屏蔽效应Shielding effect

内层电子位于外层电子与原子核之间，屏蔽作用强；同层电子相互屏蔽较弱。因此同周期从左到右增加的电子在同一层，屏蔽增加不多，有效核电荷增大；同族从上到下增加了新的电子层，外层电子受到更多屏蔽且离核更远。

Inner electrons lie between the outer electrons and the nucleus and shield strongly; electrons in the same shell shield each other weakly. Across a period the added electrons go into the same shell, so shielding grows little and the effective charge rises; down a group a new shell is added and the outer electrons are more shielded and further away.

同一电子层中，s 电子比 p 电子更能钻入内层电子区域靠近原子核，受屏蔽较少、能量较低，所以 2s 低于 2p。这也解释了为什么 B 的第一电离能比 Be 低：硼失去的是能量较高的 2p 电子。

Within a shell, s electrons penetrate the inner electron region more than p electrons, are shielded less and lie lower in energy, so 2s is below 2p. This also explains why boron's first ionisation energy is lower than beryllium's: boron loses a higher-energy 2p electron.