Which of these elements has an electron configuration of [Kr]5s2? barium

calcium

rubidium

strontium

Strontium has an electron configuration of [Kr]5s²

In an atom there are levels of energy in the shell and sub shell  

This energy level is expressed in the form of electron configurations.  

Writing electron configurations starts from the lowest to the highest sub-shell energy level. There are 4 sub-shells in the shell of an atom, namely s, p, d and f. The maximum number of electrons for each sub shell is  

s: 2 electrons  

p: 6 electrons  

d: 10 electrons and  

f: 14 electrons  

From an electron configuration of [Kr]5s² can be concluded :

If we look at the periodic system, the elements that meet the requirements in question are Strontium (Sr)

pie chart because it is in percents ( % )