Question

In which of the following, the elements belong to the same period.
A. ${\text{Ca,}}\,\,{\text{Cr,}}\,\,{\text{Zn}}$
B. ${\text{Cr,}}\,\,{\text{Na,}}\,\,{\text{Pb}}$
C. ${\text{H,}}\,\,{\text{He,}}\,\,{\text{F}}$
D. ${\text{Li,}}\,\,{\text{Na,}}\,\,{\text{K}}$

Answer 1

Hint: Elements belonging to the same period have valence electrons in the energy level of the same principal quantum number. The outermost shell is known as the valence shell.

Complete answer:
Write the electronic configuration of each set as follows:
The atomic number of calcium is 20. The atomic number of chromium is 24 and the zinc is 30.
${\text{Ca}}\,\,{\text{ = }}\,\,{\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{4}}{{\text{s}}^{\text{2}}}$
${\text{Cr}}\,\,{\text{ = }}\,\,{\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{d}}^5}{\text{4}}{{\text{s}}^1}$
${\text{Zn}}\,\,{\text{ = }}\,\,{\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{d}}^{10}}4{s^2}$
The valence shell of calcium, chromium and zinc have principal quantum number 4 so all three calcium, chromium and zinc belong to the same period. So, option (A) is correct.
The atomic number of chromium is 24. The atomic number of sodium is 11 and the lead is 82.
${\text{Cr}}\,\,{\text{ = }}\,\,{\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{d}}^5}{\text{4}}{{\text{s}}^1}$
${\text{Na}}\,\,{\text{ = }}\,\,{\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^1}$
${\text{Pb}}\,\,{\text{ = }}\,\,\left[ {{\text{Xe}}} \right]{\text{4}}{{\text{f}}^{14}}{\text{5}}{{\text{d}}^{10}}{\text{6}}{{\text{s}}^{\text{2}}}{\text{6}}{{\text{p}}^2}$
The valence shell of chromium has principal quantum number 4. The valence shell of sodium has principal quantum number 3 and the valence shell of lead has principal quantum number 6 so all three chromium, sodium and lead belong to the different periods. So, option (B) is incorrect.
The atomic number of hydrogen is 1.The atomic number of helium is 2 and fluorine is 9.
${\text{H}}\,\,{\text{ = }}\,\,{\text{1}}{{\text{s}}^1}$
${\text{He}}\,{\text{ = }}\,\,{\text{1}}{{\text{s}}^{\text{2}}}$
${\text{F}}\,\,{\text{ = }}\,\,{\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^5}$
The valence shell of hydrogen and helium has principal quantum number 1 and the valence shell of fluorine has principal quantum number 2, so hydrogen and helium belong to the same period but fluorine belongs to the different period. So, option (C) is incorrect.
The atomic number of lithium is 3.The atomic number of sodium is 11 and the potassium is 19.
\[{\text{Li}}\,{\text{ = }}\,\,{\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^1}\]
${\text{Na}}\,\,{\text{ = }}\,\,{\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^1}$
\[{\text{K}}\,{\text{ = }}\,{\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{4}}{{\text{s}}^1}\]
The valence shell of lithium has principal quantum number 2. The valence shell of sodium has principal quantum number 3 and the valence shell of potassium has principal quantum number 4 so all three lithium, sodium and potassium belong to the different periods. So, option (D) is incorrect.

So the correct option is (A).

Note: Elements belong to the same period have the same valence shell but different electrons number. Elements belonging to the same group have the same number of electrons in the valence shell. For example, option (D) represents the elements of the same group because all have one electron in the valence shell.