Question

What element has the most similar properties to sodium?

Answer 1

The properties of any element are determined based on its electronic configuration. Electronic configuration means the way in which the electrons(valence as well as inner shell) are filled in that atom. Thus, it can be said that 2 atoms which have the same configuration in their outermost shells will have similar properties.

Complete answer:
We know that electronic configuration is one of the biggest determining factors into the determination of physical as well as chemical properties of that element. Properties like melting point, boiling point, ionization enthalpy, electronegativity and so on can be easily understood once the configuration of that element is known.
Now, considering the given element, i.e. sodium, we first need to understand its configuration.
Sodium(Na) has the atomic number $11$ . When we see the electronic configuration of this atom, we find:
$Na:[1{s^2}][2{s^2}2{p^6}][3{s^1}]$
This is the electronic configuration of sodium atoms. Thus, we can infer that it has $1$ electron in its outermost shell.
Thus, it can be easily inferred that all those elements that have their electronic configuration in the same manner, i.e. outermost shell having one electron, will have their property similar to that of sodium.
Hence, since sodium belongs to group $1$ , the group $1$ elements which are above and below it will exhibit a similar type of property.
Hence, it can be concluded that lithium and potassium will show properties similar to sodium.
Also, out of sodium and lithium, potassium will show properties which are closer to sodium when compared to properties exhibited by lithium.
Thus, it is potassium (K) which will show properties similar to sodium(Na).

Note:
Keep in mind that as we go down the group, there are certain phenomena such as lanthanide contraction, increase in atomic mass and increase in shells, which would result in certain deviations from showing the similar properties. Also, lithium is not considered as the one closest to show similar properties to sodium since it doesn’t have any electrons in p-orbital and thereby shows certain deviations.