The correct increasing order of ionisation enthalpy of `He, Li ^(+), Be ^(2+)` is

Options

A. `He lt Li ^(+) lt Be ^(2+)`
B. `Be ^(2+) lt Li ^(+) lt He`
C. `Li ^(+) lt Be ^(2+) lt He`
D. `Be ^(2+)lt He lt Li ^(+)`

Answer

A

Explanation

Be electronic configuratin `= I s^(2) 2s ^(2)`
li electronic configuration `= 1s ^(2) 2s ^(1)`
He electronic configuratin `= 1s ^(2)`
For ions with 2-electrons, more be the
`(Z_(("Charge proton")))/(e_(("Charge electron")))` ratio, more is the ionisation enthalpy.
`Z/e` for `Be^(2+) =4/2 =2, Z/e ` for `Li ^(+) =3/2 =1.5`
`Z/e` for `He (2)/(2) =1.0`
So, correct order of ionisation enthalpy is `He lt Li ^(+) lt Be ^(2+).`