Answer:
D) Strontium. The electron configuration of strontium in short-hand notation is [Kr] 5s^2.
Step-by-step explanation:
(I'm assuming this comes from a multiple choice question between barium, calcium, rubidium, and strontium).
I believe it's easiest to think of electron configuration in terms of the AUFBAU principle: the notion that electrons in an atom will occupy the lowest energy orbital first (they will generally fill in from inside rings -> out). Finding electron configuration can be made easier with a tool known as an AUFBAU diagram, though it may also be found using just the periodic table itself. Shorthand notation uses a noble gas to abbreviate a longer electron configuration; [Kr] 5s^2 is equal to [1s^2 2s^2 2p^6 3s^2 3p^6 3d^10 4s^2 4p^6] 5s^2.
For a question like this, it makes sense to locate each element choice on the periodic table. This quickly narrows down our choices; short-hand notation gives us a major clue. The [Kr] in [Kr] 5s^2 indicates the noble gas krypton. As mentioned previously, short-hand notation essentially means "rounding" to the nearest noble gas; the row below the selected noble gas will contain the elements that "round" to said noble gas when orbital notation is used. In this instance, rubidium and strontium are the only two elements out of the answer choices that are located in row 5, the row below krypton, making them the only two possible answers.
To narrow it down to the final selection, we must look at the second half of the given notation, the 5s^2 in [Kr] 5s^2. This is where we use the periodic table's structure to our advantage; you may have noticed that it's organized mainly by both properties of elements and electron position. To break it down: the "5" refers to the fifth row. The "s" refers to the s orbital, which can hold only two electrons, meaning the element is within the first two rows of the periodic table. The "2" exponent reveals the identity of the element; it means that there are two electrons in the valence shell, and thus that the element is in the second column. (If there were multiple exponents, you would add them together and count out that number across the specific row of the periodic table. Because there is only one exponent in [Kr] 5s^2, we count 2 across the 5th row.)
To reiterate, [Kr] 5s^2 is strontium, the 2nd element in the 5th row, the row under Krypton.
Hope this helps! I've attached two images to illustrate, with the related noble gas outline in black, the possible answers outlined in blue, and the correct answer outlined in red.
For a more in-depth explanation on electron configuration and shorthand notation, I'd recommend checking out The Organic Chemistry Tutor's video, "Electron Configuration - How To Identify The Element."