ɢɪᴠᴇ ᴀ sʜᴏʀᴛ sᴜᴍᴍᴀʀʏ ᴏɴ ᴛʜᴇ ʟᴇssᴏɴ ᴘᴇʀɪᴏᴅɪᴄ ᴄʟᴀssɪғɪᴄᴀᴛɪᴏɴ ᴏғ ᴇʟᴇᴍᴇɴᴛs (ᴄᴇᴍɪsᴛʀʏ ᴄʟᴀss X)
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→ When elements are arranged in the order of increasing atomic masses, the element with similar properties occur at regular intervals. ...
→ Mendeleev's periodic table is based on the chemical properties of elements. It contains 7 periods (horizontal rows) and 8 groups (vertical columns).
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Early Models of Periodic Table
Dobereiner’s Triads
Dobereiner arranged a group of three elements with similar properties in the order of increasing atomic masses and called it a triad. He showed that the atomic mass of the middle element is approximately the arithmetic mean of the other two. But, Dobereiner could identify only the following three triads from the elements known at that time.
Newlands’ Law of Octaves
When the elements are arranged according to increasing atomic masses, the physical and chemical properties of every eighth element are similar to that of the first.
Newlands compared these octaves to the series of eight notes of a musical scale.
Assumptions and Limitations:
1. The law was applicable for elements with atomic masses up to 40.
2. Properties of new elements discovered did not fit into the law of octaves.
3. In a few cases, Newlands placed two elements in the same slot to fit elements in the table.
4. He also grouped unlike elements under the same slot.
Features of Mendeleev’s Periodic Table
● Twelve horizontal rows, which were condensed to 7, known as periods.
● Eight vertical columns known as groups.
● Groups I to VII subdivided into A and B subgroups.
● Group VIII doesn’t have any subgroups and contains three elements in each row.
● Elements in the same group exhibit similar properties.