For that, you need to remember the basic sequence of orbitals: 1s, 2s, 2p, 3s, 3p, 4s. Fill electrons according to the atomic number. Sulphur (16) is 1s²2s²2p⁶3s²3p⁴, so its valency is often 2, needing two electrons to complete its octet. Calcium (20) is up to 4s², so its valency is 2, as it easily loses those two electrons. I wrote these configurations on the margins of my Organic Chemistry notes, right next to my own little diagram of a tapioca plant’s root system—both need a stable structure to thrive.
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