**Class 11 Chemistry Chapter Structure of Atom Important MCQ’s:**

At infinity the energy difference between two

Adjacent orbits becomes almost:

1.Increased

2.Decreased

3.Infinite

4. Zero

Answer ) Zero

**Brief Explanation :**

**The difference of energy between two consecutive orbits in an atom decreases with increase in their distance from the nucleus i.e. E****2****>- E****1**** > E****3****-E****2**** > E****4****-E****3**** > …..**

**At infinity, this difference of energy becomes zero.**

The splitting of single spectral line into several thinner closely spaced lines in a magnetic field is known as:

1.Stark effect

2.Tunnel effect

3.Zeeman effect

4.Leveling effect

Answer ) Zeeman effect

**Brief Explanation :**

**Zeeman found the splitting of a spectral line into further thinner closely spaced lines in a magnetic field. An energy level ( s,p,d,f) gets different orientations ( orbitals) in a space along x, y and z –axes in presence of a magnetic field .This effect is known as Zeeman effect after his name.**

De-Broglie’s equation λ = h/mv describes:

1.Wave nature of light

2.Particle nature of light

3.Particle nature of matter

4.Dual nature of matter

Answer ) Dual nature of matter

**Brief Explanation :**

De- Broglie proposed that if light has a dual nature i.e. sometimes acts as a wave and sometimes as a particle ,then a material particle like electron should also have a dual nature. The de-Broglie’s equation λ = h/mv correlates the wavelength and mass , hence it describes the dual nature of matter.

An orbital may be:

1.Dumb-bell shaped

2.Double Dumb-bell shaped

3.Spherically symmetrical

4.All of these

Answer ) All of these

**Brief Explanation :**

**An s-orbital is spherically symmetrical and represented by a circle, with its center at the nucleus .A p-orbital has dumb-bell shape i.e. with 2 lobes on the opposite sides of the nucleus. Each d-orbital has double dumb-bell shape i.e. has four lobes . The shape of an f- orbital is very complicated.**

Number of orbitals in N- shell of an atom is:

- 3

2. 4

3. 9

4. 16

Answer ) 16

**Brief Explanation :**

**The number of sub-shells ( s,p,d,f) in an orbit is always equal to the shell number (n) .The number of orbitals in an orbit can be calculated by the formula “n ”**

**Where n = shell number**

**Number of orbitals for :**

**n= 1 (K –shell ) = (1) = 1**

** n= 2 (L –shell ) = (2) = 4**

** n= 3 (M –shell ) = (3) = 9**

** n= 4 (N –shell ) = (4) = 16**

An orbital that will fill first with electrons is:

1.3d

2.4p

3.5s

4.None of these

Answer ) 3d

**Brief Explanation :**

**When different orbitals have the same (n+** l **) values, then an orbital of lower n-value is more stable and filled first with electrons.**

**For example ,the sequence of filling of 3d ,4p & 5s orbital is:**

**For 3d orbital , n=3 , **l **=2 and ( n+ **l** ) = 3+2 = 5**

**For ****4p ****orbital , ****n=4 ****, **l **=1 ****and ( n+ **l** ****) = ****4+1 ****= ****5**

**For 5s orbital , n=5 , **l **=0 and ( n+ **l** ) = 5+0 = 5**

Maximum number of electrons in energy level n = 3 and l =2 is:

1.2

2.6

3.10

4.14

Answer ) 10

**Brief Explanation :**

**The maximum number of electrons in s,p,d,f subshells can be calculated by the formula**

**2 ( 2** l **+1).**

**Thus the maximum number of electrons in**

**n=3,**l **=2**

**(3d subshell) = 2 ( 2x 2+1) =10**

The number of orientations for an energy level n = 2 and l =1 is:

1.One

2.Three

3.Five

4.Seven

Answer ) Three

**Brief Explanation :**

**The total number of orientations / m-values (orbitals) for s,p,d,f energy levels can be calculated by the formula :**

** (2** l **+1)**

**When **l **=0 (s-orbital ) , m-vale = (2** l **+1) = (2×0+1) = 1**

l **=0 (s-orbital ) , m-vale = (2** l **+1) = (2×0+1) = ****1**

l **=1 (p-orbital ****) , m-vale = (2** l **+1) = (****2×1+1****) = 3**

l **=2 (d-orbital ****) , m-vale = (2** l **+1) = (****2×2+1****) = 5**

l **=3 (f-orbital ) , m-vale = (2** l **+1) = (2×3+1) = 7**

Principal quantum number (n) describes which property of an electron cloud:

1.Energy

2.Shape

3.Orientation

4.Spinning

Answer ) Energy

**Brief Explanation :**

Principal quantum number (n) describes the energy and size of an electron cloud in term of its distance from the nucleus i.e indicates shell number .

**Greater the “n” value, more is the distance of an electron cloud (energy level) from the nucleus and hence greater is the energy and size.**

Azimuthal quantum number (l) gives:

1.Shell number

2.Orientations of sub-shell in magnetic field

3.Sub-shell of the shell

4.Splitting of the spectral line in magnetic field

Answer ) Sub-shell of the shell

**Brief Explanation :**

Azimuthal quantum number (l) describes the spatial distribution (shape ) of electron cloud in space around the nucleus i.e. indicates a sub-shell.

