7.1
Discuss the general characteristics of Group 15 elements with reference to
their electronic configuration, oxidation state, atomic size, ionisation enthalp
and electronegativity.
7.2 Why does the reactivity of nitrogen differ from phosphorus?
7.3 Discuss the trends in chemical reactivity of group 15 elements.
Answers
Answer:
7.1 General trends in group 15 elements
(i) Electronic configuration: All the elements in group 15 have 5 valence electrons. Their general electronic configuration is ns2 np3.
(ii) Oxidation states: All these elements have 5 valence electrons and require three more electrons to complete their octets. However, gaining electrons is very difficult as the nucleus will have to attract three more electrons. This can take place only with nitrogen as it is the smallest in size and the distance between the nucleus and the valence shell is relatively small. The remaining elements of this group show a formal oxidation state of −3 in their covalent compounds. In addition to the −3 state, N and P also show −1 and −2 oxidation states.
All the elements present in this group show +3 and +5 oxidation states. However, the stability of +5 oxidation state decreases down a group, whereas the stability of +3 oxidation state increases. This happens because of the inert pair effect.
(iii) Ionization energy and electronegativity First ionization decreases on moving down a group. This is because of increasing atomic sizes. As we move down a group, electronegativity decreases, owing to an increase in size.
(iv) Atomic size: On moving down a group, the atomic size increases. This increase in the atomic size is attributed to an increase in the number of shells.
7.2 Nitrogen is chemically less reactive. ... It is because of nitrogen's small size that it is able to form pπ-pπ bonds with itself. This property is not exhibited by atoms such as phosphorus. Thus, phosphorus is more reactive than nitrogen.
7.3. The trends in chemical reactivity of group 15 elements are given below:
(i) Reactivity with hydrogen: Group 15 elements form hydrides of the type EH
3
where E is group 15 element. On moving down the group, the stability of hydrides decreases.
(ii) Reactivity with oxygen: Group 15 elements form the oxides E
2
O
3
and E
2
O
5
. In the oxide, when the oxidation state of group 15 element is higher, it is more acidic than the one with lower oxidation state. On moving down the group, the acidic character decreases.
(iii) Reactivity with halogens : Group 15 elements form the salts EX
3
and EX
5
. Nitrogen only forms NX
3
but not NX
5
as it lacks the d-orbital. NX
3
is unstable and other trihalides are stable.
(iv) Reactivity towards metals: Group 15 elements form binary compounds with metals. In these compounds, the oxidation state of metal is -3.
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