Question

Is $Br$ or $Cl$ more stable?

Answer 1

$Br$ and $Cl$ both belong to the same group which is group $17$ . As we move down the group or across the period there are certain trends followed by the elements. For these the stability of an atom one must be familiar with the reactivity of the elements.

Complete answer:
The group $17$ is called the halogens. Since it lacks only one electron to attain the noble gas electronic configuration, it readily adds one electron to attain the noble gas configuration.
To find which of the given element is more stable let us first find what are the factors that affect the ease of electron acceptance:
Number of protons- Since the number of protons in $Br$ is $35$ and in $Cl$ is $17$ , therefore $Br$ nucleus has stronger attraction for an electrons than the $Cl$
Atomic radius- Since $Br$ has one more electron shell than $Cl$ atom the distance between the nucleus and the outermost shell of $Br$ is larger and therefore has to attract an electron further away, and greater distance means the electron is attracted less strongly.
Shielding – Again due to one more shell of $Br$ than the $Cl$ , it has more shielding $Cl$ meaning attraction between nucleus and electron is weaker.
Due to all these factors the electron is more attracted into the $Cl$ atom than that of $Br$ , $Cl$ is more reactive than $Br$ . For an element to be more stable it should be less reactive. Since $Br$ is less reactive, therefore will be more stable.

Note:
As we move down the group the reactivity decreases. Therefore $F$ is most reactive and $At$ the least reactive. This group has highly negative electron gain enthalpy and metallic characters increases as we go down the group.