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P-1 Periodic Classification of Element — Science STD 10 — Question

Maharashtra BoardEnglish MediumSTD 10ScienceP-1 Periodic Classification of Element2 Marks
Question
The third period contains only eight elements even through the electron capacity of the third shell is $18$.
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Answer

(1) In the modern periodic table, there are seven horizontal rows called periods. In a periods elements are arranged in an increasing order of their atomic numbers. The third row contains 8 elements and the electron capacity of the third shell is $18$.
(2) In the third period, while moving from left to right, atomic number increases, number of electrons increases in the shell. The number of elements present in $3^{rd}$​​​​​​​ period is decided on the basis of electronic configuration and octet rule.
Atomic number $11$ $12$ $13$ $14$ $15$ $16$ $17$ $18$
Elements Na Mg Al Si P S Cl Ar
Argon (Ar) is the last element of the third period and has a capacity of maximum $18$ electrons. Its octet of electrons is completed and as argon belongs to zero group, the third shell contains 18 electrons.

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