Question
Explain the outline of the modern periodic table.
| Period number | Number of valence shell | Type of complete shell and elements | Number of elements |
| First | 1 | $\mathrm{K}\left({ }_1 \mathrm{H} \text { to }{ }_2 \mathrm{He}\right)$ | 2 |
| Second | 2 | $\mathrm{L}\left({ }_3 \mathrm{Li}\right.$ to $\left.{ }_{10} \mathrm{Ne}\right)$ | 8 |
| Third | 3 | $\mathrm{M}\left({ }_{11} \mathrm{Na}\right.$ to $\left.{ }_{18} \mathrm{Ar}\right)$ | 8 |
| Fourth | 4 | $\mathrm{N}\left({ }_{19} \mathrm{~K}\right.$ to $\left.{ }_{36} \mathrm{Kr}\right)$ | 18 |
| Fifth | 5 | $\mathrm{O}\left({ }_{37} \mathrm{Rb}\right.$ to $\left.{ }_{54} \mathrm{Xe}\right)$ | 18 |
| Sixth | 6 | $\mathrm{P}\left({ }_{55} \mathrm{Cs}\right.$ to $\left.{ }_{86} \mathrm{Rn}\right)$ | 32 |
| Seventh | 7 | $- (_{87}Fr$ to $...)$ | - |
| Atomic number | Symbol of element | Name of element | Electronic configuration |
| 1 | H | Hydrogen | 1 |
| 3 | Li | Lithium | 2, 1 |
| 11 | Na | Sodium | 2, 8, 1 |
| 19 | K | Potassium | 2, 8, 8, 1 |
| 37 | Rb | Rubidium | 2, 8, 8, 18, 1 |
| 55 | Cs | Cesium | 2, 8, 8, 18, 18, 1 |
| 87 | Fr | Francium | 2, 8, 8, 18, 18, 32, 1 |
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