Chemistry Class 11 NCERT Solutions: Chapter 3 Classification of Elements and Periodicity Part 2

Q: 10. How does atomic radius vary in a period and in a group? How you explain the variation?

Answer:

Atomic radius generally decreases from left to right across a period. This is because within a period, the outer electrons are present in the same valence shell and the atomic number increases from left to right across a period, resulting in an increased effective nuclear charge. As a result, the attraction of electrons to the nucleus increases.

On the other hand, the atomic radius generally increases down a group. This is because down a group, the principal quantum number (n) increases which results in an increase of the distance between the nucleus and valence electrons.

Q: 11. What do you understand by isoelectronic species? Name a species that will be isoelectronic with each of the following atoms or ions.

(i) Equation

(ii) Equation

(iii) Equation

(iv) Equation

Answer:

Atoms and ions having the same number of electrons are called isoelectronic species.

(i) Equation electrons. Thus, the species isoelectronic with it will also have Equation electrons. Some of its isoelectronic species are Equation Equation electrons), Equation

(ii) Ar has 18 electrons. Thus, the species isoelectronic with it will also have 18 electrons. Some of its isoelectronic species are Equation

(iii) Equation Thus, the species isoelectronic with it will also have 10 electrons. Some of its isoelectronic species are Equation

(iv) Equation Thus, the species isoelectronic with it will also have 36 electrons. Some of its isoelectronic species are Equation

Q: 12 consider the following species:

Equation

(a) What is common in them?

(b) Arrange them in the order of increasing ionic radii.

Answer:

(a) Each of the given species (ions) has the same number of electrons (10 electrons). Hence, the given species are isoelectronic.

(b) The ionic radii of isoelectronic species increases with a decrease in the magnitudes of nuclear charge.

The arrangement of the given species in order of their increasing nuclear charge is as follows:

Equation

Nuclear charge Equation

Therefore, the arrangement of the given species in order of their increasing ionic radii is as follows:

Equation

Q: 13. Explain why cations are smaller and anions larger in radii than their parent atoms?

Answer:

A cation has a fewer number of electrons than its parent atom, while its nuclear charge remains the same. As a result, the attraction of electrons to the nucleus is more in a cation than in its parent atom. Therefore, a cation is smaller in size than its parent atom.

On the other hand, an anion has one or more electrons than its parent atom, resulting in an increased repulsion among the electrons and a decrease in the effective nuclear charge. As a result, the distance between the valence electrons and the nucleus is more in anions than in it’s the parent atom. Hence, an anion is larger in radius than its parent atom.

Q: 14. What is the significance of the terms – ‘isolated gaseous atom’ and ground state’ while defining the ionization enthalpy and electron gain enthalpy?

Hint: Requirements for comparison purposes.

Answer:

Ionization enthalpy is the energy required to remove an electron from an isolated gaseous atom in its ground state. Although the atoms are widely separated in the gaseous state, there are some amounts of attractive forces among the atoms. To determine the ionization enthalpy, it is impossible to isolate a single atom. But, the force of attraction can be further reduced by lowering the pressure. For this reason, the term ‘isolated gaseous atom is in its ground state, then less amount energy would be required to remove an electron from it. Therefore, for comparison purposes, ionization enthalpy and electron gain enthalpy must be determined for an ‘isolated gaseous atom’ and its ‘ground state’.

Q: 15. Energy of an electron in the ground state of the hydrogen atom Equation the ionization enthalpy of atomic hydrogen in terms of J Equation

Answer:

It is given that the energy of an electron in the ground state of the hydrogen atom is Equation

Therefore, the energy required to remove that electron from the ground state of hydrogen atom is Equation

Equation

Hence, ionization enthalpy of atomic hydrogen in terms of J Equation

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