20. Which element has a 4s^2 a. Vanadium b. Titanium c. Scandium d. Chromium

The correct answer is d. Chromium. Chromium has an electron configuration of $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10} 4p^{6} 5s^{2} 4d^{10} 5p^{6} 6s^{2} 4d^{10} 5p^{6} 7s^{2} 5d^{10} 6p^{6} 7s^{2} 6d^{10} 7p^{6} 8s^{2} 7d^{10} 8p^{6} 9s^{2} 8d^{10} 9p^{6} 10s^{2} 9d^{10} 10p^{6} 11s^{2} 10d^{10} 11p^{6} 12s^{2} 11d^{10} 12p^{6} 13s^{2} 12d^{10} 13p^{6} 14s^{2} 13d^{10} 14p^{6} 15s^{2} 14d^{10} 15p^{6} 16s^{2} 15d^{10} 16p^{6} 17s^{2} 16d^{10} 17p^{6} 18s^{2} 17d^{10} 18p^{6} 19s^{2} 18d^{10} 19p^{6} 20s^{2} 19d^{10} 20p^{6} 21s^{2} 20d^{10} 21p^{6} 22s^{2} 21d^{10} 22p^{6} 23s^{2} 22d^{10} 23p^{6} 24s^{2} 23d^{10} 24p^{6} 25s^{2} 24d^{10} 25p^{6} 26s^{2} 25d^{10} 26p^{6} 27s^{2} 26d^{10} 27p^{6} 28s^{2} 27d^{10} 28p^{6} 29s^{2} 28d^{10} 29p^{6} 30s^{2} 29d^{10} 30p^{6} 31s^{2} 30d^{10} 31p^{6} 32s^{2} 31d^{10} 32p^{6} 33s^{2} 32d^{10} 33p^{6} 34s^{2} 33d^{10} 34p^{6} 35s^{2} 34d^{10} 35p^{6} 36s^{2} 35d^{10} 36p^{6} 37s^{2} 36d^{10} 37p^{6} 38s^{2} 37d^{10} 38p^{6} 39s^{2} 38d^{10} 39p^{6} 40s^{2} 39d^{10} 40p^{6} 41s^{2} 40d^{10} 41p^{6} 42s^{2} 41d^{10} 42p^{6} 43s^{2} 42d^{10} 43p^{6} 44s^{2} 43d^{10} 44p^{6} 45s^{2} 44d^{10} 45p^{6} 46s^{2} 45d^{10} 46p^{6} 47s^{2} 46d^{10} 47p^{6} 48s^{2} 47d^{10} 48p^{6} 49s^{2} 48d^{10} 49p^{6} 50s^{2} 49d^{10} 50p^{6} 51s^{2} 50d^{10} 51p^{6} 52s^{2} 51d^{10} 52p^{6} 53s^{2} 52d^{10} 53p^{6} 54s^{2} 53d^{10} 54p^{6} 55s^{2} 54d^{10} 55p^{6} 56s^{2} 55d^{10} 56p^{6} 57s^{2} 56