Question

Correct order of radius of the $1$st orbit of $H,{He}^{+},{Li}^{2+}$ and ${Be}^{3+}$ is:

• A. $H>{He}^{+}>{Li}^{2+}>{Be}^{3+}$
• B. ${Be}^{3+}>{Li}^{2+}>{He}^{+}>H$
• C. ${He}^{+}>{Be}^{3+}>{Li}^{2+}>H$
• D. ${He}^{+}>H>{Li}^{2+}>{Be}^{3+}$

Answer 1

The correct option is A $H>{He}^{+}>{Li}^{2+}>{Be}^{3+}$

Correct order of radius of the 1st orbit is $H>H{e}^{+}>L{i}^{2+}>B{e}^{3+}$
These all are isoelectronic species. They have same number of electrons (one). With increase in the atomic number (from H to Be), the effective nuclear charge increases (as more nuclear charge is acting on same number of electrons). Hence, the electron is pushed inside. This decreases the radius.