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

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

barium calcium rubidium strontium


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

The element that has an electron configuration of [Kr]5s2 is strontium. The rest of the given choices do not answer the question above.

Given this abbreviated electron configuration, we begin from the row of the periodic table that contains Krypton (Kr). Then since the remaining electrons are 5s2 (which indicates 2 electrons in the 5s orbital), we check the next row (the 5th row) and find the 2nd element, which would contain two electrons. The first element is rubidium, while the second is strontium, so this means that Sr is the final answer.
you should look for the next row after Kr and second column (s2)
The answer is strontium. 
The element is in the sub-level s, period 5, and group 2. This element is Strontium. 

Strontium has an electron configuration of [Kr]5s²

Further explanation  

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 :

element is in period 5 (5 = shell)element is group 2(valence electron =2 and block s) has atomic number 38 (Kr = 36 + 2(valence elektron from 5s²)

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

D) strontium

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