Question

Element showing spontaneous combustion is:
A. Phosphorus (P)
B. Calcium (Ca)
C. Copper (Cu)
D. Lithium (Li)

Answer 1

Hint: Combustion reaction is a process in which any substance burns in the presence of oxygen. In this process the substance gets oxidised which means it loses electrons and forms different oxides. So, for this process, the element must not have stable electronic configuration otherwise it will not lose electrons easily. Therefore, the element should have low ionisation enthalpy. We will see the electronic configuration and properties of all of the given compounds and find out which will show spontaneous combustion.

Complete answer:
We know that a combustion reaction takes place when a substance or element burns in the presence of oxygen. In this process the substance gets oxidised which means it loses electrons and forms different oxides. Spontaneous process refers to the process which takes place instantly by itself without the help of any initiator.
This means that the ionization enthalpy of the element should be less, so that it can easily lose electrons. The electronic configuration of all of the above elements are given as:
$$P = 1{s^2}2{s^2}2{p^6}3{s^2}3{p^3}$$
$$Ca = 1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}$$
$$Cu = 1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^1}3{d^{10}}$$
$$Li = 1{s^2}2{s^1}$$
In the above electronic configurations, we can see that phosphorus is stable because of half-filled p orbitals, Calcium is stable because of fully-filled orbitals. Between copper and lithium, the ionisation energy of lithium will be lower because after losing one electron it will attain noble gas configuration.
Hence, lithium will show spontaneous combustion.
Therefore, the correct option is D. Lithium (Li).

Note:
Ionisation energy also depends on various factors such as size, electronegativity of the element and its electronic configuration. Ionisation energy is inversely proportional to size. It is also directly proportional to electronegativity of the atom because if an atom is electronegative then it will not lose electrons easily and hence, it will have high ionisation energy.