**It can have value 0,1,2 & 3 denoted by s (sharp) , p ( principle) ,d (diffused) & f (fundamental) symbols respectively . It explains the fine structure of an atom in term of sub-shells.**

Magnetic quantum number (m) describes orientation of an electron cloud in magnetic field and explains:

1.Angular momentum of electron

2.Zeeman effect

3.Fine structure of an atom

4.Stark effect

Answer ) Zeeman effect

**Brief Explanation :**

Magnetic quantum number (m) describes orientation of a sub-shell in space in the presence of magnetic field.

**The number of orientations of a sub-shell indicates the number of orbitals in a sub-shell.**

**It explains the splitting of spectral lines in magnetic field (Zeeman effect) in term of different alignments in magnetic field . It is also called as orbital orientation quantum number.**

Pick up the incorrect formula:

1.Number of orbitals in a shell = n^{2}

2.Number of electrons in a shell = 2 (2 n^{2} )

3.Number of electrons in a sub-shell = 2(2 l +1)

4.Number of orientations of a sub-shell = (2 l +1)

Answer ) Number of electrons in a shell = 2 (2 n^{2} )

**Brief Explanation :**

** The maximum number of electrons in energy level is determined by the formula “ 2n ^{2 “ ,} where n= energy level / shell number.**

A set of quantum numbers for an electron of hydrogen atom in its ground state:

1.n= 1 , l = 1 , m= 0 ,s = -1/2

2.n= 1 , l = 1 , m= +1 ,s = +1/2

3.n= 1 , l = 0 , m= 0 ,s = -1/2

4.n= 1 , l = 0 , m= +1 ,s = +1/2

**Answer ) n= 1 , L = 0 , m= +1 ,s = +1/2**

**Brief Explanation :**

** The electronic configuration of hydrogen atom is 1s ^{1} . The set of quantum numbers for an electron of H-atom in its ground state is ; **

n= 1 , l = 0 , m= 0 , s = +1/2

The set of quantum numbers n, l , m and s for unpaired valence electron of chlorine atom is:

1. n= 3 , l = 3 , m= +1 ,s = +1/2

2.n= 3 , l = 2 , m= 0 ,s = -1/2

3.n= 3 , l = 1 , m= +1 ,s = +1/2

4.n= 3 , l = 2 , m= +1 ,s = -1/2

Answer ) n= 3 , l = 1 , m= +1 ,s = +1/2

**Brief Explanation :**

** **** The electronic configuration of chlorine atom is 1s**^{2}** , 2**** ****s**^{2}** , 2p**** **^{6}** , 3s**** **^{2}** , 3p**^{5 .}

**The last electron of chlorine atom is in 3p-orbital i.e. 3px**** **^{2 ,}** 3py **** **^{2}** **** , 3pz**** **^{1.}

**Thus the set of quantum numbers for the valence electron of Cl –atom is:**

n= 3 , l = 1 , m= +1 ,s = +1/2

Pick up the incorrect statement:

1. Electron are filled into various sub-energy levels in the order of their increasing energy.

2.When several degenerate orbitals are available, an electron prefers to enter an empty orbital instead of half-filled orbital. 3.Two electrons of the same orbital can not have the same set of four quantum numbers.

4.An orbital has sharp boundaries.

Answer ) An orbital has sharp

**Brief Explanation :**

**An orbital has sharp boundaries is an incorrect statement .**

**An orbital is a graphical representation of the three dimensional region in a space around the nucleus where the probability of finding the electron cloud is maximum (95%).**

** It has no physical existence & no sharp boundaries.**

Two electrons in the same orbital can not have the same value of:

1. Principal quantum number(n)

2.Azimuthal quantum number(l)

3.Magnetic quantum number(m)

4.Spin quantum number(s)

Answer ) Spin quantum number(s)

**Brief Explanation :** According to the Pauli’s exclusive principle “ An orbital can contain a maximum of two electrons with opposite spins” .It means that the two electrons in the same orbital have always the same values of principle, azimuthal and magnetic quantum numbers but different values of spins quantum number i.e. when one electron has clockwise (s=+1/2_) , the second electron must have anti-clock wise spin (s=-1/2)

A particle which has half-filled p-orbital after losing an electron:

1.C^{-1}

2.N

3.N^{-1}

4.O^{-1}

Answer ) N^{-1}

**Brief Explanation :**

N^{-1} is a particle which has half filled p-orbital after losing an electron .

N^{-1}has eight electrons. After losing one electron the remaining seven electrons may be distributed as 1s ^{2},2s ^{2},2p ^{3}.

According to Hind’s rule, the three electrons will remain unpaired in the three p-orbitals and each p-orbital will be half-filled i.e. px ^{1}, py ^{1}, pz ^{1}.

Valence shell electronic configuration of chromium (24Cr):

1.[ Ar] 4s^{2} , 3d^{4}

2.[ Ar] 4s^{2} , 3d^{5}

3.[ Ar] 4s^{1} , 3d^{5}

4.[ Ar] 4s^{0} , 3d^{6}

Answer ) [ Ar] 4s^{1} , 3d^{5}

**Brief Explanation :**

Generally the decreasing order of stability of orbitals with various configuration is:

Completely filled > Half filled > Partially filled

In Cr , one electron of 4s –orbital is shifted into 3d –orbital to make it half –filled and stable. Thus the valence shell electronic configuration of chromium (24Cr) becomes

[ Ar] 4s^{1} , 3d^{5}

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