Chemistry Class 11 NCERT Solutions: Chapter 2 Structure of Atom Part 20

Q: 64 among the following pairs of orbitals which orbital will experience the larger effective nuclear charge?

(i) 2s and 3s,

(ii) 4d and 4f,

(iii) 3d and 3p

Answer:

Nuclear charge is defined as the net positive charge experienced by an electron in the orbital of a multi-electron atom. The closer the orbital, the greater is the nuclear charge experienced by the electron (s) in it.

(i) The electron(s) present in the 2s orbital will experience greater nuclear charge (being closer to the nucleus) than the electron(s) in the 3s orbital.

(ii) 4d will experience greater nuclear charge than 4f since 4d is closer to the nucleus.

(iii) 3p will experience greater nuclear charge since it is closer to the nucleus than 3f.

Q: 65. The unpaired electrons in Al and Si are present in 3p orbital. Which electrons will experience more effective nuclear charge from the nucleus?

Answer:

Nuclear charge is defined as the net positive charge experienced by an electron in a multi-electron atom.

The higher the atomic number, the higher is the nuclear charge. Silicon has 14 protons while aluminium has 13 protons. Hence, silicon has a larger nuclear charge of (+14) than aluminium, which has a nuclear charge of (+13). Thus, the electrons in the 3p orbital of silicon will experience a more effective nuclear charge than aluminium.

Q: 66. Indicate the number of unpaired electrons in : (a) P, (b) Si, (c) Cr, (d) Fe and (e) Kr.

Answer:

(a) Phosphorus (P):

Atomic number = 15

The electronic configuration of P is:

Equation

The orbital picture of P can be represented as:

From the orbital picture, phosphorus has three unpaired electrons.

(b) Silicon (Si):

Atomic number Equation

The electronic configuration of Si is:

Equation

The orbital picture of Si can be represented as:

From the orbital picture, silicon has two unpaired electrons.

(c) Chromium (Cr):

Atomic number Equation

The electronic configuration of Cr is:

Equation

The orbital picture of chromium is:

From the orbital picture, chromium has six unpaired electrons.

(d) Equation (Fe):

Atomic number = 26

The electronic configuration is:

Equation

The orbital picture of chromium is:

From the orbital picture, iron has four unpaired electrons.

(e) Krypton (Kr):

Atomic number Equation

The electronic configuration is:

Equation

The orbital picture of krypton is:

Since all orbitals are fully occupied, there are no unpaired electrons in krypton.

Q: 67.

(a) How many sub-shells are associated with Equation ?

(b) How many electrons will be present in the sub-shells having Equation value of Equation for Equation ?

Answer:

(a) Equation (Given)

For a given value of Equation can have values from zero to Equation

Equation

Thus, four sub-shells are associated with Equation , which are s, p, d and f.

(b) Number of orbitals in the Equation shell = Equation

For Equation

Number of orbitals = 16

If each orbital is taken fully, then it will have Equation electron with Equation value of Equation .

Equation Number of electrons with Equation value of Equation is 16.

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