电离能Ionization energy

电离能取决于原子核对最外层电子的吸引：核电荷越大，吸引越强；电子离核越远，吸引越弱；内层电子的屏蔽越多，有效核电荷越小，吸引越弱。此外，从半充满或全充满的亚层中移走电子需要更多能量，而成对电子之间的排斥使其中一个较易失去。

Ionization energy depends on how strongly the nucleus attracts the outer electron: a larger nuclear charge attracts more strongly; an electron further from the nucleus is held less strongly; more shielding by inner electrons lowers the effective nuclear charge and weakens the attraction. In addition, removing an electron from a half-filled or filled subshell costs more energy, while repulsion between paired electrons makes one of them easier to remove.

同周期从左到右，核电荷增加而屏蔽变化不大，第一电离能总体增大；同主族从上到下，电子层增加、原子半径增大，第一电离能减小。同周期有两处下降：Be > B，因为 B 的 2p 电子能量较高且受 2s 屏蔽；N > O，因为 O 的 2p 亚层中有一对成对电子，相互排斥。氦的第一电离能（2372 kJ mol⁻¹）最大，铯的很小（376 kJ mol⁻¹）。

Across a period the nuclear charge rises while shielding changes little, so first ionization energy generally increases; down a group the extra shells and larger radius make it decrease. There are two dips across a period: Be > B, because boron's 2p electron is higher in energy and shielded by the 2s electrons; N > O, because oxygen has one paired 2p electron repelled by its partner. Helium has the highest first ionization energy (2372 kJ mol⁻¹); caesium's is very low (376 kJ mol⁻¹).

把一个元素的逐级电离能依次画出，会在电子层改变处出现大的跳跃。例如钠的第一电离能为 496 kJ mol⁻¹，第二电离能跃升到 4562 kJ mol⁻¹，说明钠只有一个外层电子，第二个电子来自更靠近原子核的内层。由跳跃出现的位置可以判断元素所在的族。

Plotting the successive ionization energies of an element shows large jumps where the shell changes. Sodium's first ionization energy is 496 kJ mol⁻¹ but its second jumps to 4562 kJ mol⁻¹, showing sodium has one outer electron and the second comes from an inner shell closer to the nucleus. The position of the jump reveals the element's group.