### JEE Mains Previous Years Questions with Solutions

4.5
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1

### AIEEE 2005

Of the following sets which one does NOT contain isoelectronic species?
A
$BO_3^{3 - }$, $CO_3^{2 - }$, $NO_3^{- }$
B
$SO_3^{2 - }$, $CO_3^{2 - }$, $NO_3^{-}$
C
$CN^{- }$, $N_2$, $C_2^{2 - }$
D
$PO_4^{3 - }$, $SO_4^{2 - }$, $ClO_4^{ - }$

## Explanation

In $SO_3^{2 - }$ no of electrons = 16 + 24 + 2 = 42

In $CO_3^{2 - }$ no of electrons = 6 + 24 + 2 = 32

In $NO_3^{-}$ no of electrons = 7 + 24 + 1 = 32

So they are not isoelectronic.
2

### AIEEE 2004

Which one of the following sets of ions represents the collection of isoelectronic species? (Atomic nos. : F = 9, Cl = 17, Na = 11, Mg = 12, Al = 13, K = 19, Ca = 20, Sc = 21)
A
K+, Cl-, Mg2+, Sc3+
B
Na+, Ca2+, Sc3+, F-
C
K+, Ca2+, Sc3+, Cl-
D
Na+, Mg2+, Al3+, Cl-

## Explanation

Isoelectronic means those species whose no of electrons are same.

Ions Atomic No No of Electron
K+ 19 19-1=18
Ca+2 20 20-2=18
Sc+3 21 21-3=18
Cl- 17 17+1=18

So, option $(C)$ is correct.
3

### AIEEE 2004

The wavelength of the radiation emitted when in a hydrogen atom electron falls from infinity to stationary state 1, would be (Rydberg constant = 1.097 $\times$ 107 m-1)
A
406 nm
B
192 nm
C
91 nm
D
9.1 $\times$ 10-8 nm

## Explanation

We know Rydberg formula,

${1 \over \lambda } = R \times {Z^2}\left( {{1 \over {n_1^2}} - {1 \over {n_2^2}}} \right)$

[ for hydrogen atom Z = 1 ]

= $1.097 \times {10^7}\left( {{1 \over {{1^2}}} - {1 \over {{\infty ^2}}}} \right)$

$\therefore$ $\lambda$ = ${1 \over {1.097 \times {{10}^7}}}$

= 9.11 $\times$10-8 m

= 91.1 $\times$10-9 m

= 91.1 nm

[ as 1 nm = 10-9 m ]
4

### AIEEE 2004

Consider the ground state of Cr atom (Z = 24). The number of electrons with the azimuthal quantum numbers, l = 1 and 2 are respectively
A
16 and 4
B
12 and 5
C
12 and 4
D
16 and 5

## Explanation

Electronic configuration of Cr (Z = 24) =

1s2 2s2 2p6 3s2 3p6 4s1 3d5

For 2p6 and 3p6 , l = 1. Here in 2p and 3p orbital total 6 + 6 = 12 electrons present.

For 3d5 , l = 2. Here in 3d orbital total 5 electrons present.