fixed earlier problems, like argon and potassium being in the wrong order when arranged by mass. The modern periodic table is based on Moseley’s work and remains one of the greatest tools in science.
Periodic Law
The modern periodic law states: The properties of elements are a periodic function of their atomic numbers. This means that as you move across the table, patterns in properties repeat regularly.
Why the Periodic Table Matters
The periodic table is like Chemistry’s map. It helps us understand element trends, predict chemical behaviour, and study new elements as they are discovered.
Summary
- Dobereiner: triads based on similar properties.
- Newlands: law of octaves, every 8th element similar.
- Mendeleev: arranged by atomic mass, predicted undiscovered elements.
- Moseley: modern table arranged by atomic number.
- Periodic law: properties are periodic functions of atomic number.
Evaluation
- What was Dobereiner’s contribution to the development of the periodic table?
- Why was Mendeleev considered a genius in his work on the periodic table?
- Who corrected the arrangement by using atomic number instead of atomic mass?
Well done! You’ve just walked through the history of one of science’s greatest achievements. The periodic table is like a family portrait of all known elements, and you now understand how it was carefully developed. Keep shining—Afrilearn is proud of how far you’ve come!