electronic arrangement

It is about the s,p,d,f electron orbitals which is not taught in DSE, so sad.

For your level, please know that
For Argon (element before K), has 1s2, 2s2, 2p6, 3s2, 3p6 this 5 sub-orbital (Ar is 2, 2 + 6 = 8, 2 + 6 = 8, 288)
For the correct rule, the next orbintal should be 3d, (289)
but since the orbintal 4s has a lower energy level than 3d,
the electron goes to 4s, where represent the group I and II
orbital 4s is in next group of orbital, so 2881 and 2882
So K and Ca is 2881, 2882

the series of the orbital below (ascending distance between nucleus)
1s
2s
2p
3s
3p
4s
3d

為了達到全充滿、半充滿、全空的穩定狀態，不同的原子選擇不同的方式。具有同樣價電子構型的原子，理論上得或失電子的趨勢是相同的，這就是同一族元素性質相近的原因；同一族元素中，由於周期越高，價電子的能量就越高，就越容易失去。元素周期表中的區塊是根據價電子構型的顯著區別劃分的。不同區的元素性質差別同樣顯著：如s區元素只能形成簡單的離子，而d區的過渡金屬可以形成配合物。