| Mendeleev's Periodic Table | Modern Periodic Table |
|---|---|
| $1$. Elements are arranged in the increasing order of their atomic masses. | $1$. Elements are arranged in the increasing order of their atomic numbers. |
| $2$. There are a total of $7$ groups and $6$ periods. | $2$. There are a total of $18$ groups and $7$ periods. |
| $3$. Elements with similar properties were placed in the same group. | $3$. Elements with the same number of valence electrons are placed in the same group. |
| $4$. The position of hydrogen was ambiguous. | $4$. Hydrogen is placed in group $1$ above alkali metals. |
| $5$. No clear distinction between metals and non-metals. | $5$. Metals are on the left and non-metals are on the right. |
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