今日知识点：Ionisation energy and valence electrons

例题

The table gives the successive ionisation energies for an element X.

What could be the formula of the chloride of X

A   XCl

B   XCl2

C   XCl3

D   XCl4

解析

Answer: C

Definition of Ionisation Energy

A  →  A+ + e-

The energy required for the above reaction, or removing an electron. (挪走一个电子需要的能量)

Based on the table, there is a “jump” from the 5th to the 6th ionization energies, so there are 5 valence electrons.

5th 和6th 之间的数字变化比较大, 说明挪走第6个电子的需要很高能量，很困难，即挪走5个电子后达到8电子稳定结构，所以最外层有5个电子。

If X has 5 valence electrons, it needs to get an additional 3 electrons to form an Octect. Since each chlorine can provide 1 electron to form a bonding pair, 3 Cl is needed, forming XCl3.

氯化物中心的X需要形成8电子稳定结构，所以还需要额外和Cl共用3个电子，每个Cl只能共用1个电子，所以需要3个Cl.

小试牛刀：

Three successive elements in the Periodic Table have first ionization energies which have the pattern shown in the diagram.

What could be the first element of the sequence?

A   C

B   N

C   F

D   Na

Correct Answer：C