Question

Which among the following is monovalent?
A.Sodium
B.Oxygen
C.Aluminium
D.Copper

Answer 1

Hint: Valency is defined as the tendency of an atom to gain or lose the electrons. The species which donate one electron or which gain one electron are known as monovalent species. We find the valency by checking the atomic number and then writing electronic configuration. There are K, L, M and N shells. The last shell always contains 8 electrons.

Complete step by step answer:
The atoms which have less than four electrons in their outermost shell have valency equal to the electrons present in the last shell and they donate their electrons. So they acquire positive charge. When atoms have more than 4 electrons in their outermost shell, valency equals the 8 minus the number of electrons present in the shell. So, they gain electrons to complete their outermost shell also known as octet and possess negative charge.
The first shell, the K shell contains 2 electrons in it.
Sodium: it has atomic number 11 when we write electronic configuration, we find it to be 2,8,1 So, it has 1 electron in its last shell and can donate it. Thus, it has 1 valency which means monovalent. Thus, this option is correct.
Oxygen: it has atomic number 8 when we write electronic configuration, we find it to be 2, 6 So, it has 6 electrons in its last shell and can gain 2 electrons to complete its octet. Thus, it has 2 valencies which means divalent. Thus, this option is not correct.
Aluminium: it has atomic number 13 when we write electronic configuration we find it to be 2,8,3 So, it has 3 electrons in its last shell and can donate it. Thus, it has 3 valencies that are trivalent. Thus, this option is not correct.
Copper: it has an atomic number of 29 we can write configuration as \[\mathop {[Ar]3d}\nolimits^{10} \mathop {4s}\nolimits^1 \]. The electrons from 3d get excited to 4s orbital and from 4s two orbitals get lost. So, its valency is two that is it is divalent. This option is not correct.
 Our required option is A i.e sodium.

Note:
Electronic configuration can also be written in the s , p , d and f subshell rather than K , L ,M and N subshell. This happens for atoms which have more than 20 atomic numbers; it becomes difficult to write electronic configuration in K , L , M and N subshells. As seen in copper here 29 atomic number \[\mathop {[Ar]3d}\nolimits^{10} \mathop {4s}\nolimits^1 \